With Java8 streams, we take the advantage of parallelism on multi core processor. Java8 has introduced forEach() forEachOrdered() on collections to iterate over a stream of elements, however there is a difference between the two methods.
From the forEach Javadoc:
The behavior of this operation is explicitly nondeterministic. For parallel stream pipelines, this operation does not guarantee to respect the encounter order of the stream, as doing so would sacrifice the benefit of parallelism.
Where the forEachOrdered Javadoc states
Performs an action for each element of this stream, in the encounter order of the stream if the stream has a defined encounter order.
Java8 forEach() vs forEachOrdered() example
package com.jminded.java8.foreach;
import java.util.ArrayList;
import java.util.List;
/**
* https://jminded.com
*
* @author Umashankar
* Example to iterate list through forEarch
*/
public class ListForEach {
public static void main(String[] args) {
List<String> authors = getAuthors();
/**
* Performs an action for each element of this stream. This is a
* terminal operation.
*
* The behavior of this operation is explicitly nondeterministic. For
* parallel stream pipelines, this operation does not guarantee to
* respect the encounter order of the stream, as doing so would
* sacrifice the benefit of parallelism. For any given element, the
* action may be performed at whatever time and in whatever thread the
* library chooses. If the action accesses shared state, it is
* responsible for providing the required synchronization.
*
*/
authors.stream().forEach(a -> {
System.out.println(a);
});
/**
* Performs the given action for each element of the Iterable until all
* elements have been processed or the action throws an exception.
* Unless otherwise specified by the implementing class, actions are
* performed in the order of iteration (if an iteration order is
* specified). Exceptions thrown by the action are relayed to the
* caller.
*/
authors.forEach(a -> {
System.out.println(a);
});
/**
* Performs an action for each element of this stream, in the encounter
* order of the stream if the stream has a defined encounter order. This
* is a terminal operation.
*
* This operation processes the elements one at a time, in encounter
* order if one exists. Performing the action for one element
* happens-before performing the action for subsequent elements, but for
* any given element, the action may be performed in whatever thread the
* library chooses.
*
*/
authors.stream().forEachOrdered(a -> {
System.out.println(a);
});
}
/**
*
* @return
*/
public static List<String> getAuthors() {
List<String> authors = new ArrayList<>();
authors.add("Umashankar");
authors.add("Rod Jhonson");
authors.add("Gavin King");
return authors;
}
}