Getting value of public static final field/property of a class in Java via reflection

Question

Say I have a class:

public class R {
    public static final int _1st = 0x334455;
}

How can I get the value of the "_1st" via reflection?

Solution 1

First retrieve the field property of the class, then you can retrieve the value. If you know the type you can use one of the get methods with null (for static fields only, in fact with a static field the argument passed to the get method is ignored entirely). Otherwise you can use getType and write an appropriate switch as below:

Field f = R.class.getField("_1st");
Class<?> t = f.getType();
if(t == int.class){
    System.out.println(f.getInt(null));
}else if(t == double.class){
    System.out.println(f.getDouble(null));
}...

Solution 2

 R.class.getField("_1st").get(null);

Exception handling is left as an exercise for the reader.

Basically you get the field like any other via reflection, but when you call the get method you pass in a null since there is no instance to act on.

This works for all static fields, regardless of their being final. If the field is not public, you need to call setAccessible(true) on it first, and of course the SecurityManager has to allow all of this.

Solution 3

I was following the same route (looking through the generated R class) and then I had this awful feeling it was probably a function in the Resources class. I was right.

Found this: Resources::getIdentifier

Thought it might save people some time. Although they say its discouraged in the docs, which is not too surprising.

Comments

[Matthieu]

R._1st couldn't work? If you're talking about Android development, I think the R class is always there...

[Sevastyan Savanyuk]

@Matthieu I thought so too, until this day when I had to do this very same thing, but only with the BR class.

[Viet]

thanks. I tried but it didn't work. Exception is thrown at the operation f.getInt(null). I caught it but how come there's an exception?

[M. Jessup]

What kind of exception did you receive?

[Viet]

Hi, the Exception e.getMessage() returns the field name, which is "_1st" and nothing else.

[M. Jessup]

But what is the type of the exception? (i.e. NullPointerException, SecurityException, ...)

[Viet]

I got it. The class I needed was actually R.id. Thanks for your help!

[Viet]

I got it. The class I needed was actually R.id. Thanks for your help!

[Matthieu]

So you inferred it was an Android question. Should have been indicated in the tags...

[Matthew Read]

It's not an Android question, it's a Java reflection question that uses a particular example. Questions are tagged based on their topic.

[Sevastyan Savanyuk]

How come the documentation never mentions that getInt() ignores the passed in argument? Spent hours on trying to get the instance of the class to pass there.