Java For Loop

How the for Loop Runs, Step by Step

The three header parts are written together but are not executed together. For for (int i = 1; i <= 3; i++) { body }, Java runs them in this exact order:

• 1. Initialization — int i = 1 runs exactly once, before anything else.
• 2. Condition — i <= 3 is checked. If it is false, the loop ends immediately and the body is skipped.
• 3. Body — the statements inside the braces run once.
• 4. Update — i++ runs after the body, not before it.
• 5. Repeat from step 2 — the condition is checked again with the new value, and steps 2 to 4 repeat until the condition becomes false.

Tracing an Iteration Table

The easiest way to predict what a loop does is to trace it in a small table: one row per condition check, recording the counter's value, whether the condition is true, and what the body does. For for (int i = 1; i <= 3; i++) the condition is checked four times (i = 1, 2, 3 and finally 4) but the body runs only three times, because the fourth check is the one that fails and ends the loop. The condition is always checked one more time than the body runs, and that single extra check is where most off-by-one bugs come from.

Nested for Loops

A for loop can contain another for loop. The inner loop runs through all of its iterations for every single iteration of the outer loop, so an outer loop of 3 and an inner loop of 4 execute the inner body 3 × 4 = 12 times. Nested loops are the standard tool for rows-and-columns work: grids, 2D arrays, multiplication tables, and printed patterns. Give each loop its own counter name (commonly i for rows and j for columns), and remember that a plain break only exits the innermost loop it sits in.

Multiple Expressions and Infinite Loops

Java allows comma-separated multiple initializations and multiple update expressions in a single for header, which is useful for loops that track two related counters moving in opposite directions. Any of the three header parts can also be omitted; omitting all three, as in for (;;), produces an intentional infinite loop that must be stopped from inside the body, usually with a break statement or a return.

Scope of the Loop Variable

A variable declared in the initialization section (e.g. int i) is scoped only to the for loop itself — it does not exist before the loop and cannot be referenced after it ends. This prevents loop counters from accidentally leaking into and cluttering the surrounding method's namespace.

Examples

public class ForLoopDemo {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1; i <= 5; i++) {
            sum += i;
        }
        System.out.println("Sum: " + sum);

        for (int i = 1, j = 5; i <= j; i++, j--) {
            System.out.println(i + " " + j);
        }
    }
}

Sum: 15
1 5
2 4
3 3

public class ForLoopTrace {
    static boolean check(int i) {
        boolean result = i <= 3;
        System.out.println("check i=" + i + " -> " + result);
        return result;
    }

    public static void main(String[] args) {
        for (int i = init(); check(i); i = update(i)) {
            System.out.println("  body i=" + i);
        }
        System.out.println("loop finished");
    }

    static int init() {
        System.out.println("init i=1");
        return 1;
    }

    static int update(int i) {
        System.out.println("  update i=" + i + " -> " + (i + 1));
        return i + 1;
    }
}

init i=1
check i=1 -> true
  body i=1
  update i=1 -> 2
check i=2 -> true
  body i=2
  update i=2 -> 3
check i=3 -> true
  body i=3
  update i=3 -> 4
check i=4 -> false
loop finished

public class NestedForDemo {
    public static void main(String[] args) {
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print("* ");
            }
            System.out.println();
        }

        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                System.out.print(i * j + "\t");
            }
            System.out.println();
        }
    }
}

*
* *
* * *
1	2	3
2	4	6
3	6	9	

Exercise

Use a for loop to add up all the even numbers from 2 to 100 inclusive and print the total.

Expected output: 2550

public class SumOfEvens {
    public static void main(String[] args) {
        int sum = 0;

        // TODO: loop over the even numbers from 2 to 100 and add each to sum

        System.out.println(sum);
    }
}

public class SumOfEvens {
    public static void main(String[] args) {
        int sum = 0;

        for (int i = 2; i <= 100; i += 2) {
            sum += i;
        }

        System.out.println(sum); // 2550
    }
}

Quick quiz

Interview questions