Question 1

What are the restrictions when overriding a method ?

Accepted Answer

Overridden methods must have the same name, argument list, and return type (ie, they must have the exact signature of the method we are going to override, including return type) The overriding method cannot be less visible than the method it overrides( ie, a public method cannot be override to private) The overriding method may not throw any exceptions that may not be thrown by the overridden method

Question 2

What is difference between Path and Classpath?

Accepted Answer

Path and Classpath are operating system level environment variales Path is used define where the system can find the executables(exe) files and classpath is used to specify the location class files

Question 3

What is the impact of declaring a method as final?

Accepted Answer

A method declared as final can't be overridden A sub-class can't have the same method signature with a different implementation

Question 4

What is static initializer block? What is its use?

Accepted Answer

A static initializer block is a block of code that declares with the static keyword It normally contains the block of code that must execute at the time of class loading The static initializer block will execute only once at the time of loading the class only

Question 5

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

Accepted Answer

When an exception is thrown , the catch block of the try statement are examined in the order in which they appear The first catch block that is capable of handling the exception is executed The remaining catch blocks are ignored

Question 6

What are the the different ways for creating a thread?

Accepted Answer

A thread can be created by subclassing Thread, or by implementing the Runnable interface

Question 7

What is the difference between creating a thread by extending Thread class and by implementing Runnable interface? Which one should prefer?

Accepted Answer

When creating a thread by extending the Thread class, it is not mandatory to override the run method (If we are not overriding the run method , it is useless), because Thread class have already given a default implementation for run method But if we are implementing Runnable , it is mandatory to override the run method The preferred way to create a thread is by implementing Runnable interface, because it give loose coupling

Question 8

What is Thread ?

Accepted Answer

A thread is a lightweight subprocess, a smallest unit of processing It is a separate path of execution It shares the memory area of process As shown in the above figure, thread is executed inside the process There is context-switching between the threads There can be multiple processes inside the OS and one process can have multiple threads

Question 9

What do you understand by private, protected and public?

Accepted Answer

These are accessibility modifiers Private is the most restrictive, while public is the least restrictive There is no real difference between protected and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package

Question 10

If a method is declared as protected, where may the method be accessed?

Accepted Answer

A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared

Question 11

What is the difference between method overriding and overloading?

Accepted Answer

Overriding is a method with the same name and arguments as in a parent, whereas overloading is the same method name but different arguments

Question 12

What is classloader?

Accepted Answer

The classloader is a subsystem of JVM that is used to load classes and interfacesThere are many types of classloaders eg Bootstrap classloader, Extension classloader, System classloader, Plugin classloader etc

Question 13

What will be the output of the following code snippet? Object s1 = new String("Hello"); Object s2 = new String("Hello"); if(s1 == s2) { System.out.println("s1 and s2 are =="); }else if (s1.equals(s2)) { System.out.println("s1 and s2 are equals()"); }

Accepted Answer

: s1 and s2 are equals() Explanation :

Question 14

What is JDBC ?

Accepted Answer

JDBC technology is an API (included in both J2SE and J2EE releases) that provides cross-DBMS connectivity to a wide range of SQL databases and access to other tabular data sources, such as spreadsheets or flat files With a JDBC technology-enabled driver, you can connect all corporate data even in a heterogeneous environment

Question 15

What is DriverManager ?

Accepted Answer

DriverManager is a class in javasql package It is the basic service for managing a set of JDBC drivers

Question 16

What is a ResultSet ?

Accepted Answer

A table of data representing a database result set, which is usually generated by executing a statement that queries the database ResultSet object maintains a cursor pointing to its current row of data Initially the cursor is positioned before the first row The next method moves the cursor to the next row, and because it returns false when there are no more rows in the ResultSet object, it can be used in a while loop to iterate through the result set

Question 17

What are the different JDBC drivers available ?

Accepted Answer

There are mainly four type of JDBC drivers available They are: Type 1 : JDBC-ODBC Bridge Driver - A JDBC-ODBC bridge provides JDBC API access via one or more ODBC drivers Note that some ODBC native code and in many cases native database client code must be loaded on each client machine that uses this type of driver Hence, this kind of driver is generally most appropriate when automatic installation and downloading of a Java technology application is not important For information on the JDBC-ODBC bridge driver provided by Sun Type 2: Native API Partly Java Driver- A native-API partly Java technology-enabled driver converts JDBC calls into calls on the client API for Oracle, Sybase, Informix, DB2, or other DBMS Note that, like the bridge driver, this style of driver requires that some binary code be loaded on each client machine Type 3: Network protocol Driver- A net-protocol fully Java technology-enabled driver translates JDBC API calls into a DBMS-independent net protocol which is then translated to a DBMS protocol by a server This net server middleware is able to connect all of its Java technology-based clients to many different databases The specific protocol used depends on the vendor In general, this is the most flexible JDBC API alternative It is likely that all vendors of this solution will provide products suitable for Intranet use In order for these products to also support Internet access they must handle the additional requirements for security, access through firewalls, etc, that the Web imposes Several vendors are adding JDBC technology-based drivers to their existing database middleware products Type 4: JDBC Net pure Java Driver - A native-protocol fully Java technology-enabled driver converts JDBC technology calls into the network protocol used by DBMSs directly This allows a direct call from the client machine to the DBMS server and is a practical solution for Intranet access Since many of these protocols are proprietary the database vendors themselves will be the primary source for this style of driver Several database vendors have these in progress

Question 18

Is the JDBC-ODBC Bridge multi-threaded ?

Accepted Answer

No The JDBC-ODBC Bridge does not support multi threading The JDBC-ODBC Bridge uses synchronized methods to serialize all of the calls that it makes to ODBC Multi-threaded Java programs may use the Bridge, but they won't get the advantages of multi-threading

Question 19

What is the difference between an Interface and an Abstract class?

Accepted Answer

An abstract class can have instance methods that implement a default behavior An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract An interface has all public members and no implementation An abstract class is a class which may have the usual flavors of class members (private, protected, etc), but has some abstract methods

Question 20

What is the purpose of garbage collection in Java, and when is it used ?

Accepted Answer

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used

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