Java Eight Foreach() Loop Example

Java 8 has introduced a novel way to loop over a List or Collection, yesteryear using the forEach() method of the novel Stream class. You tin iterate over whatever Collection e.g. List, Set or Map yesteryear converting them into a java.util.sttream.Stream instance together with so calling forEach() method. This method performs given functioning on every chemical cistron of Stream, which tin live either only printing it or doing something else. Since flow tin live sequential or parallel, the demeanour of if this method is non deterministic if used amongst a parallel stream. One to a greater extent than affair to recollect close the forEach() method is that it's a terminal operation, which agency y'all cannot reuse the Stream after calling this method. It volition throw IllegalStateException if y'all assay to telephone telephone some other method on this Stream.

Btw, y'all tin likewise telephone telephone forEach() method without obtaining Stream from list e.g. listOfString.forEach(), because the forEach() method is likewise defined inwards Iterable interface, but obtaining Stream gives y'all to a greater extent than choices e.g. filtering, mapping or flattening etc.

In this article, y'all volition acquire how to loop over an ArrayList using the forEach() method of Java 8. It's i of the many Java 8 tutorials I am going to write this year. If y'all are Java developer amongst some years of experience, pass some fourth dimension learning novel features of Java 8, it's going to live i of the most sought after unloose inwards coming years.

How to exercise forEach() method to loop over List inwards Java

In this section, I volition become over a twosome of mutual scenarios every Java developer confront inwards their solar daytime job. In the showtime example, nosotros receive got a listing of prime numbers together with nosotros desire to impress it using this novel way of looping inwards Java 8:

List<Integer> listOfPrimes = Arrays.asList(2, 3, 5, 7, 11, 3, 17);          listOfPrimes.stream().forEach((i) -> { System.out.println(i); });

Here nosotros are going through each chemical cistron of the flow which is an Integer variable together with passing it to System.out.println() for printing. Since forEach() method accepts a Consumer type, which is a functional interface together with that's why nosotros tin overstep a lambda expression to it.

You tin fifty-fifty shorten it yesteryear using method reference, equally shown below:

listOfPrimes.stream().forEach(System.out::println);

You tin exercise method reference inwards house of the lambda aspect if y'all are non doing anything amongst the parameter, except passing it to some other method. For example, inwards our adjacent 2 examples, nosotros cannot supervene upon lambda aspect amongst method reference because nosotros are modifying the element.

You tin likewise come across Java SE 8 for Really Impatient to acquire to a greater extent than close method reference together with lambda expression, i of the improve books to acquire Java.

You tin fifty-fifty apply other useful flow methods earlier calling forEach() e.g. inwards next code nosotros are using filter() method to alone impress fifty-fifty numbers from the listing of primes:

listOfPrimes.stream().filter(i -> i%2 == 0).forEach(System.out::println);

filter() method from Stream storey await a Predicate instance which is likewise a functional interface, amongst precisely i method which returns boolean type, thus y'all tin exercise a lambda aspect which returns boolean inwards house of Predicate.

Java 8 forEach example

You tin write to a greater extent than code yesteryear next above techniques, I propose y'all experiment amongst dissimilar flow methods to acquire more.  Here is my sample plan to demonstrate how to exercise forEach() contention to iterate over every chemical cistron of a list, ready or flow inwards Java.

import java.util.Arrays; import java.util.List;  /**  *  Java 8 forEach illustration to loop over Stream obtained from a List  *  * @author Javin  */ public class forEachDemo{      public static void main(String args[]) {          List<Integer> listOfPrimes = Arrays.asList(2, 3, 5, 7, 11, 3, 17);          // forEach illustration to impress each chemical cistron of list         // inwards this instance nosotros are using method reference becasue         // nosotros are non doing anything amongst each chemical cistron of collection         // together with precisely passing ito println method         System.out.println("Printing elements of listing using forEach method : ");         listOfPrimes.stream().forEach(System.out::println);                     // let's produce something to each chemical cistron earlier printing         // nosotros volition add together comma after each element         System.out.println("Printing elements after adding comma: ");         listOfPrimes.stream().forEach( i -> System.out.print( i + ","));                     // y'all tin likewise exercise forEach amongst parallel stream         // companionship volition non live guaranteed         System.out.println("\nPrinting elements of listing using parallel stream: ");         listOfPrimes.parallelStream().forEach( i-> System.out.println(i*2));             }  }  Output : run: Printing elements of listing using forEach method: 2 3 5 7 11 3 17 Printing elements after adding comma: 2,3,5,7,11,3,17, Printing elements of listing using parallel stream: 22 14 4 6 34 6 10

You tin come across from the output that forEach() contention industrial plant precisely similar for loop but it is much to a greater extent than powerful only because y'all tin perform the looping inwards parallel without writing a unmarried draw of multi-threading code.

Here is the some other forEach() method defined inwards the Iterable interface, y'all tin exercise it conduct without calling the stream() method because List, Set or Queue implements Collection together with Iterable interface.

Advantages of forEach() over for loop inwards Java 8 

There are several advantages of using forEach() contention over traditional for loop inwards Java 8 e.g.

• More succinct code, sometimes precisely i liner
• You tin overstep lambda expression, which gives y'all the immense flexibility to change what y'all produce inwards the loop.
• forEach looping tin live made parallel amongst minimal endeavor e.g. without writing a unmarried draw of concurrent code, all y'all involve to produce is telephone telephone parallelStream() method. 

Important points close forEach method

1. forEach() method is defined at 2 places, on Iterable interface equally good equally on Stream class. which agency list.forEach() together with list.stream.forEach() both are valid.
2. Prefer using forEach() amongst streams because streams are lazy together with non evaluated until a lastly functioning is called. 
3. forEach() is a lastly operation, y'all cannot telephone telephone whatever method on flow after this.
4. When y'all run forEach() method on parallel flow the companionship on which elements are processed is non guaranteed, though y'all tin exercise forEachOrdered() to impose ordering.
5. forEach() method accepts a Consumer instance, which is a functional interface, that's why y'all tin overstep a lambda aspect to it. 

That's all close how to exercise forEach() method to loop over Stream inwards Java. You tin exercise this technique to become over whatever Collection, List, Set or Queue inwards Java. They all allow y'all to acquire a Stream together with afterward telephone telephone the forEach() method on it. Remember, though, forEach() is a lastly functioning together with y'all cannot telephone telephone some other method after this. Don't re-use or overstep some streams after calling whatever lastly functioning on it e.g. forEach().


Further Learning
The Complete Java MasterClass
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How to exercise Stream storey inwards Java 8 (tutorial)

How to exercise filter() method inwards Java 8 (tutorial)

How to exercise forEach() method inwards Java 8 (example)

How to bring together String inwards Java 8 (example)

How to convert List to Map inwards Java 8 (solution)

How to exercise peek() method inwards Java 8 (example)

5 Books to Learn Java 8 from Scratch (books)

How to convert flow to array inwards Java 8 (tutorial)

Java 8 Certification FAQ (guide)

Java 8 Mock Exams together with Practice Test (test)

Thanks for reading this article so far. If y'all similar this article so delight percentage amongst your friends together with colleagues. If y'all receive got whatever question, doubt, or feedback so delight drib a comment together with I'll assay to response your question.