'Extends'

Say we have two classes, MainClass and SubClass in separate files:

public class MainClass {
    public MainClass() {
    // Do something in constructor.
    }

    public int add(int a, int b) {
        return a + b;
    }
}

public class SubClass extends MainClass {
    public SubClass() {
        super(); // Calls the superclass constructor, in this case the constructor in MainClass.
    }    

public int multiply(int a, int b) {
        return a * b;
    }

    public static void main(String args[]){
        SubClass subClass = new SubClass;
        int sum = subClass.add(2, 7);
        int product = subClass.multiply(3, 9);
        System.out.println("sum: "+ sum + " product: " + product);
    }
}

Now imagine that MainClass defines properties and behaviours that SubClass wants to inherit. SubClass will extend MainClass making it a child class of MainClass. A subclass, like SubClass, inherits all the members (fields, methods, and nested classes) from its superclass. It does not inherit constructors as these are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass using the keyword super.

An example of this in the android framework is the View class and the TextView class. In this example, TextView extends View and inherits all of its members. The View class only holds general members that all views share in common and the TextView is a more specific view so should still extend View and contain more members that are more specific to TextView.

Hope this helps, happy coding!