Question

In the following code I’d expect that it should not be necessary to initialise variables a and b in last else block, however the compiler does not like it.

import java.util.Random;

public class Foo {

    private void foo () {

    double a,b;
    boolean c;

    double r = (new Random()).nextDouble();

    if(r < 0.25) {
        a = 1;
        b = 2;
        c = true;
    } else if(r >= 0.25 && r < 0.75) {
        a = 3;
        b = 3;
        c = true;
    } else {
        // why is it necessary to init a and b here?
        // given that c is set to false
        c = false;
    }

    if(c) {
        double k = a + b;
    }

  }

}

With the code above, the compiler does complain.

bash-3.2$ javac Foo.java
Foo.java:25: variable a might not have been initialized
        double k = a + b;
                   ^
Foo.java:25: variable b might not have been initialized
        double k = a + b;
                       ^
2 errors

I would have thought that the compiler could do the static analysis to figure out that k will not be evaluated if c is set to false. So my question is why does the compiler demand that I initialise a and b?

Answer 1

The compiler isn’t smart enough to understand that going through the else block will set c to false, and that the next if block thus won’t ever be executed. The static analysis is more limited than what you expect, which also makes the comilation faster/

And it’s probably a good thing, because changing the code of the else block would suddenly make the next if block not compilable, which would be annoying.