Libraries - Java 8 Lambdas - page 52

Default Methods and Subclassing

There are some subtleties about the way that default methods override and can be overrid-
den by other methods. Let's look the simplest case to begin with: no overriding. In
Example 4-11 , our Parent interface defines a welcome method that sends a message when
called. The ParentImpl class doesn't provide an implementation of welcome , so it inherits
the default method.

Example 4-11. The Parent interface; the welcome method is a default
public public interface interface Parent Parent {

public public void void message ( String body );

public public default void welcome () {

message ( "Parent: Hi!" );

default void

}

public public String getLastMessage ();

}

When we come to call this code, in Example 4-12 , the default method is called and our as-
sertion passes.

Example 4-12. Using the default method from client code
@Test
public

public void void parentDefaultUsed () {

Parent parent = new new ParentImpl ();

parent . welcome ();
assertEquals ( "Parent: Hi!" , parent . getLastMessage ());

}

Now we can extend Parent with a Child interface, whose code is listed in Example 4-13 .
Child implements its own default welcome method. As you would intuitively expect, the
default method on Child overrides the default method on Parent . In this example, again,
the ChildImpl class doesn't provide an implementation of welcome , so it inherits the de-
fault method.

Example 4-13. Child interface that extends Parent
public public interface interface Child Child extends extends Parent {

@Override