Abstract class, we have heard that abstract class are classes which can have abstract methods and it can’t be instantiated. We cannot instantiate an abstract class in Java because it is abstract, it is not complete, hence it cannot be used.
Example 1
Java
// Java program to demonstrate abstract class// cannot have instance public abstract class ClassOne { public void printSomething() { System.out.println("Hello in abstract class"); }}class CreateClassOne { public static void main(String[] args) { // instance of abstract // class "ClassOne" ClassOne obj = new ClassOne(); }} |
Output
prog.java:17: error: ClassOne is abstract; cannot be instantiated
ClassOne obj = new ClassOne();
^
1 error
Example 2
Java
// Java code to demonstrate// Anonymous class public abstract class ClassOne { public void printSomething() { System.out.println("Hello in abstract class"); }} class InheritClassOne { public static void main(String[] args) { // obj points to anonymous subclass ClassOne obj = new ClassOne() {}; // calls the implementation // provided by ClassOne obj.printSomething(); }} |
Hello in abstract class
Is abstract class instantiated here! But that’s not possible!
Yes, the answer is still the same, the abstract class can’t be instantiated, here in the second example object of ClassOne is not created but the instance of an Anonymous Subclass of the abstract class. And then you are invoking the method printSomething() on the abstract class reference pointing to subclass object obj. When you have added { } while creating the object in second class, the compiler takes it as an anonymous class where { } denotes the body of the anonymous class.
Example 1
Java
// java program to demonstrate anonymous class interface NewClass { public void show();} class AnonymousClassExample { public static void main(String[] args) { // nc points to anonymous class inside // the {} NewClass nc = new NewClass() { public void show() { System.out.println( "This is an anonymous class implementing interface"); } }; // calls the implementation // method of anonymous class nc.show(); }} |
This is an anonymous class implementing interface
In this example, the interface is not instantiated but an anonymous class is implementing the interface New Class. Now, you may have understood that how the second example worked where instead of abstract class instantiation, an anonymous subclass was created or abstract class were implemented by an abstract class.
Example 2
Java
// Anonymous class implementing abstract class public abstract class ClassOne { public ClassOne() { System.out.println( "Anonymous(Unnamed) Subclass object Created"); } public void printSomething() { System.out.println( "Hello,in abstract class:printSomething method"); } public abstract void implementMethod();} class AnonymousClassExample { public static void main(String[] args) { // anonymous class ClassOne obj = new ClassOne() { @Override public void implementMethod() { System.out.println( "Implemented abstract method in anonymous class"); } }; // calls abstract class implementation obj.printSomething(); // calls anonymous class implementation obj.implementMethod(); }} |
Anonymous(Unnamed) Subclass object Created Hello,in abstract class:printSomething method Implemented abstract method in anonymous class
