'abstract' is a non-access modifier in Java for classes and methods but not variables.

Characteristic of abstract keyword :

• Can't create an object directly. You need to extend it by some other class.

• The abstract method does not have a body; a subclass needs to override it.

• It can have both abstract and normal(concrete) abstract methods.

• it can have constructors.

• it can have instance variables.

• it can implement interfaces.

An example to demonstrate the use of the 'abstract' keyword.

//define an abstrcat class
abstract class A{
abstract void m1(); //abstract method has no body
//normal methods are also allowed in abstract class
public void m2(){
System.out.println("this is an normal method");
};
}
//normal class B 
class B extends A{
    //class B must override method of Aotherwise compile time error would be shown
    void m1(){
        System.out.println("B's implementation of m1");
}
}
//test class
public class Main {
    public static void main (String[] args) {
        B b=new B();
        b.m1();
        b.m2();
            }
}

Note: it's compulsory to override your method otherwise you would get a compile-time error. Think of this as a rule.

Interfaces

Interfaces are similar to classes in many ways. Interfaces can be used to achieve 100% abstraction.

• there can only be abstract methods (which means "no content").

• Can help in achieving multiple inheritance.

• class extends class; class implements Interface; interface extends interface.

• Updates -> Since Java 8 we can have default and static methods.

  -> Science Java 9 we can have private methods in an interface.

  An example to demonstrate the use of the 'Interface' keyword.

    interface Bank
    {
  
        float rateOfInterest ();
  
    }
    class SBI implements Bank
    {
  
        public float rateOfInterest ()
        {
            return 9.15f;
        }
  
    }
  
    class PNB implements Bank
    {
  
        public float rateOfInterest ()
        {
            return 9.7f;
        }
  
    }
  
    class TestInterface2
    {
  
        public static void main (String[]args)
        {
  
            Bank b = new SBI ();
  
            System.out.println ("ROI: " + b.rateOfInterest ());
  
       }
    }
  

  PS:- Modifiers are specific keywords present in Java using which we can make changes to the characteristics of a variable, method, or class and limit its scope.

  ->Modifiers in Java are divided into two types – Access Modifiers and Non-Access modifiers.

  ->Non-access modifiers provide information about the characteristics of a class, method, or variable to the JVM. ‘static’ , ‘abstract’, ‘final’ , ‘synchronized’ are some examples of non-access modifiers.

• ->Access Modifiers - 1. public (member can be accessed by any other code) 2. private (member can only be accessed by other members of its class) 3. default (no modifier) 4. protected (allows a member (method, variable, or constructor) to be accessed within its own class, by its subclasses, and by other classes within the same package)

To the readers : If there is any further scope of improvement, then kindly bring to my attention. :)