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Types of classes in Java

Final class

A class declared with final keyword is a final class and it cannot be extended by another class, for example, java.lang.System class.

Example:-

final class BaseClass {
   void Display() {
      System.out.print("This is Display() method of BaseClass.");
   }
}
class DerivedClass extends BaseClass { //Compile-time error - can't inherit final class
   void Display() {
      System.out.print("This is Display() method of DerivedClass.");
   }
}
public class FinalClassDemo {
   public static void main(String[] arg) {
      DerivedClass d = new DerivedClass();
      d.Display();
   }
}
 
In the above example, DerivedClass extends BaseClass(final), we can't extend a final class, so compiler will throw an error. The above program doesn't execute.

Output

cannot inherit from final BaseClass Compile-time error - can't inherit final class

POJO class

A class which contains only private variables and setter and getter methods to use those variables is called POJO (Plain Old Java Object) class. It is a fully encapsulated class.

Example:-

class POJO {
  private int value=100;
  public int getValue() {
      return value;
   }
   public void setValue(int value) {
      this.value = value;
   }
}
public class Test {
   public static void main(String args[]){
      POJO p = new POJO();
      System.out.println(p.getValue());
   }
}

Output

100

Abstract class

A class declared with abstract keyword and have zero or more abstract methods are known as abstract class. The abstract classes are incomplete classes, therefore, to use we strictly need to extend the abstract classes to a concrete class.

Example:-

abstract class Animal { //abstract parent class
   public abstract void sound(); //abstract method
}
public class Dog extends Animal { //Dog class extends Animal class
   public void sound() {
      System.out.println("Woof");
   }
   public static void main(String args[]) {
      Animal a = new Dog();
      a.sound();
   }
}
   

Output

Woof

Static class

Static classes are nested classes means a class declared within another class as a static member is called a static class.

Example:-

import java.util.Scanner;

class staticclasses {
   static int s; // static variable
   static void met(int a, int b) { // static method
   System.out.println("static method to calculate sum");
   s = a + b;
   System.out.println(a + "+" + b); // print two numbers
}
   static class MyNestedClass { // static class
      static { // static block
         System.out.println("static block inside a static class");
      }
      public void disp() {
         int c, d;
         Scanner sc = new Scanner(System.in);
         System.out.println("Enter two values");
         c = sc.nextInt();
         d = sc.nextInt();
         met(c, d); // calling static method
         System.out.println("Sum of two numbers-" + s); // print the result in static variable
      }
   }
}
public class Test {
   public static void main(String args[]) {
      staticclasses.MyNestedClass mnc = new staticclasses.MyNestedClass(); // object for static class
      mnc.disp(); // accessing methods inside a static class
   }
}

Output

static block inside a static class
Enter two values 10 20
static method to calculate sum
10+20
Sum of two numbers-30

Inner Class

A class declared within another class or method is called an inner class.

Example:-

public class OuterClass {
   public static void main(String[] args) {
      System.out.println("Outer");
   }
   class InnerClass {
      public void inner_print() {
         System.out.println("Inner");
      }
   }
}

Output

Outer

 


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