Java Abstract Class and Abstract Method

In java, we have a concept called abstraction is the technique of hiding implementation specifics from the user and only displaying functionality.

In java we use, Abstract class to acheive 0 to 100% abstraction and we use interface to acheive 100% abstraction.

Abstract Class

We can create an abstract class using the abstract keyword.
The abstract class can contain both the normal methods and abstract methods.

Java does not allow the instantiation of the abstract class. Means, we cannot create object for an abstract class.

An abstract class has to be extended by other normal class.

Example for abstract class

    abstract class ProgrammingLanguage {
    
      // method of abstract class
      public void DisplayLanguage() {
        System.out.println("I'm Java Programming Language");
      }
    }
    
    class Main extends ProgrammingLanguage {
    
      public static void main(String[] args) {
        
        // create an object Main class
        Main obj = new Main();
    
        // access method of abstract class method
        // using object of Main class
        obj.DisplayLanguage();
      }
    }

Output

I'm Java Programming Language

In the above program, we have an abstract class ProgrammingLanguage with a method DisplayLanguage().

The Main class will inherit the ProgrammingLanguage and with the obj object of the Main class we call the DisplayLanguage() method of the abstract class ProgrammingLanguage.

An abstract class can have constructor of its own which can be
invoked by the sub-class using the super keyword.

Abstract Method

An abstract method does not contain any implementation or body for the method. An abstract method can be allowed only inside the abstract class.

Any class that inherits the abstract class has to implement the abstract method(s). To make a method abstract, we use the abstract keyword.

Syntax for abstract method

abstract return_type method_name();

Example to declare abstract method

abstract void DisplayInfo();

Example program for Abstract Method

  abstract class Animal {
  abstract void Sound();

  public void Category() {
    System.out.println("I am an animal.");
  }
}

class Cat extends Animal {

  public void Sound() {
    System.out.println("My sound is meow");
  }
}

class Main {
  public static void main(String[] args) {

    Cat cat = new Cat();
    cat.Category();
    cat.Sound();
  }
}

Output

I am an animal
My sound is meow

Here, we have an abstract class Animal, and this class contains an abstract method Sound() and a normal method Category().

The Cat class inherits the Animal class and implements the Sound() method.

Now it the Main class we create an object for the Cat class and call the Category() and the Sound() method.

Real-world example for Abstract Class

// Abstract Class
abstract class Bike {
    Bike() {
        System.out.println("I am a Bike");
    }
    // Abstract method
    abstract void totalGears();
    
    void runningStatus() {
        System.out.println("Safe to ride now");
    }
}

// Inherit the abstract class Bike
class Honda extends Bike {
    // Implement abstract method
    void totalGears() {
        System.out.println("Total of 5 gears");
    }
}

class Main {
    public static void main(String args[]) {
        // Create instance of Honda class
        Bike bike = new Honda();
        bike.totalGears();
        bike.runningStatus();
    }
}

Output

I am a Bike
Total of 5 gears
Safe to ride now