Java Reflection get the Instance from a Field

Question

is there any way to get the Instance from a Field?
Here's a sample code:

public class Apple {
    // ... a bunch of stuffs..
}

public class Person {
    @MyAnnotation(value=123)
    private Apple apple;
}

public class AppleList {
    public add(Apple apple) {
        //...
    }
}

public class Main {
    public static void main(String args[]) {
        Person person = new Person();
        Field field = person.getClass().getDeclaredField("apple");

        // Do some random stuffs with the annotation ...

        AppleList appleList = new AppleList();

        // Now I want to add the "apple" instance into appleList, which I think
        // that is inside of field.

        appleList.add( .. . // how do I add it here? is it possible?

        // I can't do .. .add( field );
        // nor .add( (Apple) field );
    }
}

I need to use Reflection, because I'm using it with annotations. This is just a "sample", the method AppleList.add(Apple apple) is actually called by getting the method from the class, and then invoking it.

and doing so, like: method.invoke( appleList, field );

causes: java.lang.IllegalArgumentException: argument type mismatch

*EDIT* This might be helpful for someone who's looking for the same thing.

if the class Person, had 2 or more Apple variables:

public class Person {
    private Apple appleOne;
    private Apple appleTwo;
    private Apple appleThree;
}

when I get the Field, like:

Person person = new Person();
// populate person
Field field = person.getClass().getDeclaredField("appleTwo");
// and now I'm getting the instance...
Apple apple = (Apple) field.get( person );
// this will actually get me the instance "appleTwo"
// because of the field itself...

at the beginning, by looking at the line alone: (Apple) field.get( person );
made me think that it would go and get an Instance which matches Apple class.
that's why I wondered: "which Apple will it return?"

Solution

The field isn't an apple itself - it's just a field. As it's an instance field, you need an instance of the declaring class before you can get a value. You want:

Apple apple = (Apple) field.get(person);

... after the apple field is populated for the instanced referred to be person, of course.

Comments

[sam]

oh, so that's how it is... May I ask another question?, what if there were more than one Apple variable??

[Zeki]

Not sure what you are trying to do, but if you are just trying to get all of the fields of type Apple you probably have to use Class.getDeclaredFields() to get all the fields and then loop through those to see which ones are of type Apple.

[sam]

I see, at the beginning I thought that knowing how to get one instance of one of the fields would be enough, so didn't really make my question clear. I'm actually making a class that "builds" a swing gui from a class, so it will get all the components like JButton, JTextField, etc. So I was wondering if it will work like ..: JComponent jcomponent = (JComponent) field.get(ClassExtendedFromJFrame); something like this.. (it's a little bit confusing). I'll keep trying, and see what will it return, I'll try the looping solution too, thanks for your help.