For Each Loop Java 8 Example

YatinJanuary 30th, 2018Last Updated: April 29th, 2021

2 545 9 minutes read

In this post, we feature a comprehensive For Each Loop Java 8 Example. Foreach method, it is the enhanced for loop that was introduced in Java since J2SE 5.0.

Java 8 came up with a new feature to iterate over the Collection classes, by using the forEach() method of the Iterable interface or by using the new Stream class.

In this tutorial, we will learn how to iterate over a List, Set and Map using the Java forEach method.

1. For Each Loop in Java – Introduction

As of Java 5, the enhanced for loop was introduced. This is mainly used to traverse a collection of elements including arrays.

From Java 8 and on, developers can iterate over a List or any Collection without using any loop in Java programming, because of this enhanced for loop. The new Stream class provides a forEach method, which can be used to loop over all of the selected elements of List, Set, and Map. This enhanced loop called forEach() provides several advantages over the traditional for loop e.g. We can execute it in parallel by just using a Parallel Stream instead of Regular Stream.

Since developers are working with stream, it allows them to filter and map the elements. Once they are done with filtering and mapping, they can use the forEach() method to work over them. We can even use method reference and lambda expression inside the forEach() method, resulting in more understandable and brief code.

An important thing about the forEach() method is that it is a Terminal Operation, which means developers cannot reuse the Stream after calling this method. It will throw IllegalStateException if developers try to call another method on this Stream.

Do remember, you can also call the forEach() method without obtaining the Stream from the List, e.g. sList.forEach(), because the forEach() method is also defined in Iterable interface, but obtaining Stream gives them further choices e.g. filtering, mapping or flattening etc.

1.1 forEach Signature in Java

We can write this useful tool in two ways:

As a method
As a simple for loop

As a method, in Iterable interface, the forEach() method takes a single parameter which is a functional interface. Developers can pass the Lambda Expression as an argument and it can be coded as shown below.

public interface Iterable<T> {
default void forEach(Consumer<super T> action) {
    for(T t : this) {
        action.accept(t);
    }
}

As for a simple for loop:

for(data_type item : collection) {
    ...
}

collection is an array variable or collection which you have to loop through.
item is an item from the collection.

1.2 Things to Remember

forEach() is a terminal operation, which means once calling this method on a stream, we cannot call another method. It will result in a runtime exception
When developers call the forEach() method on a parallel stream, the iteration order is not guaranteed, but developers can ensure that ordering by calling the forEachOrdered() method
There are two forEach() methods in Java8, one defined inside the Iterable and other inside the java.util.stream.Stream class. If the purpose of forEach() is just iteration then we can directly call it (i.e. list.forEach() or set.forEach() etc). But if developers want to do some operations then get the stream first and then do that operation and finally call forEach() method
Use of forEach() results in a readable and cleaner code
Favor using forEach() with Streams because streams are slow and not evaluated until a terminal operation is called

1.3 Java For Loop enhanced – advantages

There are several advantages of using the forEach() statement over the traditional for loop in Java e.g.

More manageable code
Developers can pass the Lambda Expression, which gives them the substantial flexibility to change what they do in the loop
forEach() looping can be made parallel with minimum effort i.e. Without writing a single line of parallel code, all they need to do is call a parallelStream() method

1.4 For vs forEach in Java

Use: Between for and foreach is that, in the case of indexable objects, you do not have access to the index.
Performance: When accessing collections, a foreach is significantly faster than the for loop’s array access. Αt least with primitive and wrapper-arrays when accessing arrays, access via indexes is dramatically faster.

Now, open up the Eclipse Ide and let’s see how to iterate over a List using the forEach() method of Java8.

2. For Each Loop Java 8 Example

2.1 Technologies used

We are using Eclipse Oxygen, JDK 1.8 and Maven.

2.2 Project Structure

Firstly, let us review the final project structure if you are confused about where you should create the corresponding files or folder later!

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import java.util.ArrayList; import java.util.Arrays; import java.util.HashMap; import java.util.HashSet; import java.util.List; import java.util.Map; import java.util.Set; public class For_each_loop { /**** EXAMPLE METHOD #1 ****/ private static void ListUsingForEach() { /*** List Instantiation :: Type #1 ***/ List cList = new ArrayList(); cList.add("India"); cList.add("USA"); cList.add("Japan"); cList.add("Canada"); cList.add("Singapore"); for(String clist: cList) { System.out.println(clist); } } private static void SetUsingForEach() { Set persons = new HashSet (); persons.add("Java Geek"); persons.add("Sam"); persons.add("David"); persons.add("April O' Neil"); persons.add("Albus"); for(String person: persons) { System.out.println(person); } } private static void MapUsingForEach() { Map days = new HashMap(); days.put("1", "SUNDAY"); days.put("2", "MONDAY"); days.put("3", "TUESDAY"); days.put("4", "WEDNESDAY"); days.put("5", "THURSDAY"); days.put("6", "FRIDAY"); days.put("7", "SATURDAY"); for(Map.Entry day: days.entrySet()) { System.out.println(day); } } public static void main(String[] args) { System.out.println("List using For each loop :"); ListUsingForEach(); System.out.println(); System.out.println("Set Using For Each :"); SetUsingForEach(); System.out.println(); System.out.println("Map Using For Each :"); MapUsingForEach(); } }

5. Project Demo

Developers can write more code by following the above techniques. I suggest you experiment with different stream methods to learn more. The above code shows the following logs as output.

Output of forEach() method

# Logs for 'EXAMPLE METHOD #1' #
================================
<------------Iterating List By Passing Lambda Expression-------------->
India
USA
Japan
Canada
Singapore
 
<------------Iterating List By Passing Method Reference--------------->
India
USA
Japan
Canada
Singapore
 
<------------Printing Elements Of List By Using 'forEach' Method------------>
India
USA
Japan
Canada
Singapore
 
<------------Printing Specific Element From List By Using Stream & Filter------------>
Singapore
 
<------------Printing Elements Of List By Using Parallel Stream------------>
Japan
India
Canada
USA
Singapore
 
# Logs for 'EXAMPLE METHOD #2' #
================================
<------------Iterating Set By Passing Lambda Expression-------------->
April O' Neil
Albus
Java Geek
David
Sam
 
<------------Iterating Set By Passing Method Reference--------------->
April O' Neil
Albus
Java Geek
David
Sam
 
# Logs for 'EXAMPLE METHOD #3' #
================================
<------------Iterating Map Using 'forEach' Method--------------->
1 : SUNDAY
2 : MONDAY
3 : TUESDAY
4 : WEDNESDAY
5 : THURSDAY
6 : FRIDAY
7 : SATURDAY

Output of for-each loop

List using For each loop : India USA Japan Canada Singapore Set Using For Each : April O' Neil Albus Java Geek David Sam Map Using For Each : 1=SUNDAY 2=MONDAY 3=TUESDAY 4=WEDNESDAY 5=THURSDAY 6=FRIDAY 7=SATURDAY

That’s all for this post. Happy Learning!

6. Summary

As we stated at the beginning, forEach is an enhanced for loop and it was introduced in Java since J2SE 5.0. In this article, we learned how to use the For Each loop in Java 8.

By following this example, developers can easily get to speed with respect to using the forEach() method to iterate over any Collection, List, Set or Queue in Java.