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.
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.
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.
// 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;
list.forEach(k -> {System.out.println(k);});
Java 8 forEach Map Example
Prior to java 8, we iterate over Map elements like below.
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.
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.
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.
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.
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.
//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.
Any comment on the following simple bench marking:
public class Test {
public static void main(String[] args) {
List list = new ArrayList();
for (int i = 0; i {
function(i);
});
System.out.println((System.currentTimeMillis() – time));
time = System.currentTimeMillis();
list.parallelStream().forEach(i -> {
function(i);
});
System.out.println((System.currentTimeMillis() – time));
}
private static void function(int i) {
i = i * 9 + 90 – 23 + i / 4;
}
}
Output::
23
57
43
Clearly shows the traditional forEach loop gives better performance (in terms of time duration of processing) than others, even the parallelStream one.