Object-oriented programming for Java - 4

1. Java Abstract Class

The abstract class in Java cannot be instantiated (we cannot create objects of abstract classes)

abstract class Language {
  abstract void method1();
  void method2() {
    System.out.println("This is regular method");
  }
}

2. Java Abstract Method

Though abstract classes cannot be instantiated, we can create subclasses from it. We can then access members of the abstract class using the object of the subclass.

​x
abstract class Language {
  public void display() {
    System.out.println("This is Java");
  }
}
​
class Main extends Language {
  public static void main(String[] args) {
    Main obj = new Main();
    obj.display();
  }
}
​
// output
// This is Java

Program calling the method of the abstract class using the object obj (which is the object of the child class Main)

3. Implementing Abstract Methods

If the abstract class includes any abstract method, then all the child classes inherited from the abstract superclass must provide the implementation of the abstract method

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abstract class Animal {
  abstract void makeSound();
  public void eat() {
    System.out.println("I can eat.");
  }
}
​
class Dog extends Animal {
  public void makeSound() {
    System.out.println("Bark bark");
  }
}
​
class Main {
  public static void main(String[] args) {
    Dog d1 = new Dog();
​
    d1.makeSound();
    d1.eat();
  }
}
​
// output
// Bark bark
// I can eat.

4. Java Abstraction

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abstract class MotorBike { // create an abstract super class 
  abstract void brake(); // create an abstract method
}
​
class SportsBike extends MotorBike { 
  public void brake() {
    System.out.println("SportsBike Brake");
  }
}
​
class MountainBike extends MotorBike {
  public void brake() {
    System.out.println("MountainBike Brake");
  }
}
​
class Main {
  public static void main(String[] args) {
    MountainBike m1 = new MountainBike();
    m1.brake();
    SportsBike s1 = new SportsBike();
    s1.brake();
  }
}
​
// output:
// MountainBike Brake
// SportsBike Brake