Java 8 Streams forEach Examples

forEach on an Array

Since an array in Java is not an Iterable, you cant directly use forEach() with it like you can with List, Map and other classes that implement the Iterable interface.

The workaround to this is to simply convert the array to a Stream first using the built-in Arrays.stream() function - see Converting from Streams to Arrays for more details on converting between arrays and streams.

package xyz.byexample.java8;

import java.util.Arrays;

public class forEach {
    public static void main(String[] args) {
        String[] letters = new String[]{"a", "b", "c"};
        Arrays.stream(letters).forEach((String letter) -> {
            System.out.println(letter);
        });
    }
}

Output

a
b
c

Alternatively we can use method references to simplify out code and cut out the entire anonymous function we passed in.

    String[] letters = new String[]{"a", "b", "c"};
    Arrays.stream(letters).forEach(System.out::println);

Output

a
b
c

forEach on a List

Using forEach on a List is just as simple as using it on a Stream in the above array example:

List<Integer> numbers = Arrays.asList(1,2,3);
numbers.forEach((Integer number) -> {
    System.out.println(number);
});

Output

1
2
3

Here we used a anonymous function, but we could have used a method reference had we wanted to, just like with arrays.

forEach on a Map

When using forEach on a Map, things are slightly different than with arrays or a List since we have both a key and a value so we need to use an anonymous function that implements the BiConsumer interface rather than just a simple Consumer as before:

Map<String, String> countries = new HashMap<>();
countries.put("fr", "France");
countries.put("us", "United States");
countries.put("nz", "New Zealand");

countries.forEach((String key, String value) -> {
    System.out.println("Key: " + key + " Value: " + value);
});

Output

Key: fr Value: France
Key: nz Value: New Zealand
Key: us Value: United States