Question

I’m new to Java and I’m having a bit of trouble understanding the concept of the declaration and the initialization of variables.

For example, when I do:

public class Foo {
    public static void main (String[] args) {
        int x, y;
        for (x = 0 ; x < 10 ; x++) {
           y = x + 1;
        }
        System.out.println(x);
        System.out.println(y);
    }
}

It does not compile and says that “variable y might not have been initialized.”
However, it does not have any trouble if I tell it to just print out the x value after the loop. Of course it would work if I simply declared it in the beginning (saying int y = 0; or something like that), but I wanted to know why x is printed but not y.

Thanks in advance!

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Edit:

I understand that the compiler doesn’t actually check inside the loop to see if the variable would be initialized or not so it just says it might not have been initialized, but then why does the following code work? Does the compiler check the if loop but not the for loop?

public class Foo {
    public static void main (String[] args) {
        int x = 0, y;
        if (x == 0) {
            y = 1;
        }
        else {
            y = 2;
        }
        System.out.println(y);
    }
}

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Edit 2:

It looks like it gives me the same error if I actually give another condition for the else part so that it would be:

if (x == 0) {
    y = 1;
}
else if (x == 1) {
    y = 2;
}

So I guess the other example worked since y was initialized in both the if and the else part, which means the y would always be initialized no matter what the condition given is. Now I really get it. Thank you!!

Answer 1

local variables don’t have a default value and you need to initialize them. You are certainly setting the value of x (x=0), but the compiler doesn’t check if the loop body will be actually entered. So give y a value of 0.