Java Interfaces

• Use the interface keyword
• Usually only methods
• All methods public abstract
• All attributes public static final
• No constructor, no code

Java Interface Example

public interface IMyQueue {
    void enqueue(Object o);
    Object dequeue();
    Object peek();
    int size();
}

Using a Java Interface

public class WaitingList implements IMyQueue {
    public void enqueue(Object o) {
        //code here
    }

    public Object dequeue() {
        //code here
    }

    public Object peek() {
        //code here
    }

    public int size() {
        //code here
    }
}

Interfaces are Types

IMyQueue wait = new WaitingList();
wait.enqueue(new Person());
wait.dequeue();

We can call any methods
defined by the interface

Java Inheritance

• Subclass inherits all state and behavior
• Subclass has all public and protected attributes and methods of superclass
• Subclass constructor can call super() constructor
• Subclass can override superclass methods with new code

public class Canine {
    protected name;
    public void bark() {
        //code here
    }
}

public class Dog extends Canine {
    protected owner;
    public void pet() {
        //code here
    }
}

public class GuideDog extends Dog {
    protected trainer;
    public void guide() {
        //code here
    }
}

Subclass has all Superclass behaviors

GuideDog lassie = new GuideDog();
lassie.bark();
lassie.pet();
lassie.guide();

Multiple Inheritance

Classes in Java can only
directly inherit from one superclass

Classes may implement
any number of interfaces