Constructing for Loops

Java provides two for loop forms: the basic for loop for index-controlled iteration, and the enhanced for loop (for-each) for iterating over arrays and Iterable collections without managing an index.

The Basic for Loop

The basic for loop bundles initialization, condition, and update into one line:

java
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}
// Output: 0  1  2  3  4

Three components:

Execution order: initialization → [condition → body → update] → ...

Multiple Variables in for Loop

The initialization and update sections can contain comma-separated expressions:

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

All variables declared in the initialization must be the same type.

Optional Components

All three components are optional. Omitting all three creates an infinite loop:

java
for (;;) {
    // infinite loop — equivalent to while (true)
}

Omitting only the initialization or update is legal too:

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

Variable Scope

A variable declared in the for initialization is scoped to the loop:

java
for (int i = 0; i < 3; i++) {
    System.out.println(i);
}
// System.out.println(i);  // compile error: i is out of scope

The Enhanced for Loop (for-each)

The enhanced for loop iterates over an array or any Iterable without an index:

java
int[] numbers = {10, 20, 30, 40};

for (int n : numbers) {
    System.out.println(n);
}
// Output: 10  20  30  40

Syntax: for (ElementType variable : arrayOrIterable) { body }

Iterating Over Collections

The enhanced for works with any class that implements Iterable<T>, including all standard collections:

java
List<String> names = List.of("Alice", "Bob", "Charlie");

for (String name : names) {
    System.out.println(name.toUpperCase());
}

java
Set<Integer> ids = new HashSet<>(Set.of(1, 2, 3));

for (int id : ids) {
    System.out.println("ID: " + id);
}

Enhanced for vs Basic for — When to Use Each

Modifying Arrays: Enhanced for Does Not Write Back

The enhanced for loop copies the value into the loop variable. Assigning to that variable does not change the original array:

java
int[] nums = {1, 2, 3};

for (int n : nums) {
    n = n * 2;   // modifies the copy, NOT nums[i]
}
// nums is still {1, 2, 3}

Use the basic for loop when you need to update array elements:

java
for (int i = 0; i < nums.length; i++) {
    nums[i] = nums[i] * 2;   // modifies the actual element
}

Iterating a 2D Array

java
int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6}
};

for (int[] row : matrix) {
    for (int val : row) {
        System.out.print(val + " ");
    }
    System.out.println();
}
// Output:
// 1 2 3
// 4 5 6

Common Pitfall: Off-by-One Errors

The most frequent bug in basic for loops is an incorrect boundary condition:

java
int[] arr = {10, 20, 30};

// BUG: ArrayIndexOutOfBoundsException when i == 3
for (int i = 0; i <= arr.length; i++) {
    System.out.println(arr[i]);
}

// CORRECT: use <, not <=
for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}

The enhanced for eliminates this category of error entirely.

Key Rules to Remember

• The three for components (init, condition, update) are each optional
• Variables declared in the for initialization are scoped to that loop only
• Multiple init/update expressions must be separated by commas; all declared vars must share the same type
• The enhanced for works with arrays and any Iterable<T>
• Assigning to the loop variable in an enhanced for does not modify the original array or collection
• Use < array.length (not <=) to avoid ArrayIndexOutOfBoundsException

References

• Oracle Docs — The for Statement (Java 21)
• Java Language Specification — §14.14 The for Statement