Java 8 - Features Overview
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This document provides an agenda for a Java 8 training session that covers Lambdas and functional interfaces, the Stream API, default and static methods in interfaces, Optional, the new Date/Time API, and Nashorn JavaScript engine. It includes sections on Lambda expressions and method references syntax, functional interfaces, built-in functional interfaces, streams versus collections, using Optional to avoid null checks, extending interfaces with default methods, and key concepts of the new Date/Time and Nashorn JavaScript APIs.
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Java 8 - Features Overview
- 2. 2CONFIDENTIAL • Java 8 General Overview • Lambdas and Functional Interfaces • Stream API • Method References • Interface’s Default and Static Methods • Optional • Date/Time API • Nashorn, JS Engine Agenda
- 3. 3CONFIDENTIAL Java installing problems Please, pay attention when installing Java 8. Note! Tomcat 7.0.x does not work with Java 8: So make sure your JAVA_HOME is not pointing to Java 8 SDK directory or you will get bunch of errors. Change your JAVA_HOME or configure your project properly.
- 4. 4CONFIDENTIAL Overview JAVA 8 RELEASE (March 18, 2014) Optional (value wrapper)Lambdas & Method references METHODS INSIDE JAVA INTERFACES Default methods New Utilities Stream API Date Time API Nashorn, JS Engine
- 6. 6CONFIDENTIAL Lambdas (closures) syntax Each lambda expression has two parts separated by an arrow token: the left hand side is our method arguments, and the right hand side is what we do with those arguments (business logic)
- 7. 7CONFIDENTIAL Just compare And it’s not even about lines of code
- 8. 8CONFIDENTIAL Motivation • Shorter way of writing an implementation of a method for later execution. • Provides simple horizontal solution that solves the "vertical problem" presented by inner classes. • Lambda expressions are designed to allow code to be streamlined. • Lambdas expressions do not produce any extra classes the way nested classes do.
- 9. 9CONFIDENTIAL Method reference Oracle reference guide “You use lambda expressions to create anonymous methods. Sometimes, however, a lambda expression does nothing but call an existing method. In those cases, it's often clearer to refer to the existing method by name.” Kind Example Reference to a static method ContainingClass::staticMethodName Reference to an instance method of a particular object containingObject::instanceMethodName Reference to an instance method of an arbitrary object of a particular type ContainingType::methodName Reference to a constructor ClassName::new Kinds of methods referencesExample
- 10. 10CONFIDENTIAL Functional Interfaces Conceptually, a functional interface has exactly one abstract method (e.g. Runnable, Callable, Comparator).
- 11. 11CONFIDENTIAL Pre-Built function library (java.util.function) There are a lot of re-usable functional requirements that can be captured by functional interfaces and lambdas. The designers of Java 8 have captured the common use cases and created a library of functions … etc.
- 12. STREAMS API
- 13. 13CONFIDENTIAL Hello, Streams Streams let you process data in a declarative way. We do not say how to process the data, we just say what to do with the data. Let’s say we need to find all transactions of type ‘grocery’ and return a list of transaction IDs sorted in decreasing order of transaction value.
- 14. 14CONFIDENTIAL java.util.stream.Stream: feel the power. It takes 10 seconds to process 10 filesIt takes 1 second to process 10 files
- 15. 15CONFIDENTIAL Streams versus Collections Q. What is the difference? Q. So when should I use streams and why? Q. On demand? A. In a nutshell, collections are about data and streams are about computations. A. Use streams when things are computed. A collection is an in-memory data structure, which holds all the values that the data structure currently has—every element in the collection has to be computed before it can be added to the collection. In contrast, a stream is a conceptually fixed data structure in which elements are computed on demand. A. Yes, look: limit(2) uses short- circuiting; we need to process only part of the stream, not all of it.
- 16. 16CONFIDENTIAL So… Stream as an abstraction for expressing efficient, SQL-like operations on a collection of data. The Streams API will internally decompose your query to leverage the multiple cores on your computer.
- 17. OPTIONAL
- 18. 18CONFIDENTIAL Optional A. “… a container object which may or may not contain a non-null value...” Java 8 Reference Guide Q. Of course, the null reference is the source of many problems. But how does Optional solve them? Q. What is it? Q. Why do we need them? A. Because "Null sucks." Doug Le (a professor of computer science) A. Enough talking; let's see some code!
- 19. 19CONFIDENTIAL java.util.Optional usage Do Something If a Value Is Present Default Values and Actions Creating of Optional Old style New style Old style New style Rejecting Certain Values Using the filter Method Old style New style
- 21. 21CONFIDENTIAL What are default methods? Default methods enable you to add new functionality to the interfaces of your libraries and ensure backward compatibility with code written for older versions of those interfaces. The forEach isn’t declared by java.util.List nor the java.util.Collection interface yet. The solution would be to just add the new method to the existing interface and provide the implementation where required in the JDK. But, once published, it is impossible to add methods to an interface without breaking the existing implementation Syntax
- 22. 22CONFIDENTIAL Extending interfaces with default methods When you extend an interface that contains a default method, you can do the following: Not mention the default method at all, which lets inherit the default method. Redeclare the default method, which makes it abstract. Redefine the default method, which overrides it.
- 23. 23CONFIDENTIAL Tricks “What if the class implements two interfaces and both those interfaces define a default method with the same signature?”
- 24. 24CONFIDENTIAL Static methods Like static methods in classes, you specify that a method definition in an interface is a static method with the static keyword at the beginning of the method signature. All method declarations in an interface, including static methods, are implicitly public, so you can omit the public modifier. Applications. Examples The Comparator interface has been enhanced with this ability with the default method reversed:
- 25. DATE API
- 26. 26CONFIDENTIAL Date API. Core ideas. The new API is driven by three core ideas: Domain-driven design. The new API models its domain very precisely with classes that represent different use cases for Date and Time closely. This differs from previous Java libraries that were quite poor in that regard. Immutable-value classes. One of the serious weaknesses of the existing formatters in Java is that they aren’t thread-safe. This puts the burden on developers to use them in a thread-safe manner and to think about concurrency problems in their day-to-day development of date-handling code. The new API avoids this issue by ensuring that all its core classes are immutable and represent well-defined values. Separation of chronologies. The new API allows people to work with different calendaring systems in order to support the needs of users in some areas of the world, such as Japan or Thailand, that don’t necessarily follow ISO-8601. It does so without imposing additional burden on the majority of developers, who need to work only with the standard chronology
- 27. 27CONFIDENTIAL Date API. Creating. The first classes you will probably encounter when using the new API are LocalDate and LocalTime. Standard Java getter conventions are used in order to obtain values from Java SE 8 classes
- 28. 28CONFIDENTIAL Date API. Creating (2). There are also methods for calculations based on the different fields. The new API also has the concept of an adjuster— a block of code that can be used to wrap up common processing logic
- 29. 29CONFIDENTIAL Date API. Go ahead. The new API supports different precision time points by offering types to represent a date, a time, and date with time, but obviously there are notions of precision that are more fine- grained than this. Time Zones A time zone is a set of rules, corresponding to a region in which the standard time is the same
- 31. 31CONFIDENTIAL Nashorn. JS Engine. Nashorn's goal is to implement a lightweight high-performance JavaScript runtime in Java with a native JVM. MyScript.js
Editor's Notes
- Java 8 by Sergii Stets
- Agenda
- Before starting I want pay your attention to some problems you might have when installing new Java 8 SDK. If you use any Tomcat of version 7 on your local machine you will probably fail to run it on top of Java 8. Because Apache Tomcat 7 is not compatible with Java 8. So there are several options: you have to update your Tomcat. Or if you do not want any updates, make sure your JAVA_HOME is not pointing to Java 8 SDK directory.
- So the first Java 8 release was at the 18-th of March in 2014. Later on Oracle released different security updates, different patches and so on. The main features of Java 8 are: Lambda expression - Adds functional processing capability to JAVA. Optional - Emphasis on best practices, to handle null values properly. Default method - Interface to have default method implementation. New Tools - New compiler tools and utilities are added like jdeps to figure out the dependencies. Stream API - New stream API to facilitate pipeline processing. Date Time API - Improved date time api. Nashorn , JavaScript Engine - A JAVA based engine to execute JavaScript code.
- This is Duke
- Each lambda expression has two parts separated by an arrow token: the left hand side is our method arguments, and the right hand side is what we do with those arguments (business logic) Let’s take a look at the example below. 1 2 as you may have noticed we omitted types of the arguments and that OK. You will understand it later. 3 If your business logic is much more complex than just a line of code you may use curly braces. 4 no args
- And it’s not even about lines of code. Lambdas expressions do not produce any extra classes the way inner classes do – so you don’t overheat JVM with a lot of class files it should load into memory. Lambdas has some low-level implementation. It is not just a syntax sugar over nested classes.
- Useful info: When to Use Nested Classes, Local Classes, Anonymous Classes, and Lambda Expressions As mentioned in the section Nested Classes, nested classes enable you to logically group classes that are only used in one place, increase the use of encapsulation, and create more readable and maintainable code. Local classes, anonymous classes, and lambda expressions also impart these advantages; however, they are intended to be used for more specific situations: Local class: Use it if you need to create more than one instance of a class, access its constructor, or introduce a new, named type (because, for example, you need to invoke additional methods later). Anonymous class: Use it if you need to declare fields or additional methods. Lambda expression: Use it if you are encapsulating a single unit of behavior that you want to pass to other code. For example, you would use a lambda expression if you want a certain action performed on each element of a collection, when a process is completed, or when a process encounters an error. Use it if you need a simple instance of a functional interface and none of the preceding criteria apply (for example, you do not need a constructor, a named type, fields, or additional methods). Nested class: Use it if your requirements are similar to those of a local class, you want to make the type more widely available, and you don't require access to local variables or method parameters. Use a non-static nested class (or inner class) if you require access to an enclosing instance's non-public fields and methods. Use a static nested class if you don't require this access.
- You use lambda expressions to create anonymous methods. Sometimes a lambda expression does nothing but call an existing method. In those cases, it's often clearer to refer to the existing method by name. So Oracle has just created a so called syntax sugar. There different kinds of method references…
- Lambdas expressed single method classes. They can either be assigned to a variable or passed around to other methods just like we pass data as arguments. And here where functional interfaces come into. Conceptually, a functional interface has exactly one abstract method. Well know classes: (e.g. Runnable, Callable, Comparator). In Java 8 all the mentioned classes are marked with @FunctionalInterface annotation. It just a marker you may not use it. An informative annotation used to indicate that an interface is intended to be a functional interface as defined by the Java Language Specification. Conceptually, a functional interface has exactly one abstract method 1 We tell the compiler ‘I want to implement check method from ITrade so to someone else can pass me as arguments to another method’ 2 The second one is the same. 3 We do not change the signature of our method, we just pass different trend lambdas http://radar.oreilly.com/2014/08/java-8-functional-interfaces.html
- http://radar.oreilly.com/2014/08/java-8-functional-interfaces.html
- http://www.oracle.com/technetwork/articles/java/ma14-java-se-8-streams-2177646.html Streams let you process data in a declarative way. We do not say how to process the data, we just say what to do with the data. Example … By the way Stream operations that can be connected are called intermediate operations.
- Power of parallel streams
- … (in the end) This is because limit(2) uses short-circuiting; we need to process only part of the stream, not all of it, to return a result. List<String> phrases = Arrays.asList( "sporadic perjury", "confounded skimming", "incumbent jailer", "confounded jailer"); List<String> uniqueWords = phrases .stream() .flatMap(phrase -> Stream.of(phrase.split(" +"))) .distinct() .sorted() .collect(Collectors.toList()); System.out.println("Unique words: " + uniqueWords);... and the output: Unique words: [confounded, incumbent, jailer, perjury, skimming, sporadic]
- http://www.oracle.com/technetwork/articles/java/ma14-java-se-8-streams-2177646.html
- http://geekabyte.blogspot.co.il/2015/01/using-optional-effectively-in-java-8.html http://www.oracle.com/technetwork/articles/java/java8-optional-2175753.html You use orElse() in situation where you want to have an alternative value in case T is null. orElseGet() method is similar to orElse(), but instead of providing an alternative, should in case null is encountered, you provide code which can be used to generate the alternative value . orElseThrow() comes in handy when you want to decide which exception to throw in case a null value is encountered.
- Default methods
- http://zeroturnaround.com/rebellabs/java-8-explained-default-methods/ http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html
- Just read! http://zeroturnaround.com/rebellabs/java-8-explained-default-methods/ http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html
- The primary goal of default methods is to enable an evolution of standard JDK interfaces and provide a smooth experience when we finally start using lambdas in Java 8. Who knows, maybe in the future we will see some more applications of default methods for API design.
- This example demonstrates how the Comparator interface has been enhanced with default methods, static methods, lambda expressions, and method references to create more expressive library methods whose functionality programmers can quickly deduce by looking at how they are invoked. Use these constructs to enhance the interfaces in your libraries.
- http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html
- http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html
- http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html
- http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html
- Nashorn
- http://openjdk.java.net/projects/nashorn/ http://www.oracle.com/technetwork/articles/java/jf14-nashorn-2126515.html Nashorn's goal is to implement a lightweight high-performance JavaScript runtime in Java with a native JVM.
- If you have any questions, please do not hesitate to ask me. I will send you homework later.