Java for Loop

A loop repeats a block of code. Use the for loop when you know how many times to repeat.

Syntax

for (initialization; condition; update) {
    // code to repeat
}

1. Initialization runs once at the start.
2. The condition is checked before every round. If it is false, the loop ends.
3. The update runs after every round.

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

Different steps

public class Steps {
    public static void main(String[] args) {
        for (int i = 0; i <= 20; i += 5) {
            System.out.print(i + " ");
        }
        System.out.println();
        for (int i = 5; i >= 1; i--) {       // count down
            System.out.print(i + " ");
        }
        System.out.println();
    }
}

Sum of numbers

public class Sum {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1; i <= 100; i++) {
            sum += i;
        }
        System.out.println("Sum 1 to 100 = " + sum);
    }
}

Nested loops

public class Table {
    public static void main(String[] args) {
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 5; j++) {
                System.out.print(i * j + "\t");
            }
            System.out.println();
        }
    }
}

Enhanced for loop (for each)

The easy way to go through all the items of an array or a collection:

public class Each {
    public static void main(String[] args) {
        String[] fruits = {"apple", "banana", "mango"};
        for (String fruit : fruits) {
            System.out.println(fruit);
        }

        int[] marks = {70, 85, 90};
        int total = 0;
        for (int m : marks) {
            total += m;
        }
        System.out.println("Total: " + total);
    }
}

Use the normal for loop when you need the index. Use the for each loop when you only need the values.

Loop variable scope

A variable declared in the for line (like i) exists only inside the loop.

Infinite loop

If the condition is always true, the loop never stops. This is sometimes used on purpose, with a break inside:

for (;;) {
    // runs for ever, until break
}

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