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Download as PDF, TXT or read online from Scribd. Flag for inappropriate . SEPTEMBER/OCTOBER magazine. By and for the Java community. Devices. Java Magazine is published bimonthly at no cost to qualified subscribers by. Oracle . In the March/April issue's Fix This, Stephen Chin. Java Magazine is a bi-monthly digital publication deep-dive into Java technology and the JVM. Every issue contains explanations about the language and the.
Listing 6. I use a Java 8 lambda expression. Listing 7. The cloud abstracts much of the tedious coniguration and glue to make this application functional.
The following output shows the monitoring application reacting to the various IoTCS analytics alerts. BeltSpeed was: The following listing shows some sample output from the conveyor belt device emulator as it receives commands from the monitoring application..
In reality. Conclusion As you can see. IoTCS analytics alert the monitoring application regarding when to turn the cooling fan on or of. Listening on http: It removes a level of monotony. Mon Jul 25 CoolingFanSpeed was: What should be noted from this output..
At this point. JAX-RS 2. The format of the events looks as follows: This is a message event: This message has an event name id: This message has an id which will be sent as 'last event ID' if the client reconnects This approach of asynchronous events over an established connection competes with the more powerful WebSockets standard.
This article. The server side of SSEs. The main advantages of SSEs. To follow along. These include support for server-sent events SSEs. The speciication and all the topics and examples this article covers are still subject to change. The messages represent the history of all created messages. The sseBroadcaster is initialized and enhanced with optional SseBroadcaster.
Listeners in the PostConstruct method. It contains the SseBroadcaster. SseContext is an injectable context object responsible for creating SSE broadcasters. Inject SseContext sseContext. There are two ways of creating a connection to an endpoint. This process is shown in the following code: SseEventSource ofers functionality to open a connection to an SSE endpoint by registering an event listener and.
After the output is registered to the broadcaster. I store the messages for later reuse in case any reconnecting client asks to continue the event stream from the last received message.
The client side of SSEs. The method registers the newly created output to the sseBroadcaster and then immediately resends the events if the corresponding HTTP header has been sent. SseEventSource handles all required plumbing. Consumers can call start and stop to resume and pause the stream. Because the eventInput. After successfully opening the connection.. This event input is used to receive the actual incoming events. If a more sophisticated way is needed. As an example.
Calling the endpoint via the target results in a 2-second wait time. To avoid that. To avoid this. Consumers of the StatefulSseClient class can call start and stop to resume and pause the stream. I can use constructs such as CompletableFutures together with Java 8 lambdas and streams.
Constructing invocations is enhanced with rx methods that wrap responses into a reactive invocation type and. I wrap the action in a Completable Future and extract the total value from the JSON data once the result is available. The executor is provided as a parameter so you can manage the threads for the 10 concurrent polls while most of the calling threads are waiting. The execution blocks when join is called. The stream of results is then aggregated into the average value.
Reactive Programming On the client side. Polling the services sequentially and then constructing the result would lead to long wait times while the threads are blocked.
By using NIO. I chain several actions together and CompletionStage. These callback handlers are called when data needs to be read or written without blocking. Calling the rx method conigures the builder to use an RxInvoker..
The handler method is expected to return a boolean value indicating whether there is more data to write. As in the previous example. Server-side NIO. Calling isFinished on the input stream reveals whether more data is available.
As on the server side. Similar to the reactive integration in the client that was shown earlier. The following examples write and read data from the client side in a non-blocking way. Client-side NIO. By using these approaches. Version Conclusion As of today.
He has more than six years of Java experience. Daschner evangelizes computer science practices on his blog. When not working with Java. Version 2. As of today. I try to provide beneit by picking topics that invite a deeper understanding of the conceptual background of a language construct.
In such a case. I can use it to separate speciication from implementation. Both the code and the type are inherited. I can create variables of that type. Inheritance is. These are all important.
Java interfaces are just such a topic. If Student is a subclass of Person. I will discuss why Java has two diferent inheritance mechanisms indicated by these keywords. I can use it for subtyping and dynamic dispatch. It is necessary to understand these diferences to get a good feel for inheritance and interfaces.
Objects of the subclass have all the properties of both the superclass and the subclass. It is useful to separate these two ideas conceptually. Java interfaces are closely related to inheritance. When I create a subclass using the extends keyword. As is so often the case. This leads to the introduction of default methods in Java 8. So far. A student is a person.
In Java. I assume that you have a basic understanding of inheritance. I can use inheritance to reuse some code I have written before.
I can use it to specify a contract between diferent parts of a system.
Type Inheritance Versus Code Inheritance Two main capabilities that inheritance provides are the ability to inherit code and the ability to inherit a type. It inherits this ield twice.
Languages that allow full. Type Inheritance to the Rescue When you think about these problems carefully. Let me give you an example: PhD students PhDStudent at my university also work as instructors. If the ield is. In short. This fact is coupled with another observation: The inheritance graph forms a diamond shape. Diamond Inheritance A more complicated scenario is known as diamond inheritance see Figure 2.
Conceptually this is straightforward. Multiple code inheritance is messy. In that sense. PhDStudent is a subclass of both Faculty and Student. What if both superclasses have ields with the same name?
An example of multiple inheritance Person Faculty Student The decision to link type inheritance and code inheritance in Java is a language design choice: For these cases. The answer is: I need language constructs that specify some solution to the problem of ambiguity and name overloading.
In practice. If the ield in question is. An example of diamond inheritance instructors for a class. I can model this as multiple inheritance see Figure 1. Multiple Inheritance The next idea entering the mix is multiple inheritance: But they are also students: You may then have a central collection in your program—say.
You can leave out the modiiers public static final for constants and public for methods —they are implicitly assumed. Suppose you are writing a traic simulation. Apart from cars. Java needs to be able to specify types without specifying code. But imagine now that there is also a list of all objects to be drawn on screen which is not the same as the list of actors.
Interfaces can contain constants. And I can now have diferent rules concerning multiple inheritance: Java permits multiple inheritance for types interfaces but only single inheritance for classes which contain code. No ields and no method bodies are speciied. That is what a Java interface does.
You might also want to save a simulation to permanent storage at some point. Interfaces specify a Java type the type name and the signatures of its methods without specifying any implementation. Your Car class can then implement this interface: Benefits of Multiple Inheritance for Types The beneits of allowing the inheritance of multiple types— essentially of being able to declare that one object can be viewed as having a diferent type at diferent times—is quite easy to see.
To be saved. This arrangement provides me with two types of inheritance in Java: I can inherit a class using extends. There is a way out. Abstract classes sit halfway between classes and interfaces: Situations like this are common. I can write Actor as an abstract class.
Arguing the diference between and justiication for the existence of interfaces and abstract classes.
They mark certain classes as possessing a speciic property. As I will show in a moment. Empty Interfaces Sometimes you come across interfaces that are empty— they deine only the interface name and no methods. If you are tempted. In my explanation of the issues. Java allows only single inheritance for classes that means only one class can be listed after the extends keyword. Multiple inheritance is for interfaces only.
Cloneable is another. In my example above.. You can think of them as partially implemented classes with some gaps in them code that is missing and needs to be illed in by subclasses.
A New Dawn with Java 8 So far. I have purposely ignored some new features that were introduced with Java If I want several of my interfaces to contain code. Abstract Classes I should say a few words about abstract classes.
There is little reason today to use marker interfaces in Java.. The act method itself might be abstract because it is diferent in each speciic actor and there is no reasonable default. As I stated before. These interfaces are known as marker interfaces. I did this on purpose. They are deined by using the default modiier at the beginning of the method signature.
Java 8 added lambdas and several other features to the Java language that made it desirable to adapt some of the existing interfaces in the Java library. Java 8 came up with a clever trick to get the best of both worlds: The Java language is.
Default Methods Default methods are methods in interfaces that have a method body—the default implementation. Default methods and static methods in interfaces are two mechanisms that made progress possible in Java 8. If the Java team were to design Java from scratch now. But you now come up with some really great new functionality. This is done using default methods and static methods.
Evolving Interfaces One problem in developing Java 8 was how to evolve interfaces. But how do you evolve an interface without breaking all the existing code that uses this interface? Imagine you have an interface MagicWand in your existing library: And in the real world.
There are still some diferences for instance. The new interface features in Java 8. As always. Both can contain abstract methods and methods with implementations. They are useful in many situations. Inheriting static methods and constants which are also static by deinition is not a problem. Java just tells you. At the beginning of this article I pointed out how the Java designers treaded very carefully to avoid multiple code inheritance because of possible problems.
He has published two Java textbooks and numerous papers on object orientation and computing education topics. So what is the situation now? As usual. Inheriting from diferent interfaces multiple default methods with the same signature and diferent implementations is a problem. These are deined by using the usual static modiier at the beginning of the method signature. Static Methods There is little reason today to use marker interfaces in Java.
In other words. Old code continues to work. But here Java chooses a much more pragmatic solution than some other languages: They are very often useful in your code. What About the Diamond Problem?
As you can see. Careful use of interfaces can signiicantly improve the quality of your code. Interfaces can now also contain static methods with implementations.
An actor model for concurrency leverages the type system to ensure that mutable state is never shared between threads. You will ind the goal for seamless portability woven into every aspect of the Fantom platform.
The Fantom compiler supports a pluggable architecture. Fantom was conceived to provide both compiler support and a standard library to port between three diferent runtime environments: If your language is portable but all the libraries you use are not.
The Fantom compiler and extensive standard library ensure that code written in Fantom works exactly the same on a Java server and in a browser client. These two runtimes have been used in production for commercial software for more than six years—making them very mature and robust.
Fantom also supports a novel approach to immutability and concurrency. With these APIs in place. Fantom has its own standard library. This article provides an in-depth look into several of these prominent features: These lowlevel APIs include core types Str.
Fantom is designed to be a single language for writing both server-side and browser-client-side code. On the client side. Although we prototyped a Fantom runtime for. Once we have immutable instances. Client code sends a message to an actor and is returned a Future. Deep immutability means that we not only guarantee that the pt reference is set only once. Interaction with actors is done via a message queue. Fields in Fantom may be marked const to guarantee that they are immutable: Contrast this with a Java final ield that provides only shallow immutability: Deep immutability is guaranteed by the type system using const classes: Similar APIs are available for other collection types.
Actors are lightweight objects designed to asynchronously process work on a thread pool. There is no synchronized or volatile mechanism in Fantom. Actors are guaranteed to receive their messages in order and to safely execute within one thread. Enforcing this restriction makes it much easier to reason about concurrency. Although immutable data structures are common in functional languages. Fantom achieves this goal with a built-in concurrency model based on actors.
Messages sent to actors are queued. Here is our actor class: The standard library provides many conveniences for using a mutable data structure to build up a collection and then freezing it as an immutable instance: Once the actor framework detects that an actor has pending messages. It provides a rich library to build highly polished HTML5 UIs with a design familiar to anyone who has experience with a traditional toolkit such as Swing.
Notice that receive takes and returns an Obj? Note the use of the as keyword. It provides DOM-backed widgets using a familiar widget design including Menus. What happens under the covers is that the actor queues the messages.
The third line sends a message to the actor. Fantom provides two levels of abstraction for working in HTML5. The receive method is overridden to process incoming messages. The constructor takes an ActorPool. The implementation of receive casts the object to an integer and returns the square.
Actors are required to be const classes immutable. Actors provide an elegant. I create an ActorPool instance. When the result becomes available.
The domkit module provides a much higher level of abstraction. The Fantom actor framework enforces that all messages are immutable using the const class type system. Int msg: The dom module provides a statically typed.
This syntax illustrates a nullable type. This approach ensures that data mutations are restricted to a single actor thread and avoids the need for synchronization. In Swing. It is built entirely in Fantom on top of the dom module.
The domkit Table automatically handles all the thorny issues: This only briely touches on the Fantom language. If you would like to learn more. It deines the number of rows and columns for the model and provides a callback for how to render the column headers and cells. Brian and his brother. Int c. Js class MyTableModel: Figure 1 shows what the table looks like in a browser.
I add some event handlers for action double click and selection changes that echo to stdout the selected row indexes. A sample table created with Fantom domkit just like Swing. Conclusion In this article. I examined four key features of Fantom: Andy Frank. Int r. Brian also serves as the technical lead for project-haystack.
An instance of Wrapped can be created using the expression new Wrapper. An instance of Wrapped can be created using the expression new Wrapper:: Question 1. Question 3. Choose two. Question 2. Given the following code: Choose one. You wish to calculate the sum of the numbers in a stream and also print out each one. The code compiles. Before the advent of web applications and the Meanwhile, apps that require an elabo- rate or complex UI have been steadily opting for browser-based presentation, in which UIs Java, mobile, and more.
Only a few applications have for event updates: go. Major languages all had specialized libraries UI needs or local processing requirements that that delivered the ability for rich UI experiences. C had the GTK fastly remain desktop applications. And of course Microsoft programming environments, productivity soft- had a variety of platform-speciic UI toolkits.
In addition, a decade has been matched by a surge in mobile- variety of scientiic software relies on desktop- oriented design.
Many UI metaphors today come style UIs. Whereas desktop Microsoft Windows 10 is a canonical example of applications used to be delivered as large execut- this trend.
Its easy to run, provably effective, and greatly underused. This field, which is the empirical, quantitative analysis of software-development techniques, should be the cornerstone of the trade. And yet for most development organizations, it remains a closed domain into which they never venture. Even the emerging emphasis on metrics and dashboards has not led to curiosity about what thousands of projects tell us about those very numbers. The disregard extends even to the parlance we use: In most locales, a software engineer is a seasoned programmer.
There is no implication of familiarity with software engineering. Part of this neglect is the view that practitioners are mostly academics. Their utility, however, shows up when a project with unusual requirements confronts an organization. Suppose after several years of leading various teams at your company and delivering projects roughly on time and of the desired quality, youre charged with a green-field project that requires a much lower level of defects than your organization is accustomed to.
How will you amp up the quality? This is where software engineering becomes a crucial resource: You can see which techniques deliver lower defect rates and what their impact on productivity has been historically. Informed with this data, youll be able to plan how to go about reaching the new goals for this project.
In 03 real life, of course, most organizations just call in consultants.