Getting a variable by name using reflection in Java

Question

I've got a Java class which contains a Color object called fillColor, along with several other colors;

Reflection is probably overkill for this, in fact I may just end up using a map, but either way this is worth asking.

Here's a method of that class which returns a color based on a string.

public Color getColor(String name) {
    Field field;
    Color c= new Color();
    try{
        field = getClass().getDeclaredField(name);
        System.out.println(field.get(c));
    }catch(Exception e){
        System.out.println(e.getMessage());
        throw new RuntimeException("wtf happened here?");
    }

    return c;
}

this throws an exception. It's message reads

Can not set com.badlogic.gdx.graphics.Color field com.whatever.project.Hexagon.fillColor to com.badlogic.gdx.graphics.Color

Obviously I'm confused. Is this message telling me I can't assign a Color field to a Color variable?

EDIT: stack trace

java.lang.IllegalArgumentException: Can not set com.badlogic.gdx.graphics.Color field com.whatever.project.Hexagon.fillColor to com.badlogic.gdx.graphics.Color
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:164)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:168)
    at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:55)
    at sun.reflect.UnsafeObjectFieldAccessorImpl.get(UnsafeObjectFieldAccessorImpl.java:36)
    at java.lang.reflect.Field.get(Field.java:379)
    at com.whatever.project.Hexagon.getColor(Hexagon.java:177)
    at com.whatever.project.Animator$1.animate(Animator.java:45)
    at com.whatever.project.Animator.animate(Animator.java:86)
    at com.whatever.project.Project.render(BlackDot.java:40)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:206)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:114)

fillColor is declared like this:

Color fillColor;

I bet fillColor requires another class named "Color" that is not the same — Leo
– Leo, Commented Jun 27, 2014 at 0:20
You need to call getClass() on Color instead of silent this, also what is the actual declaration fillColor? — Iłya Bursov
– Iłya Bursov, Commented Jun 27, 2014 at 0:21
getColor() is inside of the class Hexagon which contains fillColor. I'm calling getClass() on this, which is Hexagon. Is that not what I should be doing? — Jephron
– Jephron, Commented Jun 27, 2014 at 0:25

Sotirios Delimanolis · Accepted Answer · 2014-06-27 00:28:34Z

6

You get a field

field = getClass().getDeclaredField(name);

on whatever whatever type this is, presumably com.whatever.project.Hexagon. But then you try to retrieve the field on an object of type com.badlogic.gdx.graphics.Color.

System.out.println(field.get(c));

This is wrong. The javadoc states

Returns the value of the field represented by this Field, on the specified object.

Color does not have a Color field.

What you want is probably

field.get(this)

answered Jun 27, 2014 at 0:28

Sotirios Delimanolis

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