Java For Loop

Imagine printing 100 tickets, one line each. You could write System.out.println 100 times, or you could write it once and tell Java to repeat it. The Java for loop is the tool for that kind of counted repetition. In this lesson, you'll learn the three parts of a for loop, how to count up, down and in steps, where the loop variable lives, and how to use loops for totals and averages.

What is a for loop in Java?

A for loop runs a block of code repeatedly while a condition stays true, with the counting logic packed into one line at the top. Each run of the block is called an iteration.

for (initialization; condition; update) {
    // body: runs once per iteration
}

public class Laps {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.println("Lap " + i);
        }
        System.out.println("Workout done!");
    }
}

Lap 1
Lap 2
Lap 3
Lap 4
Lap 5
Workout done!

The three parts of a for loop

1. Initialization (int i = 1) runs once, before anything else. It usually creates the loop variable (also called the counter).
2. Condition (i <= 5) is checked before every iteration. If it's true, the body runs. If it's false, the loop ends and Java moves on to the code after it.
3. Update (i++) runs after every iteration. It moves the counter toward the end. i++ adds 1, as you saw in Java arithmetic operators.

The order is: initialize, then check, body, update, check, body, update... until the check fails. Watch it happen with a 3-lap version:

Notice the condition is tested 4 times for 3 laps. The last test is the one that says "stop".

How do you count down with a for loop?

Start high, keep going while the counter is above the limit, and subtract with i--:

public class RocketLaunch {
    public static void main(String[] args) {
        for (int seconds = 5; seconds >= 1; seconds--) {
            System.out.println(seconds + "...");
        }
        System.out.println("Liftoff!");
    }
}

5...
4...
3...
2...
1...
Liftoff!

The loop variable doesn't have to be called i. A descriptive name like seconds makes the loop explain itself.

Counting in steps

The update can change the counter by any amount. Use i += 2, i += 20 or even i *= 2. This shuttle bus leaves every 20 minutes between 8:00 and 9:00:

public class ShuttleTimes {
    public static void main(String[] args) {
        for (int minutes = 0; minutes <= 60; minutes += 20) {
            int hour = 8 + minutes / 60;
            int minute = minutes % 60;
            System.out.printf("Shuttle departs at %d:%02d%n", hour, minute);
        }
    }
}

Shuttle departs at 8:00
Shuttle departs at 8:20
Shuttle departs at 8:40
Shuttle departs at 9:00

Here the loop counts minutes, and integer division (/) and remainder (%) turn them into a clock time. %02d pads the minute with a zero, as covered in Java output formatting.

Where can you use the loop variable?

A variable declared in the for header only exists inside the loop. This is called its scope. Once the loop ends, the variable is gone:

for (int i = 1; i <= 3; i++) {
    System.out.println(i);   // fine: i exists here
}
System.out.println(i);       // error: cannot find symbol

If you need the final value after the loop, declare the variable before the loop:

public class PagesRead {
    public static void main(String[] args) {
        int page;
        for (page = 1; page * 30 < 100; page++) {
            System.out.println("Reading page " + page);
        }
        System.out.println("Stopped at page " + page);
    }
}

Reading page 1
Reading page 2
Reading page 3
Stopped at page 4

You'll explore scope in more depth in Java variable scope.

Using a for loop to add up totals

A very common pattern: create a total variable set to 0 before the loop, add to it inside the loop, and use it after the loop. Here, someone saves $5 in week 1, $10 in week 2, $15 in week 3, and so on for 8 weeks:

public class SavingsJar {
    public static void main(String[] args) {
        int weeks = 8;
        int total = 0;

        for (int week = 1; week <= weeks; week++) {
            int deposit = week * 5;
            total += deposit;
            System.out.println("Week " + week + ": +$" + deposit + " (jar: $" + total + ")");
        }

        double average = (double) total / weeks;
        System.out.println("Total saved: $" + total);
        System.out.println("Average per week: $" + average);
    }
}

Week 1: +$5 (jar: $5)
Week 2: +$10 (jar: $15)
Week 3: +$15 (jar: $30)
Week 4: +$20 (jar: $50)
Week 5: +$25 (jar: $75)
Week 6: +$30 (jar: $105)
Week 7: +$35 (jar: $140)
Week 8: +$40 (jar: $180)
Total saved: $180
Average per week: $22.5

The (double) cast matters. Without it, 180 / 8 is integer division and gives 22, not 22.5. See Java type casting for a refresher.

Example: one row of a multiplication table

public class TimesTable {
    public static void main(String[] args) {
        int number = 7;

        for (int i = 1; i <= 10; i++) {
            System.out.printf("%d x %2d = %2d%n", number, i, number * 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

Want the whole table, 1 to 10 on every row? That needs a loop inside a loop, which you'll build in Java nested loops.

Looping over the characters of a String

Strings have positions numbered from 0 to length() - 1, so a for loop can visit each character with charAt(), which you met in Java string methods:

public class VowelCounter {
    public static void main(String[] args) {
        String song = "Bohemian Rhapsody";
        int vowels = 0;

        for (int i = 0; i < song.length(); i++) {
            char letter = Character.toLowerCase(song.charAt(i));
            if (letter == 'a' || letter == 'e' || letter == 'i' || letter == 'o' || letter == 'u') {
                vowels++;
            }
        }

        System.out.println("\"" + song + "\" has " + vowels + " vowels");
    }
}

"Bohemian Rhapsody" has 6 vowels

Note the condition i < song.length(), with < rather than <=, because the last position is one less than the length.

Common mistakes with the Java for loop

1. Off-by-one errors

for (int i = 1; i < 10; i++) runs 9 times, not 10. Decide whether your end value should be included, then pick < or <= on purpose. A handy habit: start at 0 and use < (runs exactly n times), or start at 1 and use <=.

2. A semicolon after the header

for (int i = 1; i <= 5; i++);  // the ; is an empty loop body
{
    System.out.println("Hello");  // runs only once
}

3. Updating in the wrong direction

for (int i = 10; i >= 1; i++) counts up from 10, so i >= 1 stays true for a very long time. When counting down, use i--.

4. Changing the counter inside the body

Adding i++ inside the body as well as in the header skips iterations and makes the loop hard to follow. Let the header do the counting.

Practice exercise

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

        for (int month = 1; month <= 6; month++) {
            int price = 30 + (month - 1) * 2;
            total += price;
            System.out.println("Month " + month + ": $" + price);
        }

        System.out.println("Total: $" + total);
        System.out.println("Average: $" + (double) total / 6);
    }
}

Month 1: $30
Month 2: $32
Month 3: $34
Month 4: $36
Month 5: $38
Month 6: $40
Total: $210
Average: $35.0

What's next?

A for loop is perfect when you know how many times to repeat. When you don't, for example "keep going until the savings goal is reached", you need the Java while loop.