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Struts Framework-Interview-1

2010-11-30T01:27:00.000-08:00

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j2ee-JDBC interview questions

2008-07-29T18:10:00.000-07:00

JDBC

1) What are the steps in the JDBC connection?
While making a JDBC connection we go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();

2) What is JDBC?

JDBC is a set of Java API for executing SQL statements. This API consists of a set of classes and interfaces to enable programs to write pure Java Database applications.

3) What are drivers available?

a) JDBC-ODBC Bridge driver

b) Native API Partly-Java driver

c) JDBC-Net Pure Java driver

d) Native-Protocol Pure Java driver

4) What is the difference between JDBC and ODBC?

a) OBDC is for Microsoft and JDBC is for Java applications.

b) ODBC can't be directly used with Java because it uses a C interface.

c) ODBC makes use of pointers which have been removed totally from Java.

d) ODBC mixes simple and advanced features together and has complex options for simple queries. But JDBC is designed to keep things simple while allowing advanced capabilities when required.

e) ODBC requires manual installation of the ODBC driver manager and driver on all client machines. JDBC drivers are written in Java and JDBC code is automatically installable, secure, and portable on all platforms.

f) JDBC API is a natural Java interface and is built on ODBC. JDBC retains some of the basic features of ODBC.

5) What are the types of JDBC Driver Models and explain them?

There are two types of JDBC Driver Models and they are:

a) Two tier model

b) Three tier model

Two tier model: In this model, Java applications interact directly with the database. A JDBC driver is required to communicate with the particular database management system that is being accessed. SQL statements are sent to the database and the results are given to user. This model is referred to as client/server configuration where user is the client and the machine that has the database is called as the server.

Three tier model: A middle tier is introduced in this model. The functions of this model are:

a) Collection of SQL statements from the client and handing it over to the database,

b) Receiving results from database to the client and

c) Maintaining control over accessing and updating of the above.

6) What are the steps involved for making a connection with a database or how do you connect to a database?

a) Loading the driver : To load the driver, Class.forName( ) method is used.

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

When the driver is loaded, it registers itself with the java.sql.DriverManager class as an available database driver.

b) Making a connection with database : To open a connection to a given database,

DriverManager.getConnection( ) method is used.

Connection con = DriverManager.getConnection ("jdbc:odbc:somedb", "user", "password");

c) Executing SQL statements : To execute a SQL query, java.sql.statements class is used.

createStatement( ) method of Connection to obtain a new Statement object.

Statement stmt = con.createStatement( );

A query that returns data can be executed using the executeQuery( ) method of Statement. This method

executes the statement and returns a java.sql.ResultSet that encapsulates the retrieved data:

ResultSet rs = stmt.executeQuery("SELECT * FROM some table");

d) Process the results : ResultSet returns one row at a time. Next( ) method of ResultSet object can be called to move to the next row. The getString( ) and getObject( ) methods are used for retrieving column values:

while(rs.next( ) ) {

String event = rs.getString("event");

Object count = (Integer) rs.getObject("count");

7) What type of driver did you use in project?

JDBC-ODBC Bridge driver (is a driver that uses native(C language) libraries and makes calls to an existing ODBC driver to access a database engine).

8) What are the types of statements in JDBC?

Statement -- To be used createStatement() method for executing single SQL statement

PreparedStatement -- To be used preparedStatement() method for executing same SQL statement over and over .

CallableStatement -- To be used prepareCall( ) method for multiple SQL statements over and over.

9) What is stored procedure?

Stored procedure is a group of SQL statements that forms a logical unit and performs a particular task.

Stored Procedures are used to encapsulate a set of operations or queries to execute on database. Stored procedures can be compiled and executed with different parameters and results and may have any combination of input/output parameters.

10) How to create and call stored procedures?

To create stored procedures:

Create procedure procedurename (specify in, out and in out parameters)

BEGIN

Any multiple SQL statement;

END;

To call stored procedures:

CallableStatement csmt = con.prepareCall("{call procedure name(?,?)}");

csmt.registerOutParameter(column no., data type);

csmt.setInt(column no., column name)

csmt.execute( );

11)Why do we have index table in the database?

Because the index table contain the information of the other tables. It will be faster if we access the index table to find out what the other contain.

12) Give an example of using JDBC access the database.

Answer:

try

{

Class.forName("register the driver");

Connection con = DriverManager.getConnection("url of db", "username","password");

Statement state = con.createStatement();

state.executeUpdate("create table testing(firstname varchar(20), lastname varchar(20))");

state.executeQuery("insert into testing values('phu','huynh')");

state.close();

con.close();

}

catch(Exception e)

{

System.out.println(e);

}

13)What are the two major components of JDBC?

One implementation interface for database manufacturers, the other implementation interface for application and applet writers.

14)What is JDBC Driver interface?

The JDBC Driver interface provides vendor-specific implementations of the abstract classes provided by the JDBC API. Each vendors driver must provide implementations of the java.sql.Connection,Statement,PreparedStatement, CallableStatement, ResultSet and Driver.

15)What are the common tasks of JDBC?

1.Create an instance of a JDBC driver or load JDBC drivers through jdbc.drivers; 2. Register a driver; 3. Specify a database; 4. Open a database connection; 5. Submit a query; 6. Receive results.

16)What packages are used by JDBC?

There are 8 packages: java.sql.Driver, Connection,Statement, PreparedStatement, CallableStatement, ResultSet, ResultSetMetaData, DatabaseMetaData.

17)What are the flow statements of JDBC?

A URL string -->getConnection-->DriverManager-->Driver-->Connection-->Statement-->executeQuery-->ResultSet.

18)What are the steps involved in establishing a connection?

This involves two steps: (1) loading the driver and (2) making the connection.

19)How can you load the drivers?

Loading the driver or drivers you want to use is very simple and involves just one line of code. If, for example, you want to use the JDBC-ODBC Bridge driver, the following code will load it:

Eg.

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

Your driver documentation will give you the class name to use. For instance, if the class name is jdbc.DriverXYZ , you would load the driver with the following line of code:

E.g.

Class.forName("jdbc.DriverXYZ");

20)What Class.forName will do while loading drivers?

It is used to create an instance of a driver and register it with the DriverManager. When you have loaded a driver, it is available for making a connection with a DBMS.

21)How can you make the connection?

In establishing a connection is to have the appropriate driver connect to the DBMS. The following line of code illustrates the general idea:

E.g.

String url = "jdbc:odbc:Fred";

Connection con = DriverManager.getConnection(url, "Fernanda", "J8");

22)How can you create JDBC statements?

A Statement object is what sends your SQL statement to the DBMS. You simply create a Statement object and then execute it, supplying the appropriate execute method with the SQL statement you want to send. For a SELECT statement, the method to use is executeQuery. For statements that create or modify tables, the method to use is executeUpdate. E.g. It takes an instance of an active connection to create a Statement object. In the following example, we use our Connection object con to create the Statement object stmt :

Statement stmt = con.createStatement();

23)How can you retrieve data from the ResultSet?

First JDBC returns results in a ResultSet object, so we need to declare an instance of the class ResultSet to hold our results. The following code demonstrates declaring the ResultSet object rs.

E.g.

ResultSet rs = stmt.executeQuery("SELECT COF_NAME, PRICE FROM COFFEES");

Second:

String s = rs.getString("COF_NAME");

The method getString is invoked on the ResultSet object rs , so getString will retrieve (get) the value stored in the column COF_NAME in the current row of rs

24)What are the different types of Statements?

1.Statement (use createStatement method) 2. Prepared Statement (Use prepareStatement method) and 3. Callable Statement (Use prepareCall)

25)How can you use PreparedStatement?

This special type of statement is derived from the more general class, Statement. If you want to execute a Statement object many times, it will normally reduce execution time to use a PreparedStatement object instead. The advantage to this is that in most cases, this SQL statement will be sent to the DBMS right away, where it will be compiled. As a result, the PreparedStatement object contains not just an SQL statement, but an SQL statement that has been precompiled. This means that when the PreparedStatement is executed, the DBMS can just run the PreparedStatement 's SQL statement without having to compile it first.

E.g.

PreparedStatement updateSales = con.prepareStatement("UPDATE COFFEES SET SALES = ? WHERE COF_NAME LIKE ?");

26)How to call a Stored Procedure from JDBC?

The first step is to create a CallableStatement object. As with Statement an and PreparedStatement objects, this is done with an open Connection object. A CallableStatement object contains a call to a stored procedure;

E.g.

CallableStatement cs = con.prepareCall("{call SHOW_SUPPLIERS}");

ResultSet rs = cs.executeQuery();

27)How to Retrieve Warnings?

SQLWarning objects are a subclass of SQLException that deal with database access warnings. Warnings do not stop the execution of an application, as exceptions do; they simply alert the user that something did not happen as planned. A warning can be reported on a Connection object, a Statement object (including PreparedStatement and CallableStatement objects), or a ResultSet object. Each of these classes has a getWarnings method, which you must invoke in order to see the first warning reported on the calling object

E.g.

SQLWarning warning = stmt.getWarnings();

if (warning != null) {

while (warning != null) {

System.out.println("Message: " + warning.getMessage());

System.out.println("SQLState: " + warning.getSQLState());

System.out.print("Vendor error code: ");

System.out.println(warning.getErrorCode());

warning = warning.getNextWarning();

}

28)How to Make Updates to Updatable Result Sets?

Another new feature in the JDBC 2.0 API is the ability to update rows in a result set using methods in the Java programming language rather than having to send an SQL command. But before you can take advantage of this capability, you need to create a ResultSet object that is updatable. In order to do this, you supply the ResultSet constant CONCUR_UPDATABLE to the createStatement method.

E.g.

Connection con = DriverManager.getConnection("jdbc:mySubprotocol:mySubName");

Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,

ResultSet.CONCUR_UPDATABLE);

ResultSet uprs = ("SELECT COF_NAME, PRICE FROM COFFEES");

29). What is JDBC ? what are its advantages ?

It is an API .The latest version of jdbc api is (3.0).

The JDBC 3.0 API is divided into two packages:

(1) java.sql and (2) javax.sql.

Both packages are included in the J2SE and J2EE platforms.

Advantages:

The JDBC API can be used to interact with multiple data sources in a distributed, heterogenous environment.

It can connect to any of the database from java language.

It can switch over to any backend database without changing java code or by minute changes.

30). How many JDBC Drivers are there ? what are they?

There are 4 types of JDBC drivers.

a. JDBC-ODBC Bridge Driver(Type-1 driver)

b. Native API Partly Java Driver(Type-2 driver)

c. Net protocol pure Java Driver(Type-3 driver)

d. Native protocol Pure Java Driver(Type-4 driver)

31. Explain about JDBC-ODBC driver(Type-1) ? When this type of driver is used ?

In this mechanism the flow of execution will be

Java code(JDBC API)<------>JDBC-ODBC bridge driver<------->ODBC API<------->ODBC Layer<-------->DataBase

This type of JDBC Drivers provides a bridge between JDBC API and ODBC API.

This Bridge(JDBC-ODBC bridge) translates standard JDBC calls to Corresponding ODBC Calls, and

send them to ODBC database via ODBC Libraries.

The JDBC API is based on ODBC API.

ODBC(Open Database Connectivity)is Microsoft's API for Database drivers.

ODBC is based on X/Open Call Level Interface(CLI)specification for database access.

The URL and class to be loaded for this type of driver are

Class :- sun.jdbc.odbc.JdbcOdbcDriver

URL :- jdbc:odbc:dsnname

32. Explain about Type-2 driver ? When this type of driver is used ?

The Drivers which are written in Native code will come into this category

In this mechanism the flow of Execution will be

java code(JDBC API)<------>Type-2 driver(jdbc driver)<------->Native API(vendor specific)<------->DataBase

When database call is made using JDBC,the driver translates the request into vendor-specific API calls.

The database will process the requet and sends the results back through the Native API ,which will

forward them back to the JDBC dirver. The JDBC driver will format the results to conform to the JDBC

standard and return them to the application.

33. Explain about Type-3 driver ? When this type of driver is used ?

In this mechanism the flow of Execution will be

java code(JDBC API)<------>JDBC driver<------->JDBC driver server<-------->Native driver<------->DataBase

The Java Client Application sends the calls to the Intermediate data access server(jdbc driver server)

The middle tier then handles the requet using other driver(Type-II or Type-IV drivers) to complete the request.

34. Explain about Type-4 driver ? When this type of driver is used ?

This is a pure java driver(alternative to Type-II drivers).

In this mechanism the flow of Execution will be

java code(JDBC API)<------>Type-4 driver(jdbc driver)<------->DataBase

These type of drivers convert the JDBC API calls to direct network calls using

vendor specific networking protocal by making direct socket connection with

database.

examples of this type of drivers are

1.Tabular Data Stream for Sybase

2.Oracle Thin jdbc driver for Oracle

35. Which Driver is preferable for using JDBC API in Applets?

Type-4 Drivers.

36.Write the Syntax of URL to get connection ? Explain?

Syntax:- jdbc::

jdbc -----> is a protocal .This is only allowed protocal in JDBC.

----> The subprotocal is used to identify a database driver,or the

name of the database connectivity mechanism, choosen by the database driver providers.

-------> The syntax of the subname is driver specific. The driver may choose any

syntax appropriate for its implementation

ex: jdbc:odbc:dsn

jdbc:oracle:oci8:@ database name.

jdbc:orale:thin:@ database name:port number:SID

37.How do u Load a driver ?

Using Driver Class.forName(java.lang.String driverclass) or registerDriver(Driver driver)

38.what are the types of resultsets in JDBC3.0 ?How you can retrieve information of resultset?

ScrollableResultSet and ResultSet.We can retrieve information of resultset by using java.sql.ResultSetMetaData interface.You can get the instance by calling the method getMetaData() on ResulSet object.

39.write the steps to Connect database?

Class.forName(The class name of a spasific driver);

Connection c=DriverManager.getConnection(url of a spasific driver,user name,password);

Statement s=c.createStatement();

(or)

PreparedStatement p=c.prepareStatement();

(or)

CallableStatement cal=c.prpareCall();

Depending upon the requirement.

40.Can java objects be stored in database? how?

Yes.We can store java objects, BY using setObject(),setBlob() and setClob() methods in PreparedStatement

41.what do u mean by isolation level?

Isolation means that the business logic can proceed without

consideration for the other activities of the system.

42.How do u set the isolation level?

By using setTransactionIsolation(int level) in java.sql.Connection interface.

level MEANS:-

static final int TRANSACTION_READ_UNCOMMITTED //cannot prevent any reads.

static final int TRANSACTION_READ_COMMITTED //prevents dirty reads

static final int TRANSACTION_REPEATABLE_READ //prevents dirty reads & non-repeatable read.

static final int TRANSACTION_SERIALIZABLE //prevents dirty reads , non- repeatable read & phantom read.

These are the static final fields in java.sql.Connection interface.

43. What is a dirty read?

A Dirty read allows a row changed by one transaction to be

read by another transaction before any change in the row

have been committed.

This problem can be solved by setting the transaction isolation

level to TRANSACTION_READ_COMMITTED

44. what is a non-repeatable read ?

A non-repeatable read is where one transaction reads a row, a second

transaction alters or deletes the row, and the first transaction

re-reads the row,getting different values the second time.

This problem can be solved by setting the transaction isolation

level to TRANSACTION_REPEATABLE_READ

45. what is phantom read?

A phantom read is where one transaction reads all rows that satisfy

a WHERE condition,a second transaction inserts a row that satisfies

that WHERE condition,and the first transaction re-reads for the same

condition,retrieving the additional 'phantom' row in the second read

This problem can be solved by setting the transaction isolation

level to TRANSACTION_SERIALIZABLE

47.How to retrieve the information about the database ?

We can retrieve the info about the database by using inerface

java.sql.DatabaseMetaData

We can get this object by using getMetaData() method in

Connection interface.

48.what are the Different types of exceptions in jdbc?

BatchUpdateException

DataTruncation

SQLException

SQLWarning

49.How to execute no of queries at one go?

By using a batchUpdate's (ie throw addBatch() and executeBatch())

in java.sql.Statement interface,or by using procedures.

50. What are the advantages of connection pool.?

Performance

51.In which interface the methods commit() & rollback() are defined ?

A.java.sql.Connection interface

52.How to store images in database?

Using binary streams (i.e. getBinaryStream (), setBinaryStream ()). But it is not visible in database, it is stored in form of bytes, to make it visible we have to use any one front-end tool.

53.How to check null value in JDBC?

By using the method wasNull() in ResultSet ,it returns boolean value.

Returns whether the last column read had a value of SQL NULL.

Note that you must first call one of the getXXX methods on a column to try to read its value and then call the method wasNull to see if the value read was SQL NULL.

54.Give one Example of static Synchronized method in JDBC API?

getConnection() method in DriverManager class.Which is used to get object of Connection interface.

55.What is a Connection?

Connection is an interface which is used to make a connection between client and Database (ie opening a session with a particular database).

56.what is the difference between execute() ,executeUpdate() and executeQuery() ? where we will use them?

execute() method returns a boolean value (ie if the given query

returns a resutset then it returns true else false),so depending upon

the return value we can get the ResultSet object (getResultset())or

we can know how many rows have bean affected by our query

(getUpdateCount()).That is we can use this method for Fetching

queries and Non-Fetching queries.

Fetching queries are the queries which are used to fetch the records from database (ie which returns resutset)

ex: Select * from emp.

Non-Fetching queries are the queries which are used to update,insert,create or delete the records from database

ex: update emp set sal=10000 where empno=7809.

executeUpdate() method is used for nonfetching queries.which returns int value.

executeQuery() method is used for fetching queries which returns ResulSet object ,Which contains methods to fetch the values.

58). Using a Prepared Statement

A prepared statement should be used in cases where a particular SQL statement is used frequently. The prepared statement is more expensive to set up but executes faster than a statement. This example demonstrates a prepared statement for getting all rows from a table called ''mytable'' whose column COL_A equals ''Patrick Chan''. This example also demonstrates a prepared statement for updating data in the table. In particular, for all rows whose column COL_B equals 123, column COL_A is set to ''John Doe''.

try {

// Retrieving rows from the database.

PreparedStatement stmt = connection.prepareStatement(

"SELECT * FROM mytable WHERE COL_A = ?");

int colunm = 1;

stmt.setString(colunm, "Patrick Chan");

ResultSet rs = stmt.executeQuery();

// Updating the database.

stmt = connection.prepareStatement(

"UPDATE mytable SET COL_A = ? WHERE COL_B = ?");

colunm = 1;

stmt.setString(colunm, "John Doe");

colunm = 2;

stmt.setInt(colunm, 123);

int numUpdated = stmt.executeUpdate();

} catch (SQLException e) {

}

59.What are the types of JDBC drivers ?

Type 1- JDBC-ODBC Bridge

Type 2- Native API Driver

Type 3 - JDBC-Net,Java Pure Driver

Type 4- Native Protocol, Pure Java Driver

60.Which among the four driver is pure Java driver ?

Type 3 and Type 4

61.How do you connect without the Class.forName (" ") ?

It is not possible not connect

62.What does Class.forName return ?

Class Name

63.Why is preparedStatement,CallableStatement used for?

Preapred Stmt -SQL in the database is a precompiled, by the database for faster execution

CallableStmt-JDBC allows use of stored procedures by the CallableStatement class.

A CallableStatement object is created by the preaprecall()method in the connection object

64) Explain the role of Driver in JDBC?
The JDBC Driver provides vendor-specific implementations of the abstract classes provided by the JDBC API. Each vendors driver must provide implementations of the java.sql.Connection,Statement,PreparedStatement, CallableStatement, ResultSet and Driver.

65) Is java.sql.Driver a class or an Interface ?
It's an interface.

66) Is java.sql.DriverManager a class or an Interface ?
It's a class. This class provides the static getConnection method, through which the database connection is obtained.

67) Is java.sql.Connection a class or an Interface ?
java.sql.Connection is an interface. The implmentation is provided by the vendor specific Driver.

68) Is java.sql.Statement a class or an Interface ?
java.sql.Statement,java.sql.PreparedStatement and java.sql.CallableStatement are interfaces.

69) Which interface do PreparedStatement extend?
java.sql.Statement

70) Which interface do CallableStatement extend?
CallableStatement extends PreparedStatement.

71) What is the purpose Class.forName("") method?
The Class.forName("") method is used to load the driver.

72) Do you mean that Class.forName("") method can only be used to load a driver?
The Class.forName("") method can be used to load any class, not just the database vendor driver class.

73) Which statement throws ClassNotFoundException in SQL code block? and why?
Class.forName("") method throws ClassNotFoundException. This exception is thrown when the JVM is not able to find the class in the classpath.

74) What exception does Class.forName() throw?
ClassNotFoundException.

75) What is the return type of Class.forName() method ?
java.lang.Class

76) Can an Interface be instantiated? If not, justify and explain the following line of code:
Connection con = DriverManager.getConnection("dsd","sds","adsd");
An interface cannot be instantiated. But reference can be made to a interface. When a reference is made to interface, the refered object should have implemented all the abstract methods of the interface. In the above mentioned line, DriverManager.getConnection method returns Connection Object with implementation for all abstract methods.

77) What type of a method is getConnection()?
static method.

78) What is the return type for getConnection() method?
Connection object.

79) What is the return type for executeQuery() method?
ResultSet

80) What is the return type for executeUpdate() method and what does the return type indicate?
int. It indicates the no of records affected by the query.

81) What is the return type for execute() method and what does the return type indicate?
boolean. It indicates whether the query executed sucessfully or not.

82) Is Resultset a Classor an interface?
Resultset is an interface.

83) What is the advantage of PrepareStatement over Statement?
PreparedStatements are precompiled and so performance is better. PreparedStatement objects can be reused with passing different values to the queries.

84) What is the use of CallableStatement?
CallableStatement is used to execute Stored Procedures.

85) Name the method, which is used to prepare CallableStatement?
CallableStament.prepareCall().

86) What do mean by Connection pooling?

Opening and closing of database connections is a costly excercise. So a pool of database connections is obtained at start up by the application server and maintained in a pool. When there is a request for a connection from the application, the application server gives the connection from the pool and when closed by the application is returned back to the pool. Min and max size of the connection pool is configurable. This technique provides better handling of database connectivity.

87. Which methods and classes are used for connection pooling?

88). How will you perform transactions using JDBC?

89). What is difference between PreparedStatement and Statement?

90). What is the difference between ExecuteUpdate and ExecuteQuery?

91). How do you know which driver is connected to a database?

92). What is DSN and System DSN and differentiate these two?

93). How you can know about drivers and database information?

94). If you are truncated using JDBC, How can you know? That how much

data is truncated?

95). What are the Normalization Rules? Define the Normalization?

JAVA-EXCEPTION HANDLING

2008-07-22T08:12:00.001-07:00

EXCEPTION HANDLING

1). What is Exception?

An exception is an abnormal condition that arises on programming code at run time. An exception is a run time error.

2). What is java Exception?

A java Exception is an object that describes an exceptional condition that has occurred in a piece of code. When an exceptional condition arises, an object representing that exception is created and thrown to the method that caused error. That method may choose to handle the exception itself or pass to on.

3). How many ways the java exception occurred?

There are two ways.

1). Java run-time system. 2). Manually generated by programming code.

4). What does java run-time system exception means?

Exception thrown by java relate to fundamental errors that violate the rules of the java language. Java run-time system exception are automatically thrown by the java run-time system.

5). What does manually generated exception?

Manually generated exceptions are used to report some error condition to the caller of a method. To manually throw an exception, use keyword throw.

6). How does java exception managed or handled?

Java exception handling is managed via five keywords: try, catch, and throw, throws, finally.

7). How does exception handling work?

Program statements that monitor for exception are contained within a try block. If an exception occurs within the try block, it is thrown. The catch clause can catch this exception and handle the exception.

Note: all exception types are subclasses of built-in class Throwable.

8). What is uncaught exception?

Any exception is not caught by program will be processed by the default handler. The default handler displays a string that describes the exception, print stack trace and terminates the program.

9) What is stack trace?

The stack trace will show the sequence of method invocation that led up to the error.

10) Although java run-time systems handle exception, why we use try and catch?

Two benefits:

1). It allows to fix error. 2). It prevents the program from automatically terminating.

11). Why we use multiple catch clauses for single statement?

In some cases, more than one exception can be raised by a single piece of code. To handle this type of exception, you can specify two or more catch clauses, each catch have different type of exception. When an exception is thrown, each catch statement is inspected in order and the first one whose type matches that of the exception is executed. After one catch statement excutes, the others are bypassed, and execution continues after the try/catch block.

12). What is nested try statement?

The try statement can be nested. A try statement can be inside the block of another try. Each time a try statement is entered, the context of that exception is pushed on the stack. If an inner try statement does not have a catch handler for a particular exception, the stack is unwound and the next try statement’s catch handlers are inspected for a match. This continues until one of the catch statements succeeds, or the java run-time system will handle the exception.

13). What is throwing an Exception?.

The act of passing an Exception Object to the runtime system is called Throwing an Exception.

14) What is the use of throw statement?

It is possible to throw an exception explicitly using throw statement.

15) What is the use of throws statement?

If a method is capable of causing an exception that it does not handle, it must specify this behavior so that the callers of the method can guard themselves against that exception. We do this by using throws clause in method’s declaration. The throws clause lists the types of exceptions that a method might throw. This is necessary for all exception except the type Error or Runtime Exception or their subclasses. All exception that a method can throw must be declared in the throws clause. If they are not, the compile time error will result.

16). What is the unchecked exception?

An unchecked exception need not be included in any method’s throws list. Because the compiler dose not check if a method handles or throws these exceptions.

17). What is the checked exception?

A checked exception included in method’s declaration if the method can generated one of these exceptions and does not handle it itself.

18). What is chained exception?

The chained exception features allow to associate another exception with an exception. The second exception describes the cause of the first exception.

19) What is NullPointerException and how to handle it?

When an object is not initialized, the default value is null. When the following things happen, the NullPointerException is thrown:
--Calling the instance method of a null object.
--Accessing or modifying the field of a null object.
--Taking the length of a null as if it were an array.
--Accessing or modifying the slots of null as if it were an array.
--Throwing null as if it were a Throwable value.
The NullPointerException is a runtime exception. The best practice is to catch such exception even if it is not required by language design.

20). What is the purpose of the finally clause of a try-catch-finally statement?

The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.

21) What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

22) What is the difference between throw and throws keywords?
The throw keyword denotes a statement that causes an exception to be initiated. It takes the Exception object to be thrown as argument. The exception will be caught by an immediately encompassing try-catch construction or propagated further up the calling hierarchy.
The throws keyword is a modifier of a method that designates that exceptions may come out of the method, either by virtue of the method throwing the exception itself or because it fails to catch such exceptions that a method it calls may throw.

23) Does the code in finally block get executed if there is an exception and a return statement in a catch block?
If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(1) statement is executed earlier or the system shut down earlier or the memory is used up earlier before the thread goes to finally block.

24)When is the ArrayStoreException thrown?
When copying elements between different arrays, if the source or destination arguments are not arrays or their types are not compatible, an ArrayStoreException will be thrown.

25) WHEN IS THE ArithmeticException thrown?
The ArithmeticException is thrown when integer is divided by zero or taking the remainder of a number by zero. It is never thrown in floating-point operations.

26)What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

27)Can an exception be rethrown?
Yes, an exception can be rethrown.

28)What happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.

29)Which arithmetic operations can result in the throwing of an ArithmeticException?
Integer / and % can result in the throwing of an ArithmeticException.

30)What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination.

31)What happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.

32)How does a try statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.

33) Explain the user defined Exceptions?

User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can create by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions.
Example:
class myCustomException extends Exception {
// The class simply has to exist to be an exception
}

34) What are checked exceptions?

Checked exceptions are those that the Java compiler forces you to catch. e.g. IOException are checked Exceptions.

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35) What are runtime exceptions?

Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

36) What is the difference between error and an exception?

An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).

37) How to create custom exceptions?

Your class should extend class Exception.

38) If I want an object of my class to be thrown as an exception object, what should I do?

The class should extend from Exception class. Or you can extend your class from some more precise exception type also.

39) If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?

One cannot do anything in this scenario. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

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40). How does an exception permeate through the code?

An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

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41) What are the different ways to handle exceptions?

There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method handle those exceptions.

42) What is the basic difference between the 2 approaches to exception handling.
1. Try catch block and
2. Specifying the candidate exceptions in the throws clause?
When should you use which approach?
In the first approach as a programmer of the method, you yourself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

43) Is it necessary that a catch block must follow each try block?

It is not necessary that a catch block must follow each try block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

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44) If I write return at the end of the try block, will the finally block still execute?

Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.

45) If I write System.exit (0); at the end of the try block, will the finally block still execute?

No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

46) How does a try statement determine which catch clause should be used to handle an exception?

When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.

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47) What is the catch or declare rule for method declarations?

If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

48) Which package contains exception handling related classes?
java.lang
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49) What are the two types of Exceptions?

Checked Exceptions and Unchecked Exceptions.
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50) What is the base class of all exceptions?
java.lang.Throwable
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51) What is the difference between Exception and Error in java?
Exception and Error are the subclasses of the Throwable class. Exception class is used for exceptional conditions that user program should catch. Error defines exceptions that are not excepted to be caught by the user program. Example is Stack Overflow.

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52) What is the difference between throw and throws?
throw is used to explicitly raise a exception within the program, the statement would be throw new Exception(); throws clause is used to indicate the exceptions that are not handled by the method. It must specify this behavior so the callers of the method can guard against the exceptions. throws is specified in the method signature. If multiple exceptions are not handled, then they are separated by a comma. the statement would be as follows: public void doSomething() throws IOException,MyException{}
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53) Differentiate between Checked Exceptions and Unchecked Exceptions?
Checked Exceptions are those exceptions which should be explicitly handled by the calling method. Unhandled checked exceptions results in compilation error.

Unchecked Exceptions are those which occur at runtime and need not be explicitly handled. RuntimeException and it's subclasses, Error and it's subclasses fall under unchecked exceptions.
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54) What are User defined Exceptions?
Apart from the exceptions already defined in Java package libraries, user can define his own exception classes by extending Exception class.
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55) What is the importance of finally block in exception handling?
Finally block will be executed whether or not an exception is thrown. If an exception is thrown, the finally block will execute even if no catch statement match the exception. Any time a method is about to return to the caller from inside try/catch block, via an uncaught exception or an explicit return statement, the finally block will be executed. Finally is used to free up resources like database connections, IO handles, etc.
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56) Can a catch block exist without a try block?
No. A catch block should always go with a try block.
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57) Can a finally block exist with a try block but without a catch?
Yes. The following are the combinations try/catch or try/catch/finally or try/finally.
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58) What will happen to the Exception object after exception handling?
Exception object will be garbage collected.
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59) The subclass exception should precede the base class exception when used within the catch clause. True/False?
True.
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60) Exceptions can be caught or rethrown to a calling method. True/False?
True.
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61) The statements following the throw keyword in a program are not executed. True/False?
True.
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62) How does finally block differ from finalize() method?
Finally block will be executed whether or not an exception is thrown. So it is used to free resoources. finalize() is a protected method in the Object class which is called by the JVM just before an object is garbage collected.
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63) What are the constraints imposed by overriding on exception handling?
An overriding method in a subclass may only throw exceptions declared in the parent class or children of the exceptions declared in the parent class.
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64). If we don’t use exception handling what will happen?

If we don’t use exception handling, the exception is caught by the default handler provided by java run-time system.

65). What are the types of error?

There are two type of error.

1). Compile-time error.

2). Run-time error.

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66). What is compile time error?

All syntax error will be detected and displayed by java compiler. So they are compile time error.

If the compiler display error, it will not create .class file.

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67). What is run time error?

The program may compile successfully and create .class file but may not run properly. Such a program may produce wrong result or terminate program. Exam is stack overflow.

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JAVA-MULTI THREADING

2008-07-17T16:51:00.000-07:00

MULTI-THREADED PROGRAMMING

1) What is the difference between process and thread?

Process is a program in execution whereas thread is a separate path of execution in a program.

2) What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?

Multithreading is the mechanism in which more than one thread run independent of each other within the process.

wait (), notify () and notifyAll () methods can be used for inter-thread communication and these methods are in Object class.

wait( ) : When a thread executes a call to wait( ) method, it surrenders the object lock and enters into a waiting state.

notify( ) or notifyAll( ) : To remove a thread from the waiting state, some other thread must make a call to notify( ) or notifyAll( ) method on the same object.

3) What is the class and interface in java to create thread and which is the most advantageous method?

Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.

4) What are the states associated in the thread?

Thread contains ready, running, waiting and dead states.

5) What is synchronization?

Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.

6) When you will synchronize a piece of your code?

When you expect different threads will access your code and these threads may change a particular data causing data corruption.

7) What is deadlock?

When two threads are waiting each other and can't precede the program is said to be deadlock.

8) What are daemon thread and which method is used to create the daemon thread?

Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.

9). What are the disadvantages of using threads?

DeadLock

10).What are mutex and semaphore? What is the difference between them?

A mutex is a synchronization object that allows only one process or thread to access a critical code block. A semaphore on the other hand allows one or more processes or threads to access a critical code block. A semaphore is a multiple mutex.

11). Describe synchronization in respect to multithreading.?

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

12) What are synchronized methods and synchronized statements?

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class.

Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

13). What are three ways in which a thread can enter the waiting state?

A thread can enter the waiting state by invoking its sleep () method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait () method. It can also enter the waiting state by invoking its (deprecated) suspend () method.

14). In a lock be acquired on a class?

Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

15). What's new with the stop(), suspend() and resume() methods in JDK 1.2?

The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.

16). What state does a thread enter when it terminates its processing?

When a thread terminates it’s processing, it enters the dead state.

17). What is the difference between yielding and sleeping?

When a task invokes its yield() method, it returns to the ready state.

When a task invokes its sleep() method, it returns to the waiting state.

18). What is the difference between preemptive scheduling and time slicing?

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence.

Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

19). Can an anonymous class be declared as implementing an interface and extending a class?

An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

20). What invokes a thread's run () method?

After a thread is started via its start () method or that of the Thread class, the JVM invokes the thread's run () method when the thread is initially executed.

21). What is the purpose of the wait(), notify(), and notifyAll() methods?

The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate each other.

22). What are the high-level thread states?

The high-level thread states are ready, running, waiting, and dead.

23) How does multithreading take place on a computer with a single CPU?

The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

24). What is a thread?

It’s a single sequential stream of execution.

A thread is an independent path of execution in a system.

25). What is runnable?

It’s an Interface through which Java implements Threads. The class can extend from any class but if it implements Runnable, Threads can be used in that particular application.

26). What are the various thread priorities?.

(i) Min-Priority-value(1).

(ii) Normal-Priority-value(5).

(iii)Max-Priority-value(10).

27).What is Inter-Thread communication?.

To exchange information between two threads.

28) What do you understand by Synchronization?
Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value.
Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}

29) What is multi-threading?

Multi-threading means various threads that run in a system.

34) How to create a thread in a program?
You have two ways to do so. First, making your class "extends" Thread class. Second, making your class "implements" Runnable interface. Put jobs in a run() method and call start() method to start the thread.

35) Can Java object be locked down for exclusive use by a given thread?
Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it.

36) Can each Java object keep track of all the threads that want to exclusively access to it?
Yes. Use Thread.currentThread() method to track the accessing thread.

37) What happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

38) What is the difference between Process and Thread?
A process can contain multiple threads. In most multithreading operating systems, a process gets its own memory address space; a thread doesn't. Threads typically share the heap belonging to their parent process. For instance, a JVM runs in a single process in the host O/S. Threads in the JVM share the heap belonging to that process; that's why several threads may access the same object. Typically, even though they share a common heap, threads have their own stack space. This is how one thread's invocation of a method is kept separate from another's. This is all a gross oversimplification, but it's accurate enough at a high level. Lots of details differ between operating systems. Process vs. Thread A program vs. similar to a sequential program an run on its own vs. Cannot run on its own Unit of allocation vs. Unit of execution Have its own memory space vs. Share with others Each process has one or more threads vs. Each thread belongs to one process Expensive, need to context switch vs. Cheap, can use process memory and may not need to context switch More secure. One process cannot corrupt another process vs. Less secure. A thread can write the memory used by another thread

39)Two methods have key words static synchronized and synchronized separately. What is the difference between them?
Both are synchronized methods. One is instance method, the other is class method. Method with static modifier is a class method. That means the method belongs to class itself and can be accessed directly with class name and is also called Singleton design. The method without static modifier is an instance method. That means the instance method belongs to its object. Every instance of the class gets its own copy of its instance method.
When synchronized is used with a static method, a lock for the entire class is obtained. When synchronized is used with a non-static method, a lock for the particular object (that means instance) of the class is obtained.
Since both methods are synchronized methods, you are not asked to explain what is a synchronized method. You are asked to tell the difference between instance and class method. Of course, your explanation to how synchronized keyword works doesn't hurt. And you may use this opportunity to show your knowledge scope.

40)What's the difference between the methods sleep() and wait()
The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

41) Why do threads block on I/O?
Threads block on I/O (that is enters the waiting state) so that other threads may execute while the I/O Operation is performed.

42)When a thread blocks on I/O, what state does it enter?
A thread enters the waiting state when it blocks on I/O.

43)When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.

44)What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods.

45)What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.

46)What is an object's lock and which objects have locks?
An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

47) What happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

48) What happens when you invoke a thread's interrupt method while it is sleeping or waiting?
When a task's interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.

48)What state is a thread in when it is executing?
An executing thread is in the running state.

49)How can a dead thread be restarted?
A dead thread cannot be restarted.

50)What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

51)What are the different identifier states of a Thread?
The different identifiers of a Thread are:
R - Running or runnable thread
S - Suspended thread
CW - Thread waiting on a condition variable
MW - Thread waiting on a monitor lock
MS - Thread suspended waiting on a monitor lock

52)What is the difference between notify() and notifyAll()?
notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them. Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the change (for example, when freeing a buffer back into a pool). notifyAll() is necessary (for correctness) if multiple threads should resume (for example, when releasing a "writer" lock on a file might permit all "readers" to resume).

53)How is static Synchronization different form non-static synchronization?
When Synchronization is applied on a static Member or a static block, the lock is performed on the Class and not on the Object, while in the case of a Non-static block/member, lock is applied on the Object and not on class. [Trail 2: There is a class called Class in Java whose object is associated with the object(s) of your class. All the static members declared in your class will have reference in this class(Class). As long as your class exists in memory this object of Class is also present. Thats how even if you create multiple objects of your class only one Class object is present and all your objects are linked to this Class object. Even though one of your object is GCed after some time, this object of Class is not GCed untill all the objects associated with it are GCed.
This means that when ever you call a "static synchronized" block, JVM locks access to this Class object and not any of your objects. Your client can till access the non-static members of your objects.

54) Why Thread is faster compare to process?
A thread is never faster than a process. If you run a thread(say there's a process which has spawned only one thread) in one JVM and a process in another and that both of them require same resources then both of them would take same time to execute. But, when a program/Application is thread based(remember here there will be multiple threads running for a single process) then definetly a thread based appliation/program is faster than a process based application. This is because, when ever a process requires or waits for a resource CPU takes it out of the critical section and allocates the mutex to another process.
Before deallocating the ealier one, it stores the context(till what state did it execute that process) in registers. Now if this deallocated process has to come back and execute as it has got the resource for which it was waiting, then it can't go into critical section directly. CPU asks that process to follow scheduling algorithm. So this process has to wait again for its turn. While in the case of thread based application, the application is still with CPU only that thread which requires some resource goes out, but its co threads(of same process/apllication) are still in the critical section. Hence it directly comes back to the CPU and does not wait outside. Hence an application which is thread based is faster than an application which is process based.
Be sure that its not the competion between thread and process, its between an application which is thread based or process based.

55) Explain different way of using thread?
The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritances. The only interface can help.

56) What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

57) What is more advisable to create a thread, by implementing a Runnable interface or by extending Thread class?
Strategically speaking, threads created by implementing Runnable interface are more advisable. If you create a thread by extending a thread class, you cannot extend any other class. If you create a thread by implementing Runnable interface, you save a space for your class to extend another class now or in future.

58). Garbage collector thread belongs to which priority?

59). What is the method to find if a thread is active or not?

60) What is the difference between notify() and notifyAll()?
A notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them. Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the change (for example, when freeing a buffer back into a pool). notifyAll() is necessary (for correctness) if multiple threads should resume (for example, when releasing a "writer" lock on a file might permit all "readers" to resume).

Some basic question

1) What are the two types of multitasking ?
a. Process-based.
b. Thread-based.

2) What is a Thread?
A thread is a single sequential flow of control within a program.

3) What are the two ways to create a new thread?
a.Extend the Thread class and override the run() method.
b.Implement the Runnable interface and implement the run() method.

4) If you have ABC class that must subclass XYZ class, which option will you use to create a thread?
I will make ABC implement the Runnable interface to create a new thread, because ABC class will not be able to extend both XYZ class and Thread class.

5) Which package contains Thread class and Runnable Interface?
java.lang package

6) What is the signature of the run() mehod in the Thread class?
public void run()

7) Which methods calls the run() method?
start() method.

8) Which interface does the Thread class implement?
Runnable interface

9) What are the states of a Thread ?
Ready,Running,Waiting and Dead.

10) Where does the support for threading lie?
The thread support lies in java.lang.Thread, java.lang.Object and JVM.

11) In which class would you find the methods sleep() and yield()?
Thread class

12) In which class would you find the methods notify(),notifyAll() and wait()?
Object class

13) What will notify() method do?
notify() method moves a thread out of the waiting pool to ready state, but there is no guaranty which thread will be moved out of the pool.

14) Can you notify a particular thread?
No.

15) What is the difference between sleep() and yield()?
When a Thread calls the sleep() method, it will return to its waiting state. When a Thread calls the yield() method, it returns to the ready state.

16) What is a Daemon Thread?
Daemon is a low priority thread which runs in the backgrouund.

17) How to make a normal thread as daemon thread?
We should call setDaemon(true) method on the thread object to make a thread as daemon thread.

18) What is the difference between normal thread and daemon thread?
Normal threads do mainstream activity, whereas daemon threads are used low priority work. Hence daemon threads are also stopped when there are no normal threads.

19) Give one good example of a daemon thread?
Garbage Collector is a low priority daemon thread.

20) What does the start() method of Thread do?
The thread's start() method puts the thread in ready state and makes the thread eligible to run. start() method automatically calls the run () method.

21) What are the two ways that a code can be synchronised?
a. Method can be declared as synchronised.
b. A block of code be sychronised.

22) Can you declare a static method as synchronized?
Yes, we can declare static method as synchronized. But the calling thread should acquire lock on the class that owns the method.

23) Can a thread execute another objects run() method?
A thread can execute it's own run() method or another objects run() method.

24) What is the default priority of a Thread?
NORM_PRIORITY

25) What is a deadlock?
A condition that occurs when two processes are waiting for each other to complete before proceeding. The result is that both processes wait endlessly.

26) What are all the methods used for Inter Thread communication and what is the class in which these methods are defined?
a. wait(),notify() & notifyall()
b. Object class

27) What is the mechanisam defind in java for a code segment be used by only one Thread at a time?
Synchronisation

28) What is the procedure to own the moniter by many threads?
Its not possible. A monitor can be held by only one thread at a time.

29) What is the unit for 500 in the statement, obj.sleep(500);?
500 is the no of milliseconds and the data type is long.

30) What are the values of the following thread priority constants?
MAX_PRIORITY,MIN_PRIORITY and NORMAL_PRIORITY
10,1,5

31) What is the default thread at the time of starting a java application?
main thread

32) The word synchronized can be used with only a method. True/ False?
False. A block of code can also be synchronised.

33) What is a Monitor?
A monitor is an object which contains some synchronized code in it.

34) What are all the methods defined in the Runnable Interface?
only run() method is defined the Runnable interface.

35) How can i start a dead thread?
A dead Thread cannot be started again.

36) When does a Thread die?
A Thread dies after completion of run() method.

37) What does the yield() method do?
The yield() method puts currently running thread in to ready state.

38) What exception does the wait() method throw?
The java.lang.Object class wait() method throws "InterruptedException".

39) What does notifyAll() method do?
notifyAll() method moves all waiting threads from the waiting pool to ready state.

40) What does wait() method do?
wait() method releases CPU, releases objects lock, the thread enters into pool of waiting threads.

41) What is semaphore?

42) Can you explain how Scheduling and Priority works in threads?

JAVA-UTIL PACKAGE

2008-07-17T09:10:00.000-07:00

Util Collections

1) What is Collection API ?

The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces.

Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.

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2) Is Iterator a Class or Interface? What is its use?

Iterator is an interface which is used to step through the elements of a Collection.

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3) What is the Vector class?

The Vector class provides the capability to implement a growable array of objects.

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4) What's the main difference between a Vector and an ArrayList?

Java Vector class is internally synchronized and ArrayList isnot.

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5) What is the Properties class?

The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.

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6) What is the Map interface?

The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

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7) What are Vector, Hashtable, LinkedList and Enumeration?

Vector : The Vector class provides the capability to implement a growable array of objects.

Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable indexes and stores objects in a dictionary using hash codes as the object's keys. Hash codes are integer values that identify objects.

LinkedList: Removing or inserting elements in the middle of an array can be done using LinkedList. A LinkedList stores each object in a separate link whereas an array stores object references in consecutive Locations.

Enumeration: An object that implements the Enumeration interface generates a series of elements, one at a time. It has two methods, namely hasMoreElements( ) and nextElement( ). HasMoreElemnts( ) tests if this enumeration has more elements and nextElement method returns successive elements of the series.

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8) What is the difference between set and list?

Set stores elements in an unordered way but do not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements.

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9).What is the List interface?

The List interface provides support for ordered collections of objects.

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10). What is the Vector class?

The Vector class provides the capability to implement a growable array of objects.

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11). What is an Iterator interface?

The Iterator interface is used to step through the elements of a Collection.

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12). What is the GregorianCalendar class?

The GregorianCalendar provides support for traditional Western calendars.

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13). What is the SimpleTimeZone class?

The SimpleTimeZone class provides support for a Gregorian calendar.

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14). What is the Properties class?

The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.

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15). What is the Locale class?

The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

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16). What is the Map interface?

The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

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17). What is the Set interface?

The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

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18).Which java.util classes and interfaces support event handling?

The EventObject class and the EventListener interface support event processing.

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19). What is the Dictionary class?

The Dictionary class provides the capability to store key-value pairs.

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20) What's the main difference between a Vector and an ArrayList?

Java Vector class is internally synchronized and ArrayList is not.

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21)What is HashMap and Map?

Map is Interface and Hashmap is class that implements that.

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22)Difference between HashMap and HashTable?

The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

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Some util package interview questions

JAVA UTIL PACKAGE(java.utl.*)

1). What is Collection Framework?

2). What is ResourceBundle?

This class aids the internationalization of large program with many locale-specific resources.

3). What is ListResourceBundle?

This class aids the internationalization of large program with many locale-specific resources.

4). What is PropertyResourceBundle?

This class aids the internationalization of large program with many locale-specific resources.

5). What is Property permission?

The PropertyPermission allows you to grant a read/write permission to a system property.

6). What are the ad hoc classes?

Dictionary,Vector,Stack and Properties are ad hoc classes that are used to store and manipulate group of object.

7). When is UnspportedOperationException thrown?

If the collection cannot be modified when UnsupportedOperationException is thrown.

8). When is ClassCastException thrown?

A ClassCastException is generated when one object is incompatible with another object that means when attempt is made to add an incompatible object to a collection.

9). What is list?

The list interface extends Collection and declares the behavior of a collection that stores a sequence of elements. Elements can be inserted and accessed by their position in the list, using zero-based index. A list may contain duplicate elements.

10). What is set?

The Set interface defines a set. It extends Collection and declares the behavior of a collection. It does not allowed duplicate elements. It does not define any additional methods of its own.

11). What is SortedSet?

The SortedSet interface extends Set and declates the behavior of set that sorted elements in ascending order.

12). What is ArrayList?

The ArrayList extends AbstractList and implements List interface. ArrayList support dynamic arrays that can grow as needed.

13). What is difference between Array and ArrayList?

In java, standard array is fixed length. After array is created, we cannot increase or decrease the size of array. So we must know in advance how many elements an array will hold. But sometime, we don’t know util run time how large of array we need. To handle this situation, the collection framework defines arrayList.

14). What is linkedlist?

The LinkedList class extends AbstractSequentialList and implements List interface. It support linked list data structure.

15). What is hashSet?

The HashSet class extends AbstractSet that implements Set interface. It creates collection that uses hash table for storage.

16). What is hashing?

The hashtable stores elements by using a mechanism called hashing.

17). What is hashCode?

In hashing, the informational content of key is used to determine a unique value, called hashCode.

18). What is LinkedHashSet?

The LinkedHashSet extends HashSet class. This class maintains a linked list of entries in the set,in order in which they were inserted.

19). What is treeset?

The TreeSet class implements Set interface. It uses a tree for storage. Object are stored in sorted, ascending order. Access and retrieval time are quite fast so we use TreeSet when we store large amount of sorted information.

20). What is iterator?

The iterator are used to cycle through the elements in a collection.

21). What is listIterator?

ListIterator allow bidiractional traversal of a list.

22). What is RandomAccessInterface?

23). What is map?

The map is object that store association between keys and values,or key/value pair. We give key,we will find value. Both key and value are objects. The key is unique but the values are duplicates.

24). What is map.Entry?

The Map.Entry describes an element in a map. This is inner class of Map.

25). What is SortedMap?

The SortedMap extends Map so that the keys are maintained in ascending order.

26). What is hashMap?

The HashMap class use a hash table to implement the Map interface. This allows the execution time of basic operation such as get() and put(),to remain constant even for large set.

27). What is TreeMap?

The TreeMap implements the Map interface by using a tree. A TreeMap provides way for storing key/value pair in sorted order. This allow rapid retrieval.

28). What is LinkedHashMap?

The LinkedHashMap extends HashMap. This maintains a linked list of the entries in the map,in the order in which they are inserted.

29). What is the use of Comparator?

The comparator defines how two objects are compare.

30). What are legacy classes?

Dictionaries, Hashtable, Properties, Stack, Vector are legacy classes.

31). What are legacy interface?

There is one legacy interface called Enumeration.

32). What is the use of Enumeration?

The Enumeration interface defines the methods by which we can enumerate that means obtain one element at a time in a collection of object.

33). What is the use of Vector?

Vector implements a dynamic array.

34). What is the difference between Vector and ArrayList?

Vector implements a dynamic array. It is similar to ArrayList,but with two differences. Vector is synchronized and it contains many legacy methods that are not part of the collection framework.

35). What is the use of stack?

Stack is subclass of Vector that implements a standard last-in, first-out stack.

36). What is Dictionary?

Dictionary operates like map. Given key, we retrieve value.

37). What is HashTable?

It is similar to HashMap but it is synchronized. Like HashMap, HashTable stores key/value pairs in a hash table.

38). What is difference between Hashmap and HashTable?

HashMap contain null values. Whereas HashTable does not allowed null values.

HashMap in not synchronized. Whereas HashTable is synchronized.

39). What is the use of Properties?

Properties is a subclass of HashTable. It is used to maintain lists of values in which the key is a String and the value is String.

40). What is the use of store() and load() in properties class?

Properties object can be easily stored to or loaded from disk with the store() and load() methods.

1).