Legacy Interface

Suppose you are working on a very very big java project.
Congratulations!!!

Problem Scenario

Suddenly your senior comes to your desk and says- "Hey X, we need to add a new method to XYX interface".

Inside your brain

Holy Crap!

Impossible, I can't do this.

It will break all existing code blocks.

No, it is not possible.

Why it is not possible?

According to oracle documents, a java interface is a group of related methods with empty bodies. Like-

interface Bicycle {

    //  wheel revolutions per minute
    void changeCadence(int newValue);

    void changeGear(int newValue);

    void speedUp(int increment);

    void applyBrakes(int decrement);
}

Suppose, already 100 classes implement Bicycle interface. So, if you want to add a new method in the Bicycle interface-

• You need to modify 100 classes. Complete the method's body in each and every classes, who implements bicycle interface.

Can it be a feasible solution?

No, never.

Solution

Java 8 introduced completely a new concept to overcome this crucial situation.

It is default method

Using default keyword, you can add a complete method in a java interface(magic), this will help you maintain existing legacy code, untouched.

With a new default method in Bicycle interface-

interface Bicycle {

    void changeCadence(int newValue);

    void changeGear(int newValue);

    void speedUp(int increment);

    void applyBrakes(int decrement);

    //method with body in an interface
    default void saveTheWord(){
      System.out.println("Haha! it is a magic");
    }
}

Now, you don't need to modify 100 existing classes(if you wish you can). However, after this point, newly implemented classes can use/override this method if they want.

Extending Interfaces That Contain Default Methods

When you extend an interface that contains a default method, you can do the following:

• Not mention the default method at all, which lets your extended interface inherit the default method.
• Redeclare the default method, which makes it abstract.
• Redefine the default method, which overrides it.

Cheers