Java forEach
last modified July 8, 2023
Java forEach tutorial shows how to use Java 8 forEach method.
We work with consumers and demonstrate forEach on lists, map, and
set collections.
The forEach method was introduced in Java 8. It provides programmers
a new, concise way of iterating over a collection.
The forEach method performs the given action for each element of
the Iterable until all elements have been processed or the action
throws an exception.
void forEach(Consumer<? super T> action);
This is the syntax of the forEach method.
Consumer interface
The Consumer interface is a functional interface
(an interface with a single abstract method), which accepts a single
input and returns no result.
@FunctionalInterface
public interface Consumer {
void accept(T t);
}
This is the definition of the Consumer interface.
package com.zetcode;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
public class JavaForEachListConsumer {
public static void main(String[] args) {
List<String> items = new ArrayList<>();
items.add("coins");
items.add("pens");
items.add("keys");
items.add("sheets");
items.forEach(new Consumer<String>() {
@Override
public void accept(String name) {
System.out.println(name);
}
});
}
}
In this example, we iterate over a list of strings with forEach. This
syntax can be shortened with Java lambda expression.
Lambda expression
Lambda expressions are used primarily to define an inline implementation
of a functional interface, i.e., an interface with a single method only.
Lambda expression are created with the -> lambda operator.
package com.zetcode;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
public class JavaForEachListLambda {
public static void main(String[] args) {
List<String> items = new ArrayList<>();
items.add("coins");
items.add("pens");
items.add("keys");
items.add("sheets");
items.forEach((String name) -> {
System.out.println(name);
});
}
}
Here we have the same example. The lambda expression makes the example more concise.
Java forEach on Map
The following example uses forEach on a map.
package com.zetcode;
import java.util.HashMap;
import java.util.Map;
public class JavaForEachMap {
public static void main(String[] args) {
Map<String, Integer> items = new HashMap<>();
items.put("coins", 3);
items.put("pens", 2);
items.put("keys", 1);
items.put("sheets", 12);
items.forEach((k, v) -> {
System.out.printf("%s : %d%n", k, v);
});
}
}
We have a map of string/integer pairs. With the forEach method,
we iterate over the map and print its key/value pairs.
In the next example, we explicitly show the Consumer
and the Map.Entry in code.
package com.zetcode;
import java.util.HashMap;
import java.util.Map;
import java.util.function.Consumer;
public class ForEachMap2 {
public static void main(String[] args) {
HashMap<String, Integer> map = new HashMap<>();
map.put("cups", 6);
map.put("clocks", 2);
map.put("pens", 12);
Consumer<Map.Entry<String, Integer>> action = entry ->
{
System.out.printf("key: %s", entry.getKey());
System.out.printf(" value: %s%n", entry.getValue());
};
map.entrySet().forEach(action);
}
}
The example loops on a entry set, which is retrieved via entrySet.
Java forEach on Set
The following example uses forEach on a set.
package com.zetcode;
import java.util.HashSet;
import java.util.Set;
public class JavaForEachSet {
public static void main(String[] args) {
Set<String> brands = new HashSet<>();
brands.add("Nike");
brands.add("IBM");
brands.add("Google");
brands.add("Apple");
brands.forEach((e) -> { System.out.println(e); });
}
}
We have a set of strings. With the forEach method,
we iterate over the set and print its values.
Using forEach on Array
The following example uses forEach on an array.
package com.zetcode;
import java.util.Arrays;
public class JavaForEachArray {
public static void main(String[] args) {
int[] nums = { 3, 4, 2, 1, 6, 7 };
Arrays.stream(nums).forEach((e) -> { System.out.println(e); });
}
}
In the example, we have an array of integers. We use Arrays.stream method
to transform the array into a stream. The forEach method then iterates
over the elements and prints them to the console.
Filtering a list
We can easily filter our data before traversing them with forEach.
package com.zetcode;
import java.util.ArrayList;
import java.util.List;
public class JavaForEachListFilter {
public static void main(String[] args) {
List<String> items = new ArrayList<>();
items.add("coins");
items.add("pens");
items.add("keys");
items.add("sheets");
items.stream().filter(item -> (item.length() == 4)).forEach(System.out::println);
}
}
In this example, we filter a list of strings and print the filtered list to the console. Only strings having four characters are shown.
IntConsumer, LongConsumer, DoubleConsumer
Since Java 8, we have built-in consumer interfaces for primitive data types:
IntConsumer, LongConsumer and DoubleConsumer.
package com.zetcode;
import java.util.Arrays;
import java.util.function.DoubleConsumer;
import java.util.function.IntConsumer;
import java.util.function.LongConsumer;
public class JavaForEachConsSpec {
public static void main(String[] args) {
int[] inums = { 3, 5, 6, 7, 5 };
IntConsumer icons = i -> System.out.print(i + " ");
Arrays.stream(inums).forEach(icons);
System.out.println();
long[] lnums = { 13L, 3L, 6L, 1L, 8L };
LongConsumer lcons = l -> System.out.print(l + " ");
Arrays.stream(lnums).forEach(lcons);
System.out.println();
double[] dnums = { 3.4d, 9d, 6.8d, 10.3d, 2.3d };
DoubleConsumer dcons = d -> System.out.print(d + " ");
Arrays.stream(dnums).forEach(dcons);
System.out.println();
}
}
In the example, we create the three types of consumers and iterate
over them with forEach.
In this article we have presented the Java 8 forEach method. We have
introduced consumers and used forEach on lists, maps, and set.
Author
List all Java tutorials.
