问题
For example, consider code snap below:
public static final int a;
public static final int b;
static {
a = 8; // it's working
Test.b = 10; // compilation error Test.b cannot be assigned.
}
Why can't we use Test.b = 10; inside a static block of the Test class itself? Without the class name it's working fine.
Is there any reason behind this?
回答1:
A static final variable must be initialized before use. It may be initialized either directly at declaration time, or in a static block.
But when you use class.var = x it is not seen as an initialization but as an assignation.
With a JDK 7, the error is cannot assign a value to final variable.
That explains why it works if you remove the final keyword
class Test {
static final int a = 2; // initialization at declaration time
static final int b;
static final int c;
static {
b = 4; // initialization in static block
Test.c = 6; // error : cannot assign a value to final variable c
}
...
}
EDIT
In the JLS the correct word for initialization is definite assignement
Extract from the JLS :
For every access of a local variable or blank final field x, x must be definitely assigned before the access, or a compile-time error occurs.
Similarly, every blank final variable must be assigned at most once; it must be definitely unassigned when an assignment to it occurs.
Such an assignment is defined to occur if and only if either the simple name of the variable (or, for a field, its simple name qualified by this) occurs on the left hand side of an assignment operator.
For every assignment to a blank final variable, the variable must be definitely unassigned before the assignment, or a compile-time error occurs.
emphasize mine, but I think this is the real reason for the error.
来源：https://stackoverflow.com/questions/27506053/why-cant-we-set-the-value-of-static-final-variable-in-static-block-through-clas