Java for Loop

The for loop is the workhorse of Java iteration. It bundles three pieces — what to do before the loop, what to check before each iteration, and what to do after each iteration — into a single header. The result is more compact and harder to bungle than the same thing written with while.

Syntax

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

Three semicolon-separated parts:

1. Initialization — runs once, before the loop starts. Typically declares the counter.
2. Condition — checked before each iteration. If false, the loop ends.
3. Update — runs after each iteration. Typically increments the counter.

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

Prints 0 1 2 3 4. The header is read as: start with i = 0, run as long as i < 5, increment i each time.

Equivalent while form

A for loop is mechanically equivalent to this while:

{
  int i = 0;            // initialization
  while (i < 5) {       // condition
    System.out.println(i);
    i++;                // update
  }
}

The advantage of for is that everything controlling the loop lives in one line, making intent obvious at a glance.

Counting variations

The for header is flexible. The three pieces can be anything:

// counting down
for (int i = 10; i > 0; i--) { ... }

// stepping by 2
for (int i = 0; i < 100; i += 2) { ... }

// looping over an array by index
for (int i = 0; i < arr.length; i++) {
  System.out.println(arr[i]);
}

// nested with a separate step
for (int i = 0; i < n; i++) {
  // ...
}

If you don't need a particular part, leave it out — but keep the semicolons:

for (;;) {              // identical to while (true)
  if (done()) break;
}

Scope of the loop variable

A variable declared in the initialization clause is local to the loop. After the loop ends, it's gone:

for (int i = 0; i < 5; i++) {
  // i is visible here
}
// i is not visible here

That's by design — it stops you from accidentally relying on the counter's final value. If you need the value outside the loop, declare it beforehand:

int i;
for (i = 0; i < 5; i++) {
  if (someCondition(i)) break;
}
System.out.println("stopped at i=" + i);

Multiple variables in one header

You can declare and update more than one variable, separated by commas:

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

Useful occasionally — but if a loop needs more than two interacting variables, that's usually a sign to refactor into smaller pieces.

When to choose for over while

Use a for loop when you know — or can express in one line — how many iterations you need or which range you're scanning. Use a while loop when the stopping condition is detached from a counter (e.g. "until input runs out", "until success").

For walking an array or collection element-by-element, the enhanced for-each loop is almost always cleaner.

A worked example

What's next

When you just want to visit every element of an array or collection without managing an index, the for-each loop is the right tool.

Practice