java switch case can not resolve to variable

No you do not need a setter. Simply initialize the variable: int b = 0;

Yes you didn't initialized b. Set some default value as per your requirement, say, int b=-1;

As the exception states, you never initialize b by assigning it a value if your switch should run its default case.

int b; merely declares it.

to fix your issue, you can simply change it to int b = 0;

You need to assign a value to b, when you declare it or in the default block.

Instance fields have implicit default values when declared, but variables in methods do not.

public class Foo {
    private int age; // this defaults to 0

    public void bar() {
        int height; // this won't compile unless you follow it with an assignment
    }
}

The Java compiler doesn't know that a can only be 4 to 6, so it considers the possibility of the default case. So if the default case of the switch is taken, then, b was never initialized when you do return b.

There is chance that non of your cases in the switch gets executed and if that is the case, it ll come to the default block. In the default block, you are not setting the value of b. You never initialized b in the first place. That is why this is happening.

To avoid this situation, you can do one of the followings :

1. int b = -1; // Initialize b

2.

default: {
            System.out.println("wrong"); // even though it is impossible!
            b = -1;
            break;
          }

At the point of return b, b should be initialised. It will be initialised in case a is 4, 5 and 6; but what if a is 28? You're saying it shouldn't be able to happen, but things that shouldn't happen happen all the time, and the compiler likes to have all its bases covered.

Either initialise b at top to cover all cases, or make sure to assign it in each branch of the switch (including default).

Another possibility (which I actually like best) is to make sure your code can not reach return in the default case; you can throw an exception there (maybe IllegalStateException, "there is something wrong with the code!") and the compiler will then know it can't reach return through default, and not complain about b being unassigned. Your code should not happily proceed when your assumptions are violated, so this fits well.

public static int setMapTile() {
    int a = getArmadaLength(); // 4 to 6 
    int b;
    switch (a) {
        case 4:
            System.out.println(" recommended MapSize : 10 x 10");
            b = setSize();// method for bigger map, return int
            break;
        case 5:
            System.out.println(" recommended MapSize : 11 x 11");
            b = setSize();// method for bigger map, return int
            break;
        case 6:
            System.out.println(" recommended MapSize : 12 x 12");
            b = setSize();// method for bigger map, return int
            break;            
        default:
            throw new IllegalStateException("Armada length is " + a + "?!?");
            break;
    }
    return b;
}

In this specific case, actually, you can even factor out the b outside of the switch and completely avoid the problem. Also, that last break is redundant.

public static int setMapTile() {
    int a = getArmadaLength(); // 4 to 6 
    switch (a) {
        case 4:
            System.out.println(" recommended MapSize : 10 x 10");
            break;
        case 5:
            System.out.println(" recommended MapSize : 11 x 11");
            break;
        case 6:
            System.out.println(" recommended MapSize : 12 x 12");
            break;            
        default:
            throw new IllegalStateException("Armada length is " + a + "?!?");
    }
    return setSize();
}

Either change this line: int b; to int b = 0;

Or change your code to this shorter variant:

public static int setMapTile() {

    int a = getArmadaLength(); // 4 to 6 
    switch (a) {

        case 4:
            System.out.println(" recommended MapSize : 10 x 10");
            return setSize();// method for bigger map, return int
        case 5:
            System.out.println(" recommended MapSize : 11 x 11");
            return setSize();// method for bigger map, return int
        case 6:
            System.out.println(" recommended MapSize : 12 x 12");
            return setSize();// method for bigger map, return int           
        default:
            System.out.println("wrong"); // even though it is impossible!
    }

    return 0;
}