Interface Purpose Example

Teacher t = new Teacher(): //implements person
Worker w = new Worker( ); //implements person
Slave s = new Slave(); //implements person

ArrayList<Person> people = new ArrayList<Person>();

private void addToList(){
    people.add(t);
    people.add(w);
    people.add(s);
}

private void printAges(){
    for(People p : people){
        System.out.println(people.getAge());// valid since getAge is a part of the interface rather then the specific class
                            //if only slave had the getAge method this code would not be valid
    }
}