Java For Loop

When you know how many times something should repeat, the for loop is the clearest choice. It puts the three parts of a counting loop on one line: where the counter starts, how long the loop runs, and how the counter changes. You already know these parts from the While Loop topic; for just keeps them together.

The parts of a for loop

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.println("Line " + i);
        }
    }
}

Line 1
Line 2
Line 3
Line 4
Line 5

• int i = 1 runs once, before the loop starts.
• i <= 5 is checked before every round; when it is false, the loop ends.
• i++ runs after every round.

The variable i exists only inside the loop. After the closing bracket it is gone; this is explained further in the Scope topic.

Other steps

The step can be anything: count by twos, count down, or multiply:

public class Main {
    public static void main(String[] args) {
        for (int i = 0; i <= 10; i += 2) {
            System.out.print(i + " ");
        }
        System.out.println();
        for (int i = 5; i >= 1; i--) {
            System.out.print(i + " ");
        }
        System.out.println();
        for (int p = 1; p <= 1000; p *= 10) {
            System.out.print(p + " ");
        }
        System.out.println();
    }
}

0 2 4 6 8 10 
5 4 3 2 1 
1 10 100 1000 

A multiplication table

Loops and printf from String Formatting work well together:

public class Main {
    public static void main(String[] args) {
        int n = 7;
        for (int i = 1; i <= 10; i++) {
            System.out.printf("%d x %2d = %3d%n", n, i, n * i);
        }
    }
}

7 x  1 =   7
7 x  2 =  14
7 x  3 =  21
7 x  4 =  28
7 x  5 =  35
7 x  6 =  42
7 x  7 =  49
7 x  8 =  56
7 x  9 =  63
7 x 10 =  70

Nested loops

A loop inside another loop runs completely for every round of the outer loop. Nested loops are used for tables, grids and patterns:

public class Main {
    public static void main(String[] args) {
        for (int row = 1; row <= 4; row++) {
            for (int star = 1; star <= row; star++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

*
**
***
****

The outer loop counts the rows. The inner loop prints as many stars as the row number, then println() ends the line. With 4 rows the inner loop runs 1 + 2 + 3 + 4 = 10 times in total.

Choosing a loop

Use for when you count (from 1 to 10, over the positions of a string or an array). Use while when you repeat until something happens. Both can do the same job; pick the one that reads more clearly. To walk through all the items of an array or a list there is an even simpler form, the for-each loop, which you will meet in the Arrays topic.

Practice exercises

1. Print the squares of the numbers from 1 to 10.
2. Print every character of the word "Programming" on its own line, using charAt and length().
3. Print a 5 by 5 square of # characters with nested loops.