An encapsulation of the State Pattern in Java (http://en.wikipedia.org/wiki/State_pattern)

ContextAware.java

public interface ContextAware<T> {
	void beforeMethod(AtomicReference<T> context);
}

Stateful.java

public class Stateful {

	/**
	 * This is a type-checking method, which gets around Java's inability
	 * to handle multiple bounds such as "V extends T & ContextAware<T>".
	 *
	 * @param value initial value, which should extends T
	 * @param contextAware initial value, which should extend ContextAware<T>
	 * @return a new proxy object
	 */
	public static <T> T getProxy(Class<T> clazz, T value, ContextAware<T> contextAware) {
		if (value != contextAware) {
			throw new IllegalArgumentException("This method expects the same object for both parameters.");
		}

		return getProxy(clazz, value);
	}

	@SuppressWarnings("unchecked")
	private static <T> T getProxy(final Class<T> clazz, final Object initial) {
		if (initial == null) {
			throw new NullPointerException("needs a value to start");
		}

		return (T) Proxy.newProxyInstance(clazz.getClassLoader(), new Class[]{clazz}, new InvocationHandler() {
			Object value = initial;

			public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
				AtomicReference<Object> ref = new AtomicReference<>(value);
				((ContextAware) value).beforeMethod(ref);

				Object retval = method.invoke(value, args);
				value = ref.get();
				return retval;
			}
		});
	}
}

Test.java

interface DoesStuff {
	void doSomething(int i);
}

static class SomethingDoerA implements DoesStuff, ContextAware<DoesStuff> {
	AtomicReference<DoesStuff> context;

	@Override
	public void beforeMethod(AtomicReference<DoesStuff> context) {
		this.context = context;
	}

	@Override
	public void doSomething(int i) {
		System.out.println("hello "+i);
		context.set(new SomethingDoerB());
	}
}

static class SomethingDoerB implements DoesStuff, ContextAware<DoesStuff> {
	AtomicReference<DoesStuff> context;

	@Override
	public void beforeMethod(AtomicReference<DoesStuff> context) {
		this.context = context;
	}

	@Override
	public void doSomething(int i) {
		System.out.println("what's up "+(i+100));
		context.set(new SomethingDoerA());
	}
}

@Test
public void test() {
	SomethingDoerA initial = new SomethingDoerA();
	DoesStuff doer2 = StatefulX.getProxy(DoesStuff.class, initial, initial);

	doer2.doSomething(1);
	doer2.doSomething(2);
	doer2.doSomething(3);
}