Case 1
class Program {
static final int var;
static {
Program.var = 8; // Compilation error
}
public static void main(String[] args) {
int i;
i = Program.var;
System.out.println(Program.var);
}
}
Case 2
class Program {
static final int var;
static {
var = 8; //OK
}
public static void main(String[] args) {
System.out.println(Program.var);
}
}
Why does Case 1 cause a compilation error?
The JLS holds the answer (note the bold statement):
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. [§16]
This means that the 'simple name' must be used when assigning static final variables - i.e. the var name without any qualifiers.
Apparently this is a cheap syntactic trick to limit definite (un)assignment analysis within the class itself.
If the field is syntactically qualified with a class name, the code is usually in another class, where the analysis cannot reach.
This trick fails in your example. Other examples of oddity:
static class A
{
static final int a;
static
{
// System.out.println(a); // illegal
System.out.println(A.a); // compiles!
a = 1;
}
}
If they had more resources, they probably would've made a finer rule. But we can't change spec now.
来源:https://stackoverflow.com/questions/13778650/why-isnt-a-qualified-static-final-variable-allowed-in-a-static-initialization-b