Как можно расширить функционал класса

class Parent { void method() {} }
class Child extends Parent { void newMethod() {} }

class Original { void method() {} }
class Enhanced {
    private Original original;
    void newMethod() { original.method(); }
}

interface Component { void execute(); }
class Decorator implements Component {
    private Component component;
    Decorator(Component c) { this.component = c; }
    public void execute() { component.execute(); }
}

class StringUtils {
    public static String enhance(String str) { /*...*/ }
}