Labeled loop in Java

Question

I having been staring at my screen for a while and I do really need an explanation for labeled loop in this scenario:

package com.misterkourouma.oca8.starter.test;

public class LabeledLoop{

    public static void main(String[] args) {
        int x = 5, j = 0;
        OUTER: for (int i = 0; i < 3;) // -> This line has no curly braces but still compiles
            INNER: do {
                i++;
                x++;
                if (x > 10)
                    break INNER;
                x += 4;
                j++;
            } while (j <= 2);

        System.out.println(x);
    }
}

But this one does not compile :

package com.misterkourouma.oca8.starter.test;

public class LabeledLoop2{
    public static void main(String[] args) {
        int x = 5, j = 0;
        OUTER: for (int i = 0; i < 3;)
            System.out.println("Labeled Loop");
            INNER: do {
                i++;
                x++;
                if (x > 10)
                    break INNER;
                x += 4;
                j++;
            } while (j <= 2);
        System.out.println(x);
    }
}

All the INNER: block are considered (I guess) as a single statement but It does not end with semicolon I wonder Why?

I am preparing for OCA 8 that's one of the reason I need to understand these weirds stuffs.

EDIT:
My question is on LabeledLoop example why does it compiles
Thanks in advance for your help.

codeLover · Answer 1 · 2018-08-08T05:33:12.483

As per my understanding you would be getting the compilation error at the below statement:

 i++;

It is because of the fact that you are not creating any block at OUTER: for (int i = 0; i < 3;) statement(by using curly braces{}), thus, by default scope of the OUTER loop remains till the very next statement(in your case at System.out.println("Labeled Loop");). When you increment the variable i after three statements , it would be giving you compilation error, stating i is undeclared.

score 1 · Answer 2 · edited Aug 08 '18 at 05:58

You have missed the {} braces around the for loop. That is the reason, the code is not compiling. It is not recognising the variable i. As i scope is

OUTER: for (int i = 0; i < 3;) {
                    System.out.println("Labeled Loop") only

     public static void main(String[] args) {
                int x = 5, j = 0;
                OUTER: for (int i = 0; i < 3;) {
                    System.out.println("Labeled Loop");
                    INNER: do {
                        i++;
                        x++;
                        if (x > 10)
                            break INNER;
                        x += 4;
                        j++;
                    } while (j <= 2);
                System.out.println(x);
                }
            }

score 0 · Answer 3 · answered Aug 08 '18 at 05:29

Curly Brackets are important in Java, while indents are not.

This code:

public static void main(String[] args) {
    int x = 5, j = 0;
    OUTER: for (int i = 0; i < 3;)
        System.out.println("Labeled Loop");
        INNER: do {
            i++;
            x++;
            if (x > 10)
                break INNER;
            x += 4;
            j++;
        } while (j <= 2);
    System.out.println(x);
}

means:

public static void main(String[] args) {
    int x = 5, j = 0;
    OUTER: for (int i = 0; i < 3;)
        System.out.println("Labeled Loop");
    INNER: do {
        i++;
        x++;
        if (x > 10)
            break INNER;
        x += 4;
        j++;
    } while (j <= 2);
    System.out.println(x);
}

which, of course does not compile.

score 0 · Answer 4 · answered Aug 08 '18 at 05:33

If there are no braces after for loop - it is considered that only the first expression is inside the loop. so the first example is equivalent to

        OUTER: for (int i = 0; i < 3;){
            INNER: do {
                i++;
                x++;
                if (x > 10)
                    break INNER;
                x += 4;
                j++;
            } while (j <= 2);
        }
        System.out.println(x);

The variable i is declared in the for loop. In the first example do-while is inside of for loop so everything is ok. The second example is equivalent to

        OUTER: for (int i = 0; i < 3;){
            System.out.println(x);
        }
        INNER: do {
                i++;
                x++;
                if (x > 10)
                    break INNER;
                x += 4;
                j++;
        } while (j <= 2);

So variable i occurs out of scope and not declared in the while loop

Labeled loop in Java

4 Answers4