Part of the following post in orange-coding.blogspot.com: Enhancing (extending) a class using dynamic proxy classes in Java

A.java

public interface A {
    String foo();
}

ADecorator.java

public class ADecorator implements A {
    private final A delegate;
    public ADecorator(A a) {
        this.delegate = a;
    }
    public String foo() {
        return "*** " + this.delegate.foo() + " ***"; //will return "*** foo ***"
    }
}

AImpl.java

public class AImpl implements A {
    public String foo() {
        return "foo";
    }
}