For Each Loop Java 8 Example (original) (raw)

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.

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.

2.1 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!

Fig. 1: Application Project Structure

2.3 Project Creation

This section will show you how to create a Java-based Maven project with Eclipse. In Eclipse IDE, go to File -> New -> Maven Project.

Fig. 2: Create Maven Project

In the New Maven Project window, it will ask you to select project location. By default, ‘Use default workspace location’ will be selected. Select the ‘Create a simple project (skip archetype selection)’ check-box and just click on next button to go ahead.

Fig. 3: Project Details

It will ask you to ‘Enter the group and the artifact id for the project’. We will input the details as shown in the below image. The version number will be by default: 0.0.1-SNAPSHOT.

Fig. 4: Archetype Parameters

Click on Finish and the creation of a maven project is completed. If you see, it has downloaded the maven dependencies and a pom.xml file will be created. It will have the following code:

pom.xml

Developers can start adding the dependencies that they want. Let’s start building the application!

3. Application Building

Below are the steps involved in developing this application.

3.1 Java Class Implementation

Let’s create the required Java files. Right-click on the src/main/java folder, New -> Package.

Fig. 5: Java Package Creation

A new pop window will open where we will enter the package name as: com.jcg.java.

Fig. 6: Java Package Name (com.jcg.java)

Once the package is created in the application, we will need to create the implementation class to show the operation of forEach() method. Right-click on the newly created package: New -> Class.

Fig. 7: Java Class Creation

A new pop window will open and enter the file name as: ForEachDemo. The implementation class will be created inside the package: com.jcg.java.

Fig. 8: Java Class (ForEachDemo.java)

3.1.1 forEach function in Java 8

Here is a sample program to show how to use the forEach() statement to iterate over every element of a List, Set, Map or Stream in Java.

ForEachDemo.java

3.1.2 For Each loop before Java 8

Here is a sample program to show how to use a for-each loop with a List, Set, Map in Java.

For_each_loop.java

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();

    
}

}

4. Run the Application

To run the application, developers need to right-click on the class, Run As -> Java Application. Developers can debug the example and see what happens after every step!

Fig. 9: Run Application

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

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.

7. More articles

• For loop Java Example
• Simple while loop Java Example
• Java Switch Case Example

8. Download the Eclipse Project

This was a For Each Loop Java 8 Example.

Last updated on Apr. 29th, 2021

An experience full-stack engineer well versed with Core Java, Spring/Springboot, MVC, Security, AOP, Frontend (Angular & React), and cloud technologies (such as AWS, GCP, Jenkins, Docker, K8).