forEach Method in Java

The forEach method in Java is used to perform an action on every element of a collection. It takes a lambda (a Consumer) and calls it once for each element, so you don't have to write a loop. forEach was added in Java 8 to the Iterable interface (so every List, Set and Queue has it), to Map (with a two-argument lambda), and to streams. In this lesson, you will learn each version and when a normal loop is still the better choice.

Declaration

// in Iterable<T>
default void forEach(Consumer<? super T> action)
 
// in Map<K, V>
default void forEach(BiConsumer<? super K, ? super V> action)
 
// in Stream<T>
void forEach(Consumer<? super T> action)

Example: forEach on a List, a Set and a Map

Let's see an example of each.

import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.TreeSet;
 
public class ForEachDemo {
    public static void main(String[] args) {
        List<String> names = List.of("Navika", "Srihitha", "Advitha");
        names.forEach(n -> System.out.println("Hello " + n));
 
        new TreeSet<>(names).forEach(System.out::println);
 
        Map<String, Integer> marks = new LinkedHashMap<>();
        marks.put("Navika", 91);
        marks.put("Srihitha", 78);
        marks.forEach((name, m) -> System.out.println(name + " scored " + m));
    }
}

Hello Navika
Hello Srihitha
Hello Advitha
Advitha
Navika
Srihitha
Navika scored 91
Srihitha scored 78

Explanation: List.forEach runs the lambda for each name in order. TreeSet.forEach visits the names in sorted order, here with a method reference. Map.forEach passes the key and the value to a two-parameter lambda.

forEach vs a For-Each Loop

Example: Stream forEach and Ordering

import java.util.List;
 
public class StreamForEach {
    public static void main(String[] args) {
        List<Integer> marks = List.of(91, 38, 85, 42);
 
        marks.stream()
             .filter(m -> m >= 50)
             .forEach(m -> System.out.println("Pass: " + m));
 
        marks.stream()
             .sorted()
             .forEachOrdered(m -> System.out.print(m + ";"));
        System.out.println();
    }
}

Pass: 91
Pass: 85
38;42;85;91;

Explanation: forEach at the end of a stream acts on each element that survives the pipeline. forEachOrdered guarantees the stream's order even if the stream were parallel; on a parallel stream, plain forEach may visit elements in any order.

Common Mistakes

Calling list.remove(...) inside list.forEach(...). It throws ConcurrentModificationException; use removeIf.

Trying to break out of forEach. It isn't possible; use a loop or a stream with takeWhile or anyMatch.

Adding to an outside list from stream().forEach. Use collect or toList(), which also work correctly in parallel.

Practice

1. Easy: Use forEach to print each element of a list of prices with a currency symbol.
2. Harder: Given a Map<String, List<Integer>> of student marks, use Map.forEach to print each student's average.

Key Takeaways