Java Loops and Iteration

Iterate between two numbers

Using while loop:

int i = 0;
while (i < 10) {
  System.out.println(i);
  i++;
}

Using for loop:

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

Using do-while loop:

int i = 0;
do {
  System.out.println(i);
  i++;
} while (i < 10)

Using IntStream with range() - supported in Java 8 and higher:

import java.util.stream.IntStream;

IntStream.range(0, 10).forEach(System.out::println);

Using IntStream with iterate() and limit() - supported in Java 8 and higher:

import java.util.stream.IntStream;

IntStream.iterate(0, i -> i + 1).limit(10).forEach(System.out::println);

More about Streams in the Streams cheatsheet.

Iterate backwards

Using while loop:

int i = 9;
while (i >= 0) {
  System.out.println(i);
  i--;
}

Using for loop:

for (int i = 9; i >= 0; i--) {
  System.out.println(i);
}

Using do-while loop:

int i = 0;
do {
  System.out.println(i);
  i--;
} while (i >= 0)

Using IntStream with iterate() and limit() - supported in Java 8 and higher:

import java.util.stream.IntStream;

IntStream.iterate(9, i -> i - 1).limit(10).forEach(System.out::println);

More about Streams in the Streams cheatsheet.

Iterate through characters of a String

Using while loop:

String hello = "Hello World";
int i = 0;
while (i < hello.length()) {
  System.out.println(hello.charAt(i));
  i++;
}

Using for loop:

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

Using performance-optimized for loop - length only calculated once:

for (int i = 0, n = hello.length(); i < n; i++) {
  System.out.println(hello.charAt(i));
}

Using enhanced (for each) loop - note that it can't be applied directly on a String, conversion to an array is needed!:

for (char character : hello.toCharArray()) {
  System.out.println(character);
}

Iterate backwards

Using while loop:

// hello = "Hello World";
int i = hello.length() - 1;
while (i >= 0) {
  System.out.println(hello.charAt(i));
  i--;
}

Using for loop:

for (int i = hello.length() - 1; i >= 0; i--) {
  System.out.println(hello.charAt(i));
}

Iterate through an array

Using while loop:

String[] shoppingList = new String[]{"bread", "carrot", "chocolate", "apple"};
int i = 0;
while (i < shoppingList.length) {
  System.out.println(shoppingList[i]);
  i++;
}

Using for loop:

for (int i = 0; i < shoppingList.length; i++) {
  System.out.println(shoppingList[i]);
}

Using performance-optimized for loop - length only calculated once:

for (int i = 0, n = shoppingList.length; i < n; i++) {
  System.out.println(shoppingList[i]);
}

Using enhanced (for each) loop:

for (String item : shoppingList) {
  System.out.println(item);
}

Using Arrays.stream() - note that original array can't be modified this way! - supported in Java 8 and higher:

import java.util.Arrays;
Arrays.stream(shoppingList).forEach(item -> System.out.println(item));

More about Streams in the Streams cheatsheet.

Iterate backwards

Using while loop:

// shoppingList = new String[]{"bread", "carrot", "chocolate", "apple"};
int i = shoppingList.length - 1;
while (i >= 0) {
  System.out.println(shoppingList[i]);
  i--;
}

Using for loop:

for (int i = shoppingList.length - 1; i >= 0; i--) {
  System.out.println(shoppingList[i]);
}

Iterate through a collection

Using while loop:

import java.util.List;
import java.util.Arrays;

List<String> shoppingList = Arrays.asList("bread", "carrot", "chocolate", "apple");
int i = 0;
while (i < shoppingList.size()) {
  System.out.println(shoppingList.get(i));
  i++;
}

Using for loop:

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

Using performance-optimized for loop - size only calculated once:

for (int i = 0, n = shoppingList.size(); i < n; i++) {
  System.out.println(shoppingList.get(i));
}

Using enhanced (for each) loop:

for (String item : shoppingList) {
  System.out.println(item);
}

Using forEach() - note that collection can't be modified this way!:

shoppingList.forEach(item -> System.out.println(item));

Using Iterator with while loop, hasNext() and next():

import java.util.Iterator;

// shoppingList = Arrays.asList("bread", "carrot", "chocolate", "apple");
Iterator<String> it = shoppingList.iterator();
while (it.hasNext()) {
  System.out.println(it.next());
}

Using Iterator with forEachRemaining() - supported in Java 8 and higher:

Iterator<String> iter = shoppingList.iterator();
iter.forEachRemaining(listItem -> System.out.println(listItem));

Using ListIterator with while loop, hasNext() and next() - only works with Lists:

import java.util.ListIterator;

ListIterator<String> it = shoppingList.listIterator();
while (it.hasNext()) {
  System.out.println(it.next());
}

Using Stream - note that original collection can't be modified this way! - supported in Java 8 and higher:

shoppingList.stream().forEach(item -> System.out.println(item));

More about Streams in the Streams cheatsheet.

Iterate backwards

Using while loop:

// shoppingList = Arrays.asList("bread", "carrot", "chocolate", "apple");
int i = shoppingList.size() - 1;
while (i >= 0) {
  System.out.println(shoppingList.get(i));
  i--;
}

Using for loop:

for (int i = shoppingList.size() - 1; i >= 0; i--) {
  System.out.println(shoppingList.get(i));
}

Using enhanced (for each) loop and Collections.reverse(): - note that a separate collection is created, items of original collection can't be accessed directly - lower performance due to new (reversed) collection being created:

import java.util.Collections;

Collections.reverse(shoppingList);
for (String item : shoppingList) {
  System.out.println(item);
}

Using enhanced (for each) loop and List.reversed(): - same issues as with previous one, plus only works with Lists, only supported in Java 21 and higher:

for (String item : shoppingList.reversed()) {
  System.out.println(item);
}

Using ListIterator with while loop, hasPrevious() and previous() - note that the size of the List need to be passed when creating ListIterator, to use reverse iteration - only works with Lists:

import java.util.ListIterator;

ListIterator<String> it = shoppingList.listIterator(shoppingList.size());
while (it.hasPrevious()) {
  System.out.println(it.previous());
}

Using Stream with descendingIterator() - lower performance due to additional ArrayDeque being created:

import java.util.Collectors;
import java.util.ArrayDeque;

shoppingList.stream()
    .collect(Collectors.toCollection(ArrayDeque::new))
    .descendingIterator()
    .forEachRemaining(System.out::println);

More about Streams in the Streams cheatsheet.