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Java 8 forEach examples

In this article we present Java 8 forEach on examples. These are very convenient methods for use with Lambda Expressions and method references.

Classical List for-each loop

Since Java 5 there is for-each loop that allows to easily go through Iterables:

List<Integer> numbers = asList(1, 2, 3);
for (Integer x : numbers) {
    System.out.println(x);
}

A nice thing about the for-each loop is that JVM can optimize it.

Java 8 forEach on List

Java 8 shipped with default methods in interfaces and one of them is java.lang.Iterable.forEach(Consumer<? super T> action). It is implemented using classical for-each loop with check that action is not null. You can pass there method references like here:

List<Integer> numbers = asList(1, 2, 3);
numbers.forEach(System.out::println);

Or Java 8 Lambda Expressions thusly:

List<Integer> numbers = asList(1, 2, 3);
numbers.forEach(x -> {
    System.out.println("Number: " + x);
});

Java 8 forEach on Set

The same Java 8 forEach method works with Sets:

Set<Integer> numbers = new HashSet<>(asList(1, 2, 3));
numbers.forEach(System.out::println);

Classical Map for-each loop

Before Java 8 one could process Map using for-each loop on entry set:

Map<Integer, String> languages = new HashMap<>();
languages.put(1, "Java");
languages.put(2, "Clojure");
languages.put(3, "Scala");
for (Entry<Integer,String> lang : languages.entrySet()) {
    System.out.printf("%d: %s%n",
        lang.getKey(), lang.getValue());
}

Java 8 forEach on Map

In Java 8 a new forEach(BiConsumer<? super K, ? super V> action) has been added to the Map interface. Underneath it is using for-each loop through entry set as above, but is simpler to use:

Map<Integer, String> languages = new HashMap<>();
languages.put(1, "Java");
languages.put(2, "Clojure");
languages.put(3, "Scala");

languages.forEach((id, name) ->
    System.out.printf("%d: %s%n", id, name));

References:

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