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copied!<p>There is a sweet little class called a <code>DynamicObjectAdapterFactory</code> posted by <a href="http://www.artima.com/weblogs/viewpost.jsp?thread=109017" rel="nofollow">Heinz Kabutz</a> which uses generics and reflection to adapt an object to implement an interface by providing it with a source class that already implements the interface.</p> <p>Using it like below you can wrap your <code>INSTANCE</code> in a proxy. Of course the resulting object is no longer an <code>enum</code> but it <strong>does</strong> retain all of the singletonness of the <code>enum</code> I think. It also, obviously - can use any object to implement your interface.</p> <p>I think this is as close as you will get to an <code>enum</code> extending a class.</p> <p>Here's some test code that seems to work. Obviously the object is no longer an <code>enum</code> but as your aim was a singleton this may be acceptable.</p> <pre><code>public class Test { // To implement this. public interface Implement { public void init(); public void connect(); public void disconnect(); public boolean isConnected(); } // An implementor that does implement. public static class Implements implements Implement { @Override public void init() { } @Override public void connect() { } @Override public void disconnect() { } @Override public boolean isConnected() { return false; } } // Extend the INSTANCE in this. public enum Extend { INSTANCE; // Hold my adapted version - thus still a singleton. public final Implement adaptedInstance; Extend () { // Use the constructor to adapt the instance. adaptedInstance = DynamicObjectAdapterFactory.adapt(this, Implement.class, new Implements()); } } // Provides an INSTANCE that has been extended by an Implements to implement Implement. public static Implement getInstance () { return Extend.INSTANCE.adaptedInstance; } public void test() { System.out.println("Hello"); Implement i = getInstance(); } public static void main(String args[]) { new Test().test(); } } </code></pre> <p>Here's the DynamicObjectAdapterFactory - I've tweaked it a little from the original - I hope Dr. Kabutz does not object.</p> <pre><code>public class DynamicObjectAdapterFactory { // Use methods in adaptee unless they exist in target in which case use adapter. // Implement target in passing. public static <T> T adapt(final Object adaptee, final Class<T> target, final Object adapter) { return (T) Proxy.newProxyInstance( Thread.currentThread().getContextClassLoader(), new Class[]{target}, new InvocationHandler() { private final String name = adaptee.getClass().getSimpleName() + "(" + adaptee.toString() + ")" + "+" + adapter.getClass().getSimpleName() + "(" + adapter.toString() + ")"; // The methods I wish to adapt. private Map<MethodIdentifier, Method> adaptedMethods = new HashMap<>(); { // initializer block - find all methods in adapter object Method[] methods = adapter.getClass().getDeclaredMethods(); for (Method m : methods) { // Keep a map of them. adaptedMethods.put(new MethodIdentifier(m), m); } } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { // Has it been adapted? Method otherMethod = adaptedMethods.get(new MethodIdentifier(method)); if (otherMethod != null) { return otherMethod.invoke(adapter, args); } else { return method.invoke(adaptee, args); } } catch (InvocationTargetException e) { throw e.getTargetException(); } } @Override public String toString() { StringBuilder s = new StringBuilder(); // Really simple. May get more flexible later. s.append("Adapted: ").append(name); return s.toString(); } }); } private static class MethodIdentifier { private final String name; private final Class[] parameters; public MethodIdentifier(Method m) { name = m.getName(); parameters = m.getParameterTypes(); } @Override public boolean equals(Object o) { // I am always equal to me. if (this == o) { return true; } // I cannot be equal to something of a different type. if (!(o instanceof MethodIdentifier)) { return false; } MethodIdentifier mid = (MethodIdentifier) o; return name.equals(mid.name) && Arrays.equals(parameters, mid.parameters); } @Override public int hashCode() { return name.hashCode(); } } } </code></pre>

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