For any class using an interface you can utilize a Java Dynamic Proxy to implement the standard NullObject pattern for you. This is a basic implementation.

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class NullProxy implements InvocationHandler {

    public static Object create(Class interfaceClass) {
        return create(new Class[]{interfaceClass});
    }

    public static Object create(Class[] interfaceClass) {
        return Proxy.newProxyInstance(interfaceClass[0].getClassLoader(), interfaceClass, new NullProxy());
    }

    public Object invoke(Object proxy, Method method, Object[] args) {
        return getDefaultInstance(method.getReturnType());
    }

    public static Object getDefaultInstance(Class param) {
        if (param == int.class)
            return new Integer(0);
        if (param == boolean.class)
            return Boolean.FALSE;
        if (param == byte.class)
            return new Byte("0");
        if (param == short.class)
            return new Short("0");
        if (param == long.class)
            return new Long("0");
        if (param == float.class)
            return new Float("0");
        if (param == double.class)
            return new Double("0");
        return null;
    }
}

Here is one of the more interesting things you can do with a Null Dynamic Proxy. The object can dynamically implement completely unrelated interfaces. Method calls do absolutely nothing. 

        Object obj = NullProxy.create(new Class[]{List.class, Connection.class});
        assertNotNull(obj);
        List list = (List) obj;
        list.size();
        Connection con = (Connection) obj;
        con.commit();


--MikeRettig

I have done this too some time ago. But I added special cases for equals(), hashCode() and toString() to invoke() above to make it really usable. 

-- GunnarZarncke

You could also add a special case that returns a new null proxy when the return type is an interface.

''And how shall all these various instances of JavaNullProxy respond when compared to each other?''

Answer:

http://www.digi-hound.com/wp/img_wp2/fireworks_dual2_small.jpg

More seriously, there's a strong case to be made that a null is not equal to any value, including itself.  --cwillu

See NullObjectImplementation.

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CategoryNull