Java 8 forEach

The Java forEach is a utility method to iterate over a collection such as (list, set or map) and stream and perform a certain action on each element of it.

1. Java 8 forEach method

1.1. Iterable.forEach()

Below code snippet shows the default implementation of forEach in Iterable interface. It makes Iterable.forEach() method available to all collection classes except Map. forEach() method in Map is discussed in next section.

default void forEach(Consumer<? super T> action) {
    Objects.requireNonNull(action);
    for (T t : this) {
        action.accept(t);
    }
}

Above method performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.

The action represents an operation that accepts a single input argument and returns no result. It is an instance of Consumer interface.

List<String> names = Arrays.asList("Alex", "Brian", "Charles");
	
names.forEach(System.out::println);

//Console output
Alex
Brian
Charles

A custom consumer action can be created using this simple syntax. Here Object type shall be replaced with the type of elements in the collection or stream.

List<String> names = Arrays.asList("Alex", "Brian", "Charles");

Consumer<String> makeUpperCase = new Consumer<String>()
{
    @Override
    public void accept(String t) 
    {
    	System.out.println(t.toUpperCase());
    }
};

names.forEach(makeUpperCase);	

//Console output
ALEX
BRIAN
CHARLES

1.2. Map.forEach()

This method performs the given BiConsumer action for each entry in this map until all entries have been processed or the action throws an exception.

default void forEach(BiConsumer<? super K, ? super V> action) {
    Objects.requireNonNull(action);
    for (Map.Entry<K, V> entry : entrySet()) {
        K k;
        V v;
        try {
            k = entry.getKey();
            v = entry.getValue();
        } catch(IllegalStateException ise) {
            // this usually means the entry is no longer in the map.
            throw new ConcurrentModificationException(ise);
        }
        action.accept(k, v);
    }
}

Map<String, String> map = new HashMap<String, String>();

map.put("A", "Alex");
map.put("B", "Brian");
map.put("C", "Charles");

map.forEach((k, v) -> 
	System.out.println("Key = " + k + ", Value = " + v));

//Console Output

Key = A, Value = Alex
Key = B, Value = Brian
Key = C, Value = Charles

Similar to List example, we can create a custom biconsumer action taking key-value pairs from map and proces each entry one at a time.

BiConsumer<String, Integer> action = (a, b) -> 
{ 
    System.out.println("Key is : " + a); 
    System.out.println("Value is : " + b); 
}; 

Map<String, Integer> map = new HashMap<>();
    
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

map.forEach(action);

Program output.

Key is : A
Value is : 1

Key is : B
Value is : 2

Key is : C
Value is : 3

2. Java forEach example using List

Java program to iterate over all elements of a stream and perform an action. In this example, we are printing all even numbers.

List<Integer> numberList = Arrays.asList(1,2,3,4,5);
    
Consumer<Integer> action = System.out::println;

numberList.stream()
	.filter(n -> n%2  == 0)
	.forEach( action );

Program output.

2
4

3. Java forEach example using Map

We already saw above program to iterate over all entries of a HashMap and perform an action.

We can also iterate over map keys and values and perform any action on all elements.

HashMap<String, Integer> map = new HashMap<>();
    
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

//1. Map entries
Consumer<Map.Entry<String, Integer>> action = System.out::println;

map.entrySet().forEach(action);

//2. Map keys
Consumer<String> actionOnKeys = System.out::println;

map.keySet().forEach(actionOnKeys);

//3. Map values
Consumer<Integer> actionOnValues = System.out::println;

map.values().forEach(actionOnValues);

Program output.

A=1
B=2
C=3

A
B
C

1
2
3

Happy Learning !!