‘abstract’ – a non access modifier

A non-access modifier is a keyword that changes the default behaviour of the Java Class.

An _ ‘abstract’ _ modifier can be applied to

1)  ‘abstract’- Method modifier

• When an ‘abstract‘ keyword is prefixed to a method declaration, then the method becomes an abstract method:

public abstract void execute();

• A method with an empty body is not considered as an abstract method.

public void execute(){}

• An abstract method must appear in an abstract Class only.

abstract class Perform{ int x, y; public abstract String execute(); 
}

Abstract methods will be implemented by the derived Classes.

public class Action extends Perform{ public String execute(){ 
System.out.println("I am executing");
} 
}

• If the derived class is not implementing the abstract method, then that Class also must be declared as an abstract Class.

public abstract class Action extends Perform{ public abstract String execute();
}

• Abstract methods can be used in the enums as well.

enum Model{
    BENZ{
        public String getModel(){ return "BENZ"; }
    },
    LAMBORGHINI{
        public String getModel(){ return "LAMBORGHINI"; }
    },
    FERRARI{
        public String getModel(){ return "FERRARI"; }
    },
    BUGATTI{
        public String getModel(){ return "BUGATTI"; }
    }
    public abstract String getModel(); 
}

2) ‘abstract’ – Class Modifier

• When an ‘abstract‘ keyword is prefixed to a class definition, then the class becomes an abstract class.

abstract class Perform{    
}

• An abstract Class may or may not have any abstract methods

abstract class Perform{ 
   int x, y; 
   public abstract String execute(); 
}

(or)

abstract class Perform{ 
   int x, y; 
   public String execute(); 
}

• An abstract class can’t be instantiated. If an attempt is made to instantiate the abstract class,  ‘Perform is abstract, cannot be instantiated’ compilation error is thrown

3) ‘abstract’- Interface Modifier

• Every interface is implicitly abstract. Both the definitions below are the same.

interface Model{ 
   
}

(or)

abstract interface Model{ 
  
}

• Explicit use of an abstract keyword to an interface is redundant.
• Thus ‘abstract’ interface modifier is obsolete and not recommended to use in new programs.