Java For Loop — Counting, Iterating & Enhanced For-Each

Loops let you repeat a block of code multiple times. The for loop is the most commonly used loop when you know how many times to repeat.

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The Classic for Loop

for (int i = 0; i < 5; i++) {
    System.out.println("i = " + i);
}

Output:

i = 0
i = 1
i = 2
i = 3
i = 4

Anatomy

for (initialization; condition; update) {
    // body
}

Part	Purpose	Example
Initialization	Runs once before the loop	int i = 0
Condition	Checked before each iteration	i < 5
Update	Runs after each iteration	i++

Execution Flow

1. Initialize → 2. Check condition → 3. Execute body → 4. Update → back to 2.

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Counting Examples

// Count 1 to 10
for (int i = 1; i <= 10; i++) {
    System.out.print(i + " ");
}
// 1 2 3 4 5 6 7 8 9 10

// Count down
for (int i = 10; i >= 1; i--) {
    System.out.print(i + " ");
}
// 10 9 8 7 6 5 4 3 2 1

// Step by 2
for (int i = 0; i <= 10; i += 2) {
    System.out.print(i + " ");
}
// 0 2 4 6 8 10

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Enhanced For-Each Loop

For iterating over arrays and collections:

String[] fruits = {"Apple", "Banana", "Cherry"};

for (String fruit : fruits) {
    System.out.println(fruit);
}

Output:

Apple
Banana
Cherry

Syntax: for (Type variable : collection) { ... }

Use for-each when you don’t need the index. It’s cleaner and less error-prone.

When You Need the Index

String[] fruits = {"Apple", "Banana", "Cherry"};

for (int i = 0; i < fruits.length; i++) {
    System.out.println(i + ": " + fruits[i]);
}

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Nested Loops

A loop inside a loop:

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

Output:

(1,1) (1,2) (1,3)
(2,1) (2,2) (2,3)
(3,1) (3,2) (3,3)

Multiplication Table

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= 10; j++) {
        System.out.printf("%4d", i * j);
    }
    System.out.println();
}

Triangle Pattern

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

Output:

*
* *
* * *
* * * *
* * * * *

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Common Patterns

Sum of Numbers

int sum = 0;
for (int i = 1; i <= 100; i++) {
    sum += i;
}
System.out.println("Sum: " + sum);  // 5050

Finding Maximum

int[] numbers = {23, 1, 56, 12, 89, 34};
int max = numbers[0];

for (int num : numbers) {
    if (num > max) {
        max = num;
    }
}
System.out.println("Max: " + max);  // 89

Fibonacci Sequence

int n = 10, a = 0, b = 1;
for (int i = 0; i < n; i++) {
    System.out.print(a + " ");
    int temp = a + b;
    a = b;
    b = temp;
}
// 0 1 1 2 3 5 8 13 21 34

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Infinite Loop

for (;;) {
    // runs forever — use break to exit
}

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Multiple Variables in a For Loop

for (int i = 0, j = 10; i < j; i++, j--) {
    System.out.println("i=" + i + " j=" + j);
}

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Common Mistakes

Off-By-One Errors

// Prints 0 to 9 (10 iterations)
for (int i = 0; i < 10; i++) { }

// Prints 0 to 10 (11 iterations!)
for (int i = 0; i <= 10; i++) { }

// Prints 1 to 10 (10 iterations)
for (int i = 1; i <= 10; i++) { }

Modifying Collection During For-Each

List<String> names = new ArrayList<>(List.of("A", "B", "C"));

// WRONG — ConcurrentModificationException
for (String name : names) {
    names.remove(name);
}

// Use iterator or removeIf instead
names.removeIf(name -> name.equals("B"));

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Quick Exercises

1. Print all even numbers from 2 to 50.
2. Calculate the factorial of 10 using a for loop.
3. Print a right-aligned triangle pattern of * with 5 rows.
4. Find the sum of all odd numbers between 1 and 100.

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Summary

Loop Type	Use When
for (init; cond; update)	Known number of iterations
for (Type x : collection)	Iterating all elements
Nested loops	Multi-dimensional patterns

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What’s Next?

The for loop is great when you know the count. But sometimes you need to repeat until a condition changes — that’s the while loop: While Loop.