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In this post, experts will explain the features of Java 8 update in detail. If you have any requirement of java development, hire java developers who are having knowledge of these features and can use…

It's relatively easy to convert a Stream to an array in Java 8 by using the toArray() method of java.util.Stream class. By using this method you can convert any type of Stream to a corresponding array like a Stream of Strings can be converted into an array of String, or a Stream of integers can be converted into an array of Integers. The Stream.toArray() method is also overloaded, the one which doesn't take any parameter returns an Object[] which might not be very useful, particularly if you want to convert Stream of T to an array of T. On the other hand, the overloaded version of the toArray(IntFunction[] generator) returns an array containing the elements of this stream, using the provided generator function to allocate the returned array, as well as any additional arrays that might be required for a partitioned execution or for resizing. This toArray() method accepts a function that produces a new array of the desired type. The generator function takes an integer, which is the size of the desired array, and produces an array of the desired size. If you are familiar with method reference then this can be expressed even more concisely with an array constructor reference. You can pass this method a lambda expression or use the array reference to further shorten it as shown by a couple of examples in this article. Btw, let me tell you that we'll cover converting Stream to both array and ArrayList, the dynamic array of JDK library. Even though you can still use all the tricks I have shown before to convert an array to ArrayList, there are added benefits of using new methods provides by Java 8 as they minimize additional intermediate steps. 2 Ways to convert Stream to Array in Java 8 Now, let's see a couple of ways to convert a Java 8 Stream into an array in Java, using this way you can convert a Stream of Integer into Integer array and so on. You can even transform object using map() and flatMap() function before you convert them into an array. Btw, if you are wondering how Stream works in Java and how to use Stream to transform object and collect object into array or list, here is a nice diagram which explains a common Stream pipeline 1. Stream to Array using a lambda expression in Java 8 Here is the simplest way to convert a Stream of objects into an array of Objects. Suppose, the library object is a list of books and we need to convert that into an array. Of course, you can directly convert a list to an array without going via Stream as shown here, but for the purpose of this example, we'll use Stream to perform that conversion. Another reason to use Stream is filtering and lazy evaluation, for example, if you just want fiction books from a list of books, if you directly convert a list to an array, then you won't be able to filter it, but by using Stream you can just use the filter() method for lazy filtering. Once you got the stream with the interesting value, just call the toArray() method and passing them a generator function which will be used to convert each element of Stream to the corresponding object as shown in the following example: Book[] books = library.stream() .filter(b -> p.getTopic() == FICTION) .toArray(size -> new Integer[size]); Now, the book's array contains all books from the list library where the topic is FICTION. You can see that we pass the generator function using a lambda expression. If you want to learn more about new features introduced in Java 8 like lambdas, Stream, and other enhancements then I also suggest you take a look at the Functional Programming in Java course on Udemy. It's again a nice little course that provides an overview of all important Java 8 features like Stream and Lambdas. 2. Stream to an Array using a Method Reference Now, you can make the above example, even more, shorter by using the method reference in form of a constructor reference. This will eliminate the lambda expression we have passed in the above example: Book[] books = library.stream() .filter(b -> p.getTopic() == FICTION) .toArray(Book[]::new); You can see that this example is even more elegant, classic, and easy to understand than the previous example. If you are not very familiar with method reference and constructor reference then I suggest you join a good course on Java 8 which is focused on stream and lambda expression like From Collections to Streams in Java 8 Using Lambda Expressions on Pluralsight by Jose Paurmand, a Java Champion. 3. Stream to ArrayList in Java 8 Now, you have seen how to convert a Java 8 Stream to the array, you can use all the techniques you already know to convert an array to ArrayList to get an ArrayList, but what is the fun if you still have to do it in the old Java way? So, let's find out the new way to convert Java 8 Stream to ArrayList. If you recall our previous examples e.g. converting a Stream to List and converting Stream to Map, then you know that you can use the collect() method to accumulate stream elements in the choice of your collection. You can use the utility class Collectors and its conversion methods like toList() and toMap() to get the List and Map from a Stream in Java 8. This is fine, but in order to get ArrayList from Stream, you just need to know a little bit more. You need to know that the Collectors.toList() method can return any type of list, as it doesn't provide any guarantee on the type, thread-safety, or immutability of the returned List. Though, there is a similar method called toCollection() which accepts a supplier, which can be used to convert Stream to ArrayList. All you need to do is provide the ArrayList::new as supplier and toCollection() method will wrap all elements of a stream in an ArrayList and return its reference to you. You can read Java SE 8 for the Really Impatient bookto learn more about this behavior of Collector. Now, let's see a real-life example of how to convert Java 8 Stream to ArrayList in Java. ArrayList listOfBooks = library.stream() .filter(b -> p.getTopic() == BIOGRAPHY .toCollection(ArrayList::new); You can see that by using the Collectors or toCollection() method it's extremely easy to convert a Stream of values into an ArrayList of objects. Here is also a Java program to demonstrate various ways to convert a Stream to Array and ArrayList in Java 8: That's all about how to convert a Java 8 Stream to Array and ArrayList in Java. It's not that difficult, you just need to know the right methods from Stream API to do the conversion. Use the Stream.toArray(IntFunction) method when you want a typed array from Streams like an Integer array from a stream of Integer objects or a String array from a stream of Strings. Alternatively you can use toArray() method to get an Object[]. Similarly, In order to convert Stream to ArrayList, you can use toCollection() method by passing the ArrayList constructor reference. Don't use the toList() method because it doesn't specify which type of List it will return. You can further see the following resources to learn more about Stream and interoperability with the Java Collection framework. Related Java 8 Tutorials and Examples you may like 5 Free Courses to learn Java 8 and 9 (courses) 5 Books to Learn Java 8 Better? (read here) 7 Best Courses to learn Data Structure and Algorithms (best courses) 5 Best Courses to learn Java Collections and Streams (best courses) 10 Examples of converting a List to Map in Java 8 (see here) 5 lambda expression and Stream courses for beginners (online courses) Java 8 Comparator Example (check here) 10 Data Structure Courses for Coding interviews (online courses) Collection of best Java 8 tutorials (click here) Free Courses to learn Spring Framework (free courses) How to use debug Stream in Java 8 (tutorial) Free Courses to learn Spring Boot in-depth (free courses) Difference between abstract class and interface in Java 8? (answer) Difference between Stream.map() and Stream.flatMap() in Java 8? (answer) How to sort the may by values in Java 8? (example) How to format/parse the date with LocalDateTime in Java 8? (tutorial) Top 5 Course to master Java 8 Programming (courses) Thanks for reading this article so far. If you like this Java 8 Stream to Array conversion tutorial and example then please share it with your friends and colleagues. If you have any questions or feedback then please drop a note. P.S.: If you just want to learn more about new features in Java 8 then please see these Java 9 to Java 13 courses from Pluralsight. It explains all the important features of Java 8 like lambda expressions, streams, functional interfaces, Optional, new Date Time API, and other miscellaneous changes.

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Coding and testing are generally considered separate areas of expertise. In this practical book, Java expert Scott Oaks takes the approach that anyone who works with Java should be adept at understanding how code behaves in the Java Virtual Machine--including the tunings likely to help performance. This updated second edition helps you gain in-depth knowledge of Java application performance using both the JVM and the Java platform. Developers and performance engineers alike will learn a variety of features, tools, and processes for improving the way the Java 8 and 11 LTS releases perform. While the emphasis is on production-supported releases and features, this book also features previews of exciting new technologies such as ahead-of-time compilation and experimental garbage collections. Understand how various Java platforms and compilers affect performance Learn how Java garbage collection works Apply four principles to obtain best results from performance testing Use the JDK and other tools to learn how a Java application is performing Minimize the garbage collector's impact through tuning and programming practices Tackle performance issues in Java APIs Improve Java-driven database application performance

Ever since JDK 8 has allowed concrete (non-abstract) methods on the interface like default and static methods, many of my readers have asked me how should they answer the classical abstract class vs interface questions. Earlier, an interface cannot have any concrete methods and that was the main difference between abstract class and interface but now that is not the case. In this post, I'll revisit this hugely popular Java interview question in light of Java 8 changes. This is also a popular Java interview question and knowing the difference will help you to answer this question in a real interview. As I said, before JDK 8, the level of abstraction was the clear-cut difference between abstract class and interface like interface was the purest form of Abstraction which only defines what interface is supposed to do without specifying how they should be implemented, in other words, it only declares API methods and leaves implementation to its subclasses. But, the main difference between an abstract class and an interface in Java 8 is the fact that an abstract class is a class and an interface is an interface. A class can have a state which can be modified by non-abstract methods but an interface cannot have the state because they can't have instance variables. The second difference is that an interface cannot have a constructor even in Java 8 but you may remember that abstract class can also have a constructor in Java. All methods of an interface were abstract but since Java 8 you can define non-abstract methods in the form of default and static methods inside the interface in Java. This brings many questions to come to mind, which we'll discuss in the next section, btw, if you are new to the Java world and just started learning Java, I suggest you use a course that is up-to-date like The Complete Java MasterClass on Udemy because Java is changing very fast. In just a couple of years we have moved from Java 9 to Java 21. Thankfully this course is up-to-date and the author tries to update it with every Java release. Difference between Abstract class vs Interface in Java 8 Prima facia, in Java 8, an interface looks like an abstract class and one can reason about, can we use an interface with default methods in place of an abstract class in Java? Well, I believe they are for two different purposes and we will learn more once we start using Java 8 regularly, but following the semantics difference between abstract class and interface with default method will guide you further : 1) Abstract classes are classes, so they are not restricted to other restrictions of the interface in Java, like abstract class can have the state, but you cannot have the state on the interface in Java. 2) Another semantic difference between an interface with default methods and an abstract class is that you can define constructors inside an abstract class, but you cannot define constructors inside an interface in Java. In reality, default or defender methods are introduced to maintain backward compatibility and the same time making Collection API more suitable to be used inside key Java 8 features like lambda expressions. Without adding default methods, it wasn't possible to declare any new method on the existing interface in Java without breaking all classes which implement it, but because of the default method, you can now better evolve your API. They defend your code against implementing new methods hence they are also called defender methods. If you want to know more about default methods or new changes in Java 8 in general, I suggest you check out these Java 8 to Java 21 courses from sites like Udemy and Pluralsight. That's all about the difference between an Abstract class and an Interface in Java 8. Though I certainly agree that the difference between abstract class and the interface has reduced with the introduction of default methods, as well as allowing static methods inside the interface and their usage will evolve once Java 8 becomes a mainstream Java development version, but you must remember that an abstract class is a class and an interface is an interface. The key difference is that an abstract class can maintain a state but the interface cannot, and an abstract class can also have a constructor that is not allowed inside the interface even in Java 8. If you are designing API then use interface and if you are abstracting both state and behavior then use an abstract class. Other Java 8 Tutorials you may like If you are interested in learning more about the new features of Java 8, here are my earlier articles covering some of the important concepts of Java 8: 5 Books to Learn Java 8 from Scratch (books) How to join String in Java 8 (example) Top 5 Courses to learn Full Stack Java development (courses) How to use filter() method in Java 8 (tutorial) 10 Advanced Core Java courses for Programmers (courses) How to format/parse the date with LocalDateTime in Java 8? (tutorial) My favorite free courses to learn Java in-depth (courses) How to use Stream class in Java 8 (tutorial) Top 5 Courses to learn Lambdas and Stream in Java (courses) How to use forEach() method in Java 8 (example) How to convert List to Map in Java 8 (solution) 20 Examples of Date and Time in Java 8 (tutorial) How to use peek() method in Java 8 (example) How to sort the map by keys in Java 8? (example) How to sort the may by values in Java 8? (example) 10 examples of Options in Java 8? (example) How to convert lambda expression to method reference in Java 8 (tutorial) 5 Free Java 8 and Java 9 courses for Programmers (courses) Thanks for reading this article so far. If you like this article then please share it with your friends and colleagues. If you have any questions or feedback then please drop a comment. P. S. - If your goal is to learn new features introduced in Java 8 then you can also check out these Java Functional Programming and Stream API courses which only focuses on the most important new features (lambda expressions and stream) and nothing else. Now, one question for you? When do you use Abstract class over interface in Java? This question is easy to answer if you have read Effective Java but you can still try, its not that tough.

Publisher's Note: Products purchased from Third Party sellers are not guaranteed by the publisher for quality, authenticity, or access to any online entitlements included with the product. The Definitive Java Programming GuideIn Java: The Complete Reference, Eighth Edition, bestselling programming author Herb Schildt shows you everything you need to develop, compile, debug, and run Java programs. Updated for Java Platform, Standard Edition 7 (Java SE 7), this comprehensive volume covers the entire Java language, including its syntax, keywords, and fundamental programming principles. You'll also find information on key elements of the Java API library. JavaBeans, servlets, applets, and Swing are examined and real-world examples demonstrate Java in action. In addition, new Java SE 7 features such as try-with-resources, strings in switch, type inference with the diamond operator, NIO.2, and the Fork/Join Framework are discussed in detail.Coverage includes: Data types and operatorsControl statementsClasses and objectsConstructors and methodsMethod overloading and overridingInterfaces and packagesInheritanceException handlingGenericsAutoboxingEnumerationsAnnotationsThe try-with-resources statementVarargsMultithreadingThe I/O classesNetworkingThe Collections FrameworkApplets and servletsJavaBeansAWT and SwingThe Concurrent APIMuch, much more

Android ไม่ได้รองรับทุกความสามารถของ Java 8 แต่รองรับแค่บางส่วนเท่านั้น และเมื่อเราทำการพัฒนาเราต้องทำการตั้ง targetSdkVersion ไว้ที่ Android 7.0 (API Level 24) ขึ้นไป การที่เราจะใช้งานความสามารถของ…

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Hello folks, the map-reduce concept is one of the powerful concepts in computer programming, particularly on functional programming which utilizes the power of distributed and parallel processing to solve a big and heavy problem in a quick time. From Java 8 onwards, Java also got this powerful feature from the functional programming world. Many of the services provided on the internet like Google Search are based on the concept of the map and reduce. In map-reduce, a job is usually split from the input data-set into independent chunks which are processed by the map tasks in a completely parallel manner. The framework then sorts the outputs of the map operation, which are then supplied to the reduce tasks. For example, suppose you want to calculate the average age of all the people in a given town, instead of counting sequentially you can divide the problem into locality or zip code and then calculate the average age for each locality and then combine them using a reduction operation to calculate the average age for the town. It wasn't possible earlier to perform functional programming in Java earlier, at least, not so easy, but Java 8 also introduced a powerful Stream API that provides many functional programming operations like map, filter, flatmap, reduce, collect, and so on. There are many such scenarios where map-reduce can really be a game-changer and solve a problem in quick time which wasn't possible earlier due to the sheer scale of data. As I have said, earlier Java didn't have the support of performing bulk data operation and it wasn't possible to load all the data into memory because of their size but from Java 8 onwards, we have an abstraction called Stream which allows us to perform the bulk data operation on a large chunk of data without loading them into memory. By using these methods, together with implicit parallelism provided by Stream API, you can now write code in Java that can work efficiently with Big data which wasn't that easy earlier. If you want to learn more about Stream API features and how to use them efficiently in your code, I suggest you take a look at these best Java Stream and Lambda Courses. It will provide you with first-hand experience of using Stream API in your code. Map Reduce Example in Java 8 In this Java 8 tutorial, we will go over the map function in Java 8. It is used to implement MapReduce type operations. Essentially we map a set of values then we reduce it with a function such as average or sum into a single number. Let's take a look at this sample which will do an average of numbers in the old and new ways. Again, the old method is many lines of code which is designed to do this all very sequentially. This code doesn't take advantage of a multi-core processor, which is available in all modern servers. Sure, the example is simple, but imagine if you have millions of items you are processing and you have an 8 core machine or 32 core server. Most of those CPU cores would be completely wasted and idle during this long calculation and it will frustrate your clients who are waiting for the response. Now if we look at the simple one-line Java 8 way of doing it: double average = peoples.stream().mapToInt(p-> p.getAge()) .average().getAsDouble(); This uses the concept of parallelism, where it creates a parallel stream out of the array, which can be processed by multiple cores and then finally joined back into to map the results together. The map function will create a stream containing only the values with meet the given criteria, then the average function will reduce this map into a single value. Now all 8 of your cores are processing this calculation so it should run much faster. As some wise man has said, a picture is worth more than a thousand words, here is a picture that shows how the map-reduce concept works in practice. In this, the map function could be a cutting operation that applies to each vegetable and then their result is combined using Reduce to prepare a nice Sandwich you can enjoy. If you want to learn more about the map and reduce operation, or in general, functional programming features introduced in Java 8 then I suggest you see the Java Functional Programming using the Lambdas and Stream course by Ranga Rao Karnam on Udemy. An interactive, hands-on, and useful course to learn all the Java 8 features that matter. Java Program to demonstrate Map-reduce operation Here is our Java program which will teach you how to use the map and reduce it in Java 8. The reduced operation is also known as the fold in the functional programming world and is very helpful with the Collection class which holds a lot of items. You can perform a lot of bulk operations, calculating stats using the map and reduce in Java 8. As a Java developer you must know how to use map(), flatMap(), and filter() method, these three are key methods for doing functional programming in Java 8. If you want to learn more about functional programming in Java 8, I also suggest joining From Collections to Streams in Java 8 Using Lambda Expressions course on Pluralsight, one of the best courses to start with functional programming in Java. package test; import java.util.ArrayList; import java.util.List; /** * Java Program to demonstrate how to do map reduce in Java. Map, reduce also * known as fold is common operation while dealing with Collection in Java. * @author Javin Paul */ public class Test { public static void main(String args[]) { List peoples = new ArrayList<>(); peoples.add(new Employee(101, "Victor", 23)); peoples.add(new Employee(102, "Rick", 21)); peoples.add(new Employee(103, "Sam", 25)); peoples.add(new Employee(104, "John", 27)); peoples.add(new Employee(105, "Grover", 23)); peoples.add(new Employee(106, "Adam", 22)); peoples.add(new Employee(107, "Samy", 224)); peoples.add(new Employee(108, "Duke", 29)); double average = calculateAverage(peoples); System.out.println("Average age of employees are (classic way) : " + average); average = average(peoples); System.out.println("Average age of employees are (lambda way) : " + average); } /** * Java Method to calculate average from a list of object without using * lambdas, doing it on classical java way. * @param employees * @return average age of given list of Employee */ private static double calculateAverage(List extends Employee> employees){ int totalEmployee = employees.size(); double sum = 0; for(Employee e : employees){ sum += e.getAge(); } double average = sum/totalEmployee; return average; } /** * Java method which uses map reduce to calculate average of list of * employees in JDK 8. * @param peoples * @return average age of given list of Employees */ private static double average(List extends Employee> peoples){ return peoples.stream().mapToInt(p-> p.getAge()) .average() .getAsDouble(); } } class Employee{ private final int id; private final String name; private final int age; public Employee(int id, String name, int age){ this.id = id; this.name = name; this.age = age; } public int getId(){ return id; } public String getName(){ return name; } public int getAge(){ return age; } } Output: Average age of employees are (classic way) : 49.25 Average age of employees are (lambda way) : 49.25 You can see that the Java 8 way is much more succinct and readable than the iterative version of the pre-Java 8 code. Though the code doesn't tell you the impact when the amount of data increases, you can easily make the Java 8 version parallel by just replacing stream() with the parallelStream() but you need to do a lot of hard work to parallelize the iterative version of the code. That's all about how to do map-reduce in Java 8. This is rather a simple example of a powerful concept like map-reduce but the most important thing is that now you can use this concept in Java 8 with a built-in map() and reduce() method of Stream class. If you are interested in learning more about Stream API or Functional Programming in Java, here are some of the useful resources to learn further and strengthen your knowledge. Other Java 8 tutorials you may like 10 Example of Lambda Expression in Java 8 (see here) 5 Courses to learn Java 8 and Java 9 features (courses) 10 Example of forEach() method in Java 8 (example) Top 10 Courses to learn Java for Beginners (courses) 10 Example of Joining String in Java 8 (see here) Top 5 Courses to become a full-stack Java developer (courses) 10 Example of Stream API in Java 8 (see here) How to use the peek() method of Stream in Java 8? (example) 10 Free Courses to learn Spring Framework (courses) 10 Example of converting a List to Map in Java 8 (example) How to use Stream.flatMap in Java 8(example) How to use Stream.map() in Java 8 (example) 5 Books to Learn Java 8 and Functional Programming (list) 5 Free Courses to learn Java 8 and 9 (courses) 20 Example of LocalDate and LocalTime in Java 8 (see here) Difference between map() and flatMap() in Java 8? (answer) Thanks for reading this article so far. If you liked this Java Map and Reduce tutorial and example then please share it with your friends and colleagues. If you have any questions or feedback then please drop a note.

Hello guys, I have been writing about some important methods from Java SE 8 like map(), flatMap(), collect(), etc for quite some time, and today I'll share my experience with another useful method peek() from java.utill.stream.Stream class. The peek() method of the Stream class can be very useful to debug and understand streams in Java 8. You can use the peek() method to see the elements as they flow from one step to another like when you use the filter() method for filtering, you can actually see how filtering is working like lazy evaluation as well as which elements are filtered. The peek() method returns a stream consisting of the elements of this stream and performs the action requested by the client. The peek() method expects a Consumer functional interface to perform a non-interfering action on the elements of this stream, usually printing them using the forEach() method. Btw, the sole reason for using peek() is debugging the Stream pipeline, even the API itself says that peek() is only there for debugging, but it does help to understand the lazy evaluation technique Stream uses to improve performance. Lazy evaluation means nothing is evaluated in the Stream until a terminal method like forEach(), collect(), or reduce() is called and processing stops as soon as the result is obtained, which means not all the elements of Stream is processed always. It all depends upon what kind of result you want from Stream. For example, if you call the findFirst() method then as soon as it finds the first element fulling the criterion, processing stops. If you want to understand lazy evaluation in-depth and other Stream features then I highly recommend you check out these Java collections and Stream courses from Udemy and Pluralsight. Java 8 Stream peek() method Example In order to understand the peek() method better, let's see some code in action. How about using the filter and map methods in a chained pipeline? This is a very common code in Java 8 and will help you to learn how stream pipeline processing works in Java 8? What happens in each step? What is the output or data in the stream after each step etc? Consider the following example, which calls the peek() method after each step in a Stream pipeline involving filter() and map() methods: List result = Stream.of("EURO/INR", "USD/AUD", "USD/GBP", "USD/EURO") .filter(e -> e.length() > 7) .peek(e -> System.out.println("Filtered value: " + e)) .map(String::toLowerCase) .peek(e -> System.out.println("Mapped value: " + e)) .collect(Collectors.toList()); In this example, we have a Stream of String and then we are filtering all Strings whose length is greater than 7 and then we are converting them to lowercase using the map() function. Now, what do you think, how will this program execute? top to bottom or bottom to top? Many of you will think that after the first filter() execution you will get a Stream containing two elements "EURO/INR" and "USD/EURO" and peek() will print those two elements. Well, that's not the case, since Streams are executed lazily, nothing will happen until the collect() method will execute, which is the terminal method. This is proved by the following output from running the above code into Eclipse IDE or command prompt, it will print the following lines: Filtered value: EURO/INR Mapped value: euro/inr Filtered value: USD/EURO Mapped value: usd/euro The key point to note here is that values are filtered and mapped one by one, not together. It means the code is executed backward when the collect() method calls the Collectors.toList() to get the result in a List, it asks map() function which in turn asks the filter() method. Since filter() is lazy it returns the first element whose length is greater than 7 and sits back until map() asks again. You can see that peek() method clearly prints the value of the stream in the pipeline after each call to filter() method. You can further join From Collections to Streams in Java 8 Using the Lambda Expressions course on Pluralsight to learn more about different types of operation with Streamlike intermediate and terminal operation. How to use peek() method in Java 8 As I said, the Stream.peek() method is very useful for debugging and understating the stream-related code in Java. Here is a couple of more example of using peek() to understand how bulk data operations are processed by Stream utility. package test; import java.util.ArrayList; import java.util.List; import java.util.stream.Collectors; /** * Java 8 peek() method example */ public class Test { public static void main(String[] args) { List versions = new ArrayList<>(); versions.add("Lollipop"); versions.add("KitKat"); versions.add("Jelly Bean"); versions.add("Ice Cream Sandwidch"); versions.add("Honeycomb"); versions.add("Gingerbread"); // filtering all vaersion which are longer than 7 characters versions.stream() .filter(s -> s.length() > 7) .peek(e -> System.out.println("After the first filter: " + e)) .filter(s -> s.startsWith("H")) .peek(e -> System.out.println("After the second filter: " + e)) .collect(Collectors.toSet()); } } Output After the first filter: Lollipop After the first filter: Jelly Bean After the first filter: Ice Cream Sandwich After the first filter: Honeycomb After the second filter: Honeycomb After the first filter: Gingerbread By looking at this output, can you explain how the code would have been executed? Well, it seems that when to collect() ask the second filter() method, it further asks the first filter() and you can see that the first element Lollipop passed the first filter but couldn't pass the second one because it doesn't start with letter "H". So, the second filter() again asks the first() filter for an element, it returns Jelly Bean, Ice Cream Sandwich, and HoneyComb one by one. Since HoneyComb made past the second filter it is collected by Collector and again the same process happens but aborted after GingerBread because all elements in Stream are already processed. This clearly explains the lazy execution behavior of Stream as opposed to eager iterative implementation and the peek() method definitely helps you to understand this better, but if you want to learn Stream in-depth, I suggest you further check these Java Functional Programming and Stream API courses. Important points 1) The peek() method of Stream class is an intermediate method, hence you can call other stream methods after this. 2) It returns a new Stream, which is basically the stream it got. 3) It accepts an object of functional interface Consumer to perform non-interfering action e.g. printing values. 4) For parallel stream pipelines, the action may be called at whatever time and whatever thread the element is made available by the upstream operation. Btw, peek() is not the only way to figure out what goes inside a Stream pipeline, you can also use your IDEs to do the heavy work. For example, If you are using IntelliIDEA from JetBrains, you can also use their Java Stream Debugger Plugin to easily debug Java 8 code using map, filter, and collect in IDE itself, like shown in the following GIF diagram: That's all about how to use the peek() method in Java 8. You can use the peek() method for debugging. It allows you to see the elements as they flow past a certain point in the pipeline. By using this you can check whether your filter() method is working properly or not. You can see exactly which elements are got filtered by using peek() in Java 8. Related Java 8 Tutorials and Examples you may like 5 Free Courses to learn Java 8 and 9 (courses) 5 Books to Learn Java 8 Better? (read here) 10 Examples of converting a List to Map in Java 8 (see here) Java 8 Comparator Example (check here) 10 Advanced Core Java courses for Programmers (courses) Collection of best Java 8 tutorials (click here) Best Courses to learn Java Programming for Beginners (best courses) 10 Examples of Stream in Java 8 (example) 7 best Courses to learn Data structure and Algorithms (best courses) Difference between abstract class and interface in Java 8? (answer) Difference between Stream.map() and Stream.flatMap() in Java 8? (answer) How to sort the may by values in Java 8? (example) 10 Free Courses for Experienced Java Programmers (courses) How to format/parse the date with LocalDateTime in Java 8? (tutorial) Top 5 Course to master Java 8 Programming (courses) Thanks for reading this article so far. If you like this Java 8 filter method tutorial then please share it with your friends and colleagues. If you have any questions or feedback then please drop a note. P. S. - If you just want to learn more about new features in Java 8 then please see these Java 8 Online courses. It explains all the important features of Java 8 like lambda expressions, streams, functional interfaces, Optional, new Date Time API, and other miscellaneous changes.

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Article explains the concept of Function Descriptors in Java 8 with Examples.|Article explains the concept of Function Descriptors in Java 8 with Examples.

CHAPTER 1 The OCPJP 8 Exam: FAQ The acronym OCPJP 8 exam stands for Java SE 8 Programmer II exam (exam number 1Z0-809). In this first chapter, we address the … - Selection from Oracle Certified Professional Java SE 8 Programmer Exam 1Z0-809: A Comprehensive OCPJP 8 Certification Guide [Book]

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Java libraries play a very important role on a Java programmer’s daily work. Here is a list of 10 most useful libraries for java programmers

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