Question

A service from external API which I am not allowed to modify returns me

  MyClass instance = ServiceUtil.getThing();

I would like to extend this returned class and Add/Override a method but leave intacts the others, say 150 methods.

  private class MyWrapperClass extends MyClass(){
      public MyWrapperClass(){super();}
      @Override public String toString(){ return "Blocked toString"; }
  }

Is there any way to force this “casting” from the returned MyClass instance to my newly particular subtype??

NOTE: Please, not suggest the approach of making a constructor, passing the original object and having to copy and implement the 150 methods to call the wrapped object

Answer 1

If MyClass is an interface look at java.lang.reflect.Proxy and java.lang.reflect.InvocationHandler.

You can implement a dynamic proxy that always does the same. It is, always pass control to your original implementation… except when method X is invoked.

Something like:

class MyHandler implements InvocationHandler {
  Object invoke(Object proxy, Method method, Object[] args) {
    if (method is the one you want to change) {
       do whatever
    else
       return method.invoke(originalObject, args);
  }
}

Note: you must create this proxy implementation anyway:

MyClass original = ServiceUtil.getThing();
MyClass proxy = (MyClass) Proxy.newProxyInstance(
                                          MyClass.class.getClassLoader(), // classloader to use
                                          new Class[] { MyClass.class }, // interfaces to implement
                                          new MyHandler()); // who does the dirty work when methods are invoked