Updating interfaces by using default methods

Take the following interface:

public interface Cooking {
  public void fry();
  public void boil();
  public void chop();
}

To add new functionality, simply adding a new method to Cooking called microwave() will cause problems. Any class that previously implemented Cooking will now have to be updated in order to function again.

To avoid this, give microwave() a default implementation:

public interface Cooking {
  public void fry();
  public void boil();
  public void chop();
  default void microwave() {
    //some code implementing microwave
  }
}

As microwave() already has a default implementation defined in the Cooking interface definition, classes that implement it now don’t need to implement microwave() in order to work.

This allows us to add functionality without breaking old code.

Note: This has been possible since Java 8.