通过Java Android中的Reflection使用对象的功能

Question

I have a little problem on a Java Android application, I'm developing a little game and I'm loading there dynamically some pictures throw reflection.

I'm having an Asset class with a lot of objects called Image (for each resource in my game I have an object of Image class), this Image objects in fact are bitmaps with some extra functions. Exactly they have a function called Resize that I need to use.

At the moment I'm loading all the images dynamically throw reflection in a loop, inside that loop I have more or less something like this (not exactly this but I have wrote it in that way so you can understand how it works):

    Assets assetsRef = new Assets();
    String name = "picture97";
    picture = assetsRef.getClass().getField(name);
    picture.set(picture, g.newImage(name, ImageFormat.RGB565));

However after loading all those pictures I want to apply the function called Resize. In other and taking into consideration the previous code lines, I would like to use the function that is contained on the object called picture97, that is a variable of type Image inside the Assets class.

Anyone can help me? I'm quite sure that there should be an easy way to maybe select an object through reflection, but by the moment I have not been able to do so.

Lot of thanks

as

Answer 1

Image image = (Image) picture.get(); // from Field to the object which is stored in the field


Method  method = image.getClass().getDeclaredMethod("Resize",  (Class<?>[]) null);
method.invoke(image, (Object[]) null);

This applies to calling methods without parameters. If your method "Resize" requires paramters, you got to supply the types of them to the call for retrieving the method as well as passing the arguments on invoke.

Method is the reflection object used for investigating methods of classes, calling "invoke" on a Method - object requires the targeted object to be passed as argument to invoke. The Class[] argument passed to getDeclaredMethod represent the signature of the method you are trying to invoke as there maybe more then one method with the same name but different parameters and parameter types (overloading) , say void Resize(), void Resize(int x, int y). The call to invoke is similar to directly calling the method on the object, so you got to provide the parameters you would in your normal call. Because the reflection api must provide a generic way of calling your method, this arguments must be cast to Oject[], imagine that otherwise an overload of Method.invoke would have to be available for any methods signature, and because this includes your own defined class types, this wouldn't be possible. (Well, it would, if the class would be generated at runtime and after investigating the code in some cases, but then dynamic class loading would have to retrigger the generation, loading and compiling as well as linking of the Method - class, which would be some kind of overkill).

But indeed in this situation, where you both know the exact type of your object and the method you want to invoke (which is not changing), I would rather recommend you to cast the value of the field to Image and then invoke Resize on that object, like so:

Image image = (Image) picture.get(); image.Resize();