FAQ

Java

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Why the fully qualified name of a static final variable is not allowed in static initialization block?

Let's start with the following example:

public class Program {
static final int var;
static {
Program.var = 8; // Compilation error
}
public static void main(String[] args) {
System.out.println(Program.var);
}
}

And,

public class Program {
static final int var;
static {
var = 8; //OK
}
public static void main(String[] args) {
System.out.println(Program.var);
}
}

Why the fully qualified name of the static final variable is not allowed in static initialization block?

The rules governing definite assignment in Chapter 16. Definite Assignment:

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 its simple name qualified by this, occurs on the left hand side of an assignment operator. A Java compiler must carry out a specific conservative flow analysis to make sure that, for every assignment to a blank final variable, the variable is definitely unassigned before the assignment; otherwise a compile-time error must occur

According to the above rules governing definite assignment, we can have the following code:

public class Program {
static final int var1;
final int var2;
static {
var1 = 8; //OK
}
{
this.var2=10; //OK
}
public static void main(String[] args) {
.....
}
}

Further Reading


Chapter 16. Definite Assignment


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