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for / while

By · Reviewed & last updated · Part of Java Tutorial

Java has four loop forms: classic for, enhanced for-each, while, and do-while. Modern code uses for-each + streams for most jobs; the classic for survives when you need the index.

for, for-each, while, streams

EXAMPLE
import java.util.*;
import java.util.stream.*;

public class Loops {
    public static void main(String[] args) {
        var xs = List.of(10, 20, 30, 40, 50);

        // 1) Classic for — when you need the index
        for (int i = 0; i < xs.size(); i++) {
            System.out.println(i + " -> " + xs.get(i));
        }

        // 2) Enhanced for-each — when you don't
        for (var n : xs) System.out.println(n);

        // Maps
        var ages = Map.of("Ada", 36, "Bo", 28);
        for (var e : ages.entrySet()) {
            System.out.println(e.getKey() + " = " + e.getValue());
        }

        // 3) while + do-while
        var n = 0;
        while (n < 3) n++;

        var queue = new ArrayDeque<>(xs);
        while (!queue.isEmpty()) {
            var head = queue.poll();
            System.out.println(head);
        }

        // 4) Labelled break / continue (rarely needed)
        outer:
        for (var a : xs) {
            for (var b : xs) {
                if (a * b > 1000) break outer;
            }
        }

        // 5) Modern alternative — streams
        var total = xs.stream().mapToInt(Integer::intValue).sum();
        var evens = xs.stream().filter(x -> x % 2 == 0).toList();
        var max   = xs.stream().max(Comparator.naturalOrder()).orElseThrow();

        // 6) IntStream — index + value WITHOUT autoboxing penalty
        IntStream.range(0, xs.size()).forEach(i ->
            System.out.println(i + " -> " + xs.get(i)));

        IntStream.iterate(1, i -> i + 2)
                 .limit(5)
                 .forEach(System.out::println);
    }
}

Why it matters

For sums, filters, group-bys — reach for streams; the code is shorter and easier to parallelise. Reach for loops when you need break, side effects, or early-return control flow.

Tip: Tweak the snippet with Try it Yourself », then sit the quiz at the bottom of the page.

Example

Example
for (int i = 0; i < 3; i++) System.out.println(i);

int n = 0;
while (n < 3) { System.out.println(n); n++; }

for (String s : new String[]{"a","b"}) System.out.println(s);
Try it Yourself »

Discussion

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