Loops

Loops let you repeat a block of code without writing it out multiple times. Java provides four looping constructs: for, while, do-while, and the enhanced for-each loop, plus break and continue to control their flow.

The for Loop

Best when you know in advance how many times you want to repeat something. It bundles initialization, condition, and update into one line.

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

The while Loop

Repeats as long as its condition stays true. The condition is checked before each iteration, so the body might not run at all.

int count = 0;

while (count < 3) {
  System.out.println("count = " + count);
  count++;
}

The do-while Loop

Similar to while, but the condition is checked after each iteration. That means a do-while loop's body always runs at least once, even if the condition is false from the start.

int count = 10;

do {
  System.out.println("This runs once, count = " + count);
  count++;
} while (count < 3);

The Enhanced for-each Loop

A cleaner way to iterate over every element of an array or collection when you don't need the index.

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

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

break and continue

break exits a loop immediately. continue skips the rest of the current iteration and moves on to the next one.

for (int i = 0; i < 10; i++) {
  if (i == 3) {
    continue; // skip printing 3
  }
  if (i == 6) {
    break; // stop the loop entirely
  }
  System.out.println(i);
}
// 0, 1, 2, 4, 5

Nested Loops

A loop can contain another loop inside it. This is common when working with grids or 2D data.

for (int row = 1; row <= 3; row++) {
  for (int col = 1; col <= 2; col++) {
    System.out.println("row " + row + ", col " + col);
  }
}