为什么静态最终变量不是默认值？

Question

Why aren't static final variables given default values, whereas static (but non-final variables are given default values).

What is the reason that such behavior was implemented in Java?

Answer 1

Of course static final variables are given default values, see for example this:

class Test {
    static final int x;
    static {
        printX();
        x = 42;
        printX();
    }

    static void printX() {
        System.out.println("Here x is "+x);
    }
    public static void main(String[] args) { }
}

The output is:

Here x is 0
Here x is 42

If x wasn't given the default value of 0 as specified in JLS 4.12.5 , the output would depend on the JVM used. You might see some random number.

Update : Now that we have demonstrated that static final fields do get a default value, you may want to know why the default value is not enough. There is no good answer to that question, besides the obvious one: "The spec says so" . Excerpt from 8.3.1.2:

It is a compile-time error if a blank final (§4.12.4) class variable is not definitely assigned (§16.8) by a static initializer (§8.7) of the class in which it is declared.

We can only guess at the motivation behind such a restriction, but I think it's to make programs easier to understand. If you want to set the variable to 0 it's clearer to do it explicitly.

Answer 2

Simple. Since they are final , you would not be able to modify them later, so the default value would also be, well, final. You would not be allowed to modify it later. Not very useful.