Java Null Proxy

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:

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

See NullObjectImplementation.


CategoryNull

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