Example showing how existing interfaces can be extended to provide additional features by using the Function interface, introduced in Java 8, without breaking existing code.

ExtensionUsingJava8Funtion.java

import java.util.function.Function;

public class ExtensionUsingJava8Funtion {

    public static void main(String[] args) {
        SomeInterface someInterface = new SomeInterfaceImpl();
        demoAdaptingInput(someInterface);
        demoAdaptingOutput(someInterface);
        demoAdaptingBothInputAndOutput(someInterface);
    }

    private static void demoSimpleUsage(SomeInterface someInterface) {
        SomeInput someInput = new SomeInput();

        SomeOutput someOutput = someInterface.transform(someInput);
    }

    private static void demoAdaptingInput(SomeInterface someInterface) {
        SomeOtherInput someOtherInput = new SomeOtherInput();

        SomeOutput someOutput = someInterface
                .compose(SomeOtherInput::toSomeInput)
                .apply(someOtherInput);
    }

    private static void demoAdaptingOutput(SomeInterface someInterface) {
        SomeInput someInput = new SomeInput();

        SomeOtherOutput someOtherOutput = someInterface
                .andThen(SomeOtherOutput::fromSomeOutput)
                .apply(someInput);
    }

    private static void demoAdaptingBothInputAndOutput(SomeInterface someInterface) {
        SomeOtherInput someOtherInput = new SomeOtherInput();

        SomeOtherOutput someOtherOutput = someInterface
                .compose(SomeOtherInput::toSomeInput)
                .andThen(SomeOtherOutput::fromSomeOutput)
                .apply(someOtherInput);
    }

    // ****************** Core Classes **********************

    interface SomeInterface extends Function<SomeInput, SomeOutput> {
        default SomeOutput apply(SomeInput someInput) {
            return transform(someInput);
        }

        SomeOutput transform(SomeInput someInput);
    }

    static class SomeInterfaceImpl implements SomeInterface {
        @Override
        public SomeOutput transform(SomeInput someInput) {
            return new SomeOutput();
        }
    }

    static class SomeInput {

    }

    static class SomeOutput {

    }

    // ****************** Other Classes ***********************

    static class SomeOtherInput {
        SomeInput toSomeInput() {
            return new SomeInput();
        }
    }

    static class SomeOtherOutput {
        static SomeOtherOutput fromSomeOutput(SomeOutput someOutput) {
            return new SomeOtherOutput();
        }
    }
}