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Technical Sample Questions : Java Sample Questions : Core Java Sample Questions

  1. What is similarities/difference between an Abstract class and Interface?

    Answer: Differences are as follows:

    • Interfaces provide a form of multiple inheritance.A class can extend only one other class.
    • Interfaces are limited to public methods and constants with no implementation.Abstract classes can have a partial implementation, protected parts, static methods, etc.
    • A Class may implement several interfaces.But in case of abstract class, a class may extend only one abstract class.
    • Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class.Abstract classes are fast.


    • Neither Abstract classes or Interface can be instantiated.

  2. How to define an Abstract class?

    Answer: A class containing abstract method is called Abstract class.An Abstract class can't be instantiated.
    Example of Abstract class:

    abstract class testAbstractClass {
    protected String myString;
    public String getMyString() {
    return myString;
    public abstract string anyAbstractFunction();

  3. How to define an Interface?

    Answer: In Java Interface defines the methods but does not implement them.Interface can include constants.A class that implements the interfaces is bound to implement all the methods defined in Interface.
    Emaple of Interface:

    public interface sampleInterface {
    public void functionOne();
    public long CONSTANT_ONE = 1000;

  4. Explain the user defined Exceptions?

    Answer: User defined Exceptions are the separate Exception classes defined by the user for specific purposed.An user defined can created 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.

    class myCustomException extends Exception {
    // The class simply has to exist to be an exception

  5. Explain the new Features of JDBC 2.0 Core API?

    Answer: The JDBC 2.0 API includes the complete JDBC API, which includes both core and Optional Package API, and provides inductrial-strength database computing capabilities.
    New Features in JDBC 2.0 Core API:

    • Scrollable result sets- using new methods in the ResultSet interface allows programmatically move the to particular row or to a position relative to its current position
    • JDBC 2.0 Core API provides the Batch Updates functionality to the java applications.
    • Java applications can now use the ResultSet.updateXXX methods.
    • New data types - interfaces mapping the SQL3 data types
    • Custommapping of user-defined types (UTDs)
    • Miscellaneous features, including performance hints, the use of character streams, full precision for java.math.BigDecimal values, additional security, and support for time zones in date, time, and timestamp values.

  6. Explain garbage collection?

    Answer: Garbage collection is one of the most important feature of Java.Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory.User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program.Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists.In Java, it is good idea to explicitly assign null into a variable when no more in use.I Java on calling System.gc() and Runtime.gc(),JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.

  7. How you can force the garbage collection?

    Answer: Garbage collection automatic process and can't be forced.

  8. What is OOPS?

    Answer: OOP is the common abbreviation for Object-Oriented Programming.

  9. Explain the different forms of Polymorphism.

    Answer: From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:

    • Method overloading
    • Method overriding through inheritance
    • Method overriding through the Java interface

  10. What are Access Specifiers available in Java?

    Answer: Access specifiers are keywords that determines the type of access to the member of a class.These are:

    • Public
    • Protected
    • Private
    • Defaults

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