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

ABFactory.java

public abstract class ABFactory {
    private ABFactory() { /* prevent instantiation */ }

    public static AB createAB(A a) {
        B b = new BImpl(a);
        return (AB) Proxy.newProxyInstance(a.getClass().getClassLoader(),
                new Class[]{AB.class}, (proxy, method, args) -> {
            Class<?>[] declaringClassInterfaces = method.getDeclaringClass().getInterfaces();
            List<Class<?>> interfaces = new ArrayList<>(Arrays.asList(declaringClassInterfaces));
            interfaces.add(method.getDeclaringClass());
            if (interfaces.contains(A.class)) {
                return method.invoke(a, args);
            } else if (interfaces.contains(B.class)) {
                return method.invoke(b, args);
            } else {
                throw new NoSuchMethodException("Could not find proxied method: " +
                        method.toGenericString());
            }
        });
    }

    private static class BImpl implements B {
        private final A a;

        private BImpl(A a) {
            this.a = a;
        }

        @Override
        public String bar() {
            return a.foo() + "bar";
        }
    }
}

ABFactoryUsageExample.java

A a = new AImpl();
...
AB ab = ABFactory.create(a);
System.out.println(ab.foo()); //prints "foo"
System.out.println(ab.bar()); //prints "foo & bar"