Question

I wrote the following class:

  public class TestOne {
     public static void main(String[] args) {
        int count = 0;
        for (int i = 0; i < 100; i++) {
          count++;
        }
        System.out.println(count);
     }
   }

The output is 100.

Then I added a semicolon:

    public class TestOne {
     public static void main(String[] args) {
        int count = 0;
        for (int i = 0; i < 100; i++); {     // <-- Added semicolon
          count++;
        }
        System.out.println(count);
     }
   }

The output is 1.

The result is unbelievable. Why does this added semicolon change the meaning of my program so dramatically?

Answer 1

This is not a bug. The semicolon becomes the only “statement” in the body of your for loop.

Write this another way to make it easier to see:

for (int i = 0; i < 100; i++)
    ;

{
    count++;
}

The block with count++ becomes a bare block with a single statement, which isn’t associated with the for loop at all, because of the semicolon. So this block, and the count++ within it, is only executed once.

This is syntactically valid java. for (int i = 0; i < 100; i++); is equivalent to:

for (int i = 0; i < 100; i++)
{ ; } // no statement in the body of the loop.

for loops of this form can be useful because of the side-effects within the loop increment statement or termination condition. For instance, if you wanted to write your own indexOfSpace to find the first index of a space character in a String:

int idx;

// for loop with no body, just incrementing idx:
for (idx = 0; string.charAt(idx) != ' '; idx++);

// now idx will point to the index of the ' '