Wang Kai

The “static” and “final” modifier in Java are somehow confusing, especially when they interact. Here is a good explanation on how to understand these two modifiers:

You DO NOT have to assign a value to a final variable when you declare it. The “final” keyword just means that you can (and must) assign its value exactly once. Where you are allowed to assign the value depends on if it is static or not (as well as its access modifier).

private final int j;
The value MUST be assigned ONCE, either here in the declaration or in each constructor. It is NOT automatically assigned a value of 0 (zero). Also, since it is not static, there is a distinct value for each instance object, so it makes sense that it must be assigned in the constructor.

private static int k;
The value may be assigned from anywhere in this class. Since it is not final, the value may be assigned any number of times, but since it is static, this value is stored at the class-level and universally available to all instances. If left unassigned, it is assumed to be 0 (zero).

private static final int i;
The value MUST be assigned ONCE, either here in the declaration or in a “static” class-level block. It is NOT automatically assigned a value of 0 (zero).

So, to review:
“final”: per-instance value, must be assigned once per instance.
“final” + “static”: per-class value, must be assigned once per class.
“static”: per-class value, can be assigned any number of times per class.
[neither modifier]: per-instance value, can be assigned any number of times per instance.

Before Java can create an instance of a class, it must first load and initialize that class. One step of class initialization is assigning values to all its static fields. If they’re static AND final, those values can’t change once they’re assigned.