Core Java Interview Question

What is the difference between the instanceof and getclass, these two are same or not ?

instanceof is a operator, not a function while getClass is a method of java.lang.Object class. Consider a condition where we use

if(o.getClass().getName().equals("java.lang.Math")){ }

This method only checks if the classname we have passed is equal to java.lang.Math. The class java.lang.Math is loaded by the bootstrap ClassLoader. This class is an abstract class.This class loader is responsible for loading classes. Every Class object contains a reference to the ClassLoader that defines. getClass() method returns the runtime class of an object. It fetches the java instance of the given fully qualified type name. The code we have written is not necessary, because we should not compare getClass.getName(). The reason behind it is that if the two different class loaders load the same class but for the JVM, it will consider both classes as different classes so, we can't compare their names. It can only gives the implementing class but can't compare a interface, but instanceof operator can.

The instanceof operator compares an object to a specified type. We can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface. We should try to use instanceof operator in place of getClass() method. Remember instanceof opeator and getClass are not same. Try this example, it will help you to better understand the difference between the two.

Interface one{

}

Class Two implements one {

}

Class Three implements one {

}

public class Test {

public static void main(String args[]) {

one test1 = new Two();

one test2 = new Three();

System.out.println(test1 instanceof one); //true

System.out.println(test2 instanceof one); //true

System.out.println(Test.getClass().equals(test2.getClass())); //false

}

}

When you declare a method as abstract method ?

When i want child class to implement the behavior of the method.

Can I call a abstract method from a non abstract method ?

Yes, We can call a abstract method from a Non abstract method in a Java abstract class

What is the difference between an Abstract class and Interface in Java ? or can you explain when you use Abstract classes ?

Abstract classes let you define some behaviors; they force your subclasses to provide others. These abstract classes will provide the basic funcationality of your applicatoin, child class which inherited this class will provide the funtionality of the abstract methods in abstract class. When base class calls this method, Java calls the method defined by the child class.

An Interface can only declare constants and instance methods, but cannot implement default behavior.

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 find corresponding method in the actual class. Abstract classes are fast.

What is user-defined exception in java ?

User-defined expections are the exceptions defined by the application developer which are errors related to specific application. Application Developer can define the user defined exception by inherite the Exception class as shown below. Using this class we can throw new exceptions.

What is the difference between checked and Unchecked Exceptions in Java ?

All predefined exceptions in Java are either a checked exception or an unchecked exception. Checked exceptions must be caught using try .. catch() block or we should throw the exception using throws clause. If you dont, compilation of program will fail.

Java Exception Hierarchy +--------+ | Object | +--------+ | | +-----------+ | Throwable | +-----------+ / \ / \ +-------+ +-----------+ | Error | | Exception | +-------+ +-----------+ / | \ / | \ \________/ \______/ \ +------------------+ unchecked checked | RuntimeException | +------------------+ / | | \ \_________________/ unchecked

 

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