Extends vs Implements in Java

1. Inherit a Class Using the extends Keyword in Java
2. Inherit an Interface Using the implements Keyword in Java

To reuse the properties and behaviors of a given parent class in Java, we use the concept of inheritance that is an important part of an object-oriented programming language. The idea behind this is code reusability. Both implements and extends keywords are used to create a new class in Java that has the attributes of the parent class.

We can have a better understanding of extends and implements keywords using the table given below.

Inherit a Class Using the extends Keyword in Java

In the code below, we have a Parent, which is the base class, and Child as the subclass. The Child class extends the Parent class, which means that the Child class now has access to the fields and methods defined inside the Parent class.

In our Test class, we create an object of the subclass class child and call the method displayNumber() on it. The method displayString(), which is defined inside the parent class, can be executed by calling it on the same child object. Hence, it fulfills the purpose of code reusability.

public class Test {
  public static void main(String args[]) {
    Child child = new Child();
    System.out.println("Calling Child class method : " + child.displayNumber());
    System.out.println("Calling Parent class method from child : " + child.displayString());
  }
}
class Parent {
  String str = "Hello there!";
  public String displayString() {
    return str;
  }
}
class Child extends Parent {
  int number = 25;
  public int displayNumber() {
    return number;
  }
}

Output:

Calling Child class method : 25
Calling Parent class method from child : Hello there!

Inherit an Interface Using the implements Keyword in Java

We have two interfaces, Cat and Dog, in the code below, each containing an abstract method inside it. A class Animals implement both interfaces, so it must implement the method defined in the interfaces.

Hence the Animals class implemented both interfaces and defined their behaviors which is the whole purpose of the interface in Java.

interface Cat {
  public void meow();
}

interface Dog {
  public void bark();
}

class Animals implements Dog, Cat {
  @Override
  public void meow() {
    System.out.println("Cat meows");
  }

  @Override
  public void bark() {
    System.out.println("Dog barks");
  }
}
public class Test1 {
  public static void main(String args[]) {
    Animals animals = new Animals();
    animals.bark();
    animals.meow();
  }
}

Output:

Dog barks
Cat meows