public static final variable in an imported java class

Question

I happen to come across a Java code at my work place. Here's the scenario: There are 2 classes - ClassA and ClassB.

ClassA has nothing except 4 public static final string values inside it. Its purpose is to use those values like ClassA.variable (don't ask me why, it's not my code).

ClassB imports ClassA. I edited the string values in ClassA and compiled it. When I ran ClassB I could see it was using the old values - not the new values. I had to recompile ClassB to make it use new values from ClassA! (I had to recompile other classes that imports ClassA!)

Is this just because of JDK 1.6 or I should have known earlier to recompile ClassB also! Enlighten me. :)

Answer 1

If the values of the final variables from class ClassA happen to be compile-time constants, the compiler might have inlined them into the classes using ClassA instead of generating a run-time reference. I think, this is what happened in the case you described.

Example:

public class Flags {
    public static final int FOO = 1;
    public static final int BAR = 2;
}

public class Consumer {
    public static void main(String[] args) {
         System.out.println(Flags.FOO);
    }
}

In this example, the compiler will likely incorporate the value of FOO into the code generated for Consumer instead of generating the equivalent run-time reference. If the value of FOO changes later on, you will have to re-compile Consumer in order to have it use the new value.

This is an optimization, which has a few advantages with respect to efficiency and speed of the program compiled. For example, inlining the value might enable further optimizations in the expressions, which use it, for example:

int x = Flags.FOO * 10;

In this example, inlining the value (here: 1) enables the compiler to notice, that the multiplication makes no difference, and can be omitted alltogether.

Answer 2

It's a binary compatibility issue. References to constant fields are resolved at compile time. The behaviour you are seeing is correct; if you change the values in class A then you will have to re-compile the client (class B). To avoid such problems consider adding constants using an enum type, introduced in Java release 5.0.