An odd case of initialization

January 04, 2012

Overview

You might assume that once a variable has been initialized it stays that way. However, the javac compiler, treats a variable as initialized only when it can determine a variable has been set, and exception handling can confuse the compiler.

But I know its initialised because I used it

In the following example, a variable is treated as initialized but after a catch block, the compiler isn't sure, and the variable can assigned again and used later.

int a;
try {
    a = 1;
    // a is initialised and can be read.
    System.out.println(a); // compiles ok.
} catch (RuntimeException ignored) {
}
// a is not considered initialised as the compiler 
// doesn't know the catch block couldn't be thrown before a is set.
System.out.println(a); // Variable 'a' might not have been initialized

a = 2;
// a is definitely initialised.
System.out.println(a); // compiles ok.

The problem here is that you know that the variable cannot throw an exception before the variable a is set, but the compiler doesn't know this. The simplest solution is to move the initialization out of the try/catch block.

Note: If you make the variable a final, the second a = 2 gets an error "Variable 'a' might have been initialized"

Vanilla Java