for, while, and do-while Loops

Overview

Java has three loop constructs:

Basic for Loop

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

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

• Initialization runs once before the loop starts.
• Condition is checked before each iteration; if false, the loop ends.
• Update runs after each iteration.

All three parts are optional:

java
for (;;) {
    // infinite loop
}

Multiple variables and updates separated by commas:

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

Scope: Variables declared in the initialization clause exist only within the loop.

Enhanced for Loop (for-each)

Iterates over arrays and Iterable collections without an index:

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

for (int n : numbers) {
    System.out.println(n);
}

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

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

The loop variable is a copy — modifying it does not change the original array element (for primitives).

while Loop

java
while (condition) {
    // body
}

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

The body may never execute if the condition is false from the start.

do-while Loop

java
do {
    // body
} while (condition);

java
int count = 0;
do {
    System.out.println(count);
    count++;
} while (count < 5);

The body always executes at least once, then the condition is checked.

java
// Typical use: input validation
Scanner sc = new Scanner(System.in);
int value;
do {
    System.out.print("Enter a positive number: ");
    value = sc.nextInt();
} while (value <= 0);

Comparing the Three Loops

java
// All three print 0..4

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

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

// do-while
int j = 0;
do { System.out.println(j); j++; } while (j < 5);

Infinite Loops

java
while (true) { /* ... */ }
for (;;)     { /* ... */ }
do { /* ... */ } while (true);

Use break to exit, or return/throw from the enclosing method.

Key Rules