Java forEach – Java 8 forEach

Java forEach method was introduced in Iterable interface in Java 8. Java 8 forEach method is another way that we can use to traverse through a collection.

Java forEach

Below code snippet shows the default implementation of java forEach method in Iterable interface. Read java 8 interface changes to learn more about default interface methods.

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

Java forEach method performs the given action for each element of the Iterable until all elements have been processed or exception is thrown.

Java 8 forEach List Example

Before java 8, We could iterate over a list by using for loop or iterator.

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List<String> list = getList();
		
//prior to java 8
//using enhanced for loop
for(String s : list){
	System.out.println(s);
}

//using iterator
Iterator<String> it = list.iterator();
while(it.hasNext()){
	System.out.println(it.next());
}

Now same thing can be performed using java forEach method as shown below.

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// Create Consumer instance
Consumer<String> action = new Consumer<String>(){

	@Override
	public void accept(String t) {
		System.out.println(t);
	}
	
};

//java 8 forEach
list.forEach(action);

Since Consumer is a functional interface, we can use lambda expression and write above code in one line as;

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list.forEach(k -> {System.out.println(k);});

Java 8 forEach Map Example

Prior to java 8, we iterate over Map elements like below.

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Map<Integer,String> map = getMap();

//iteration prior to java 8
Set<Integer> keySet = map.keySet();

//using enhanced for loop
for (Integer i : keySet){
	System.out.println(map.get(i));
}

//using iterator
Iterator<Integer> it = keySet.iterator();
while(it.hasNext()){
	System.out.println(map.get(it.next()));
}

Since Map doesn’t extend Iterable, forEach method is added into Map interface in java 8 and below shows the default implementation.

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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);
        }
    }

Let’s look at a simple example how we can use java 8 forEach with Map.

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BiConsumer<Integer,String> action = new BiConsumer<Integer, String>(){

	@Override
	public void accept(Integer t, String u) {
		System.out.println(u);
	}
	
};
//java 8 forEach with Map
map.forEach(action);

Same code can be written using lambda expressions as below.

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map.forEach((k,v) -> {System.out.println(v);});

Java forEach Benefits

I don’t see too much benefit of forEach loop except when you are using it with parallel stream. A new method was added in Collection interface to get the parallel stream.

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default Stream<E> parallelStream() {
        return StreamSupport.stream(spliterator(), true);
    }

So if we have to iterate over a collection and we are not bothered about sequential iteration, then we can use parallel stream with forEach loop as shown below.

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//parallel operation using stream
list.parallelStream().forEach(action);

That’s all for Java forEach method. I hope you will find some use case for java 8 forEach method in parallel processing.

About Pankaj

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