Sub-Class or Derived Class or Child Class is a class that inherits fields and methods from another class called the super-class or parent class.

class SuperClass {
    // fields and methods
}

class SubClass extends SuperClass {
    // additional fields and methods
}

Features:

1. Sub-classes inherits all non-private fields and methods from their super-class.
2. Method Overriding: Sub-classes can provide specific implementations for methods that are already defined in the super-class.
3. Accessing Super-Class members: Sub-classes can access the fields and methods of the super-class using the super keyword.
4. Private members of the super-class are not accessible directly by the sub-class. However, they can be accessed through public or protected getter and setter methods.

// Super-class
class Animal {
    void sound() {
        System.out.println("Animal makes a sound");
    }
}

// Sub-class
class Dog extends Animal {
    @Override
    void sound() {
        super.sound(); // Calls the sound() method of Animal
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.sound();
    }
}