why we need abstract class in java code example

Example 1: abstract class in java

Sometimes we may come across a situation where we cannot provide 
implementation to all the methods in a class. We want to leave the 
implementation to a class that extends it. In such case we declare a class
as abstract.To make a class abstract we use key word abstract. 
Any class that contains one or more abstract methods is declared as abstract. 
If we don’t declare class as abstract which contains abstract methods we get 
compile time error.
  
  1)Abstract classes cannot be instantiated
  2)An abstarct classes contains abstract method, concrete methods or both.
  3)Any class which extends abstarct class must override all methods of abstract
    class
  4)An abstarct class can contain either 0 or more abstract method.

Example 2: What are abstract methods in java

An abstract method is the method which does’nt have any body. 
Abstract method is declared with
keyword abstract and semicolon in place of method body.

  public abstract void <method name>();
Ex : public abstract void getDetails();
It is the responsibility of subclass to provide implementation to 
abstract method defined in abstract class

Example 3: when we should use abstract class

When to use an abstract class
An abstract class is a good choice if we are using the inheritance concept since it provides a common base class implementation to derived classes.
An abstract class is also good if we want to declare non-public members. In an interface, all methods must be public.
If we want to add new methods in the future, then an abstract class is a better choice. Because if we add new methods to an interface, then all of the classes that already implemented that interface will have to be changed to implement the new methods.
If we want to create multiple versions of our component, create an abstract class. Abstract classes provide a simple and easy way to version our components. By updating the base class, all inheriting classes are automatically updated with the change. Interfaces, on the other hand, cannot be changed once created. If a new version of an interface is required, we must create a whole new interface.
Abstract classes have the advantage of allowing better forward compatibility. Once clients use an interface, we cannot change it; if they use an abstract class, we can still add behavior without breaking the existing code.
If we want to provide common, implemented functionality among all implementations of our component, use an abstract class. Abstract classes allow us to partially implement our class, whereas interfaces contain no implementation for any members.

Example 4: abstract classes and interfaces in java

abstract class have no implementation of methods functions inside it. the classes which extending abstract class have to implement it

Example 5: how to use an abstract class in java

interface methods{
    public void hey();
    public void bye();
}

//unable to implement all the abstract methods in the interface so 
// the other class automatically becomes abstract
abstract class other implements methods{
    public void hey(){
        System.out.println("Hey");
    }
}

//able to implement all the methods so is not abstract
class scratch implements methods {
    public void hey(){
        System.out.println("Hey");
    }
    public void bye() {
        System.out.println("Hey");
    }
}

Example 6: can abstract class have non abstract methods in java

abstract class AbstractDemo { // Abstract class
   private int i = 0;
   public void display() { // non-abstract method
      System.out.print("Welcome to Tutorials Point");
   }
}
public class InheritedClassDemo extends AbstractDemo {
   public static void main(String args[]) {
      AbstractDemo demo = new InheritedClassDemo();
      demo.display();
   }
}