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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.

Java ecosystem. However, Java Web Start to ship in September. JavaFX is will not be included in Java 11 or already open source, but by split- later releases. Venkat this book gets you started Updated for Java SE 9, this book Get complete coverage of all Adopt a high-performance programming in Java right away.

Electronic practice exam modernization strategy. Available in print and eBook formats. Its release at the end the language and are using it daily. Streams and the correct use. And that is precisely to apply that knowledge to write how long this edition has been what gained this book praise and better and clearer code.

For so what makes this suggestion clear, and the code examples are example, here are some of the best important? It is one of return type a functional-style operation. That its tight relationship with opera- ner stage. Listing 3. The remaining analytics are similar. The alternative would be the much less satisfactory hard-coding of values and limits. The analytics can be set and adjusted instead by someone who understands the mechanical properties of the belt motor.

To receive notiication that the monitoring application changed the belt or fan speed. Listing 5. The callback method needs to deine the parameters to match those deined in the IoTCS device model. Listing 8.

Under the covers. To receive device attribute updates. Of course. To enable them to do so. Listing 9.

You do this through the concept of a virtual device. The code in Listing 9 shows this as well as how to call actions and change an attribute value on a device. The callback method—not shown—simply switches on the attribute name to react to the value change accordingly.

The code in Listing 8 starts the process with a call to VirtualDevice. For cleaner code. To listen for IoTCS action requests such as the power action. 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.

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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. The type system and runtime work in concert to provide guaranteed immutability. The primary design goal from its start in was portability between heterogeneous runtime environments.

Once a Fantom module is built. Portability True portability requires both compiler support and library support. A single pass of the compiler is used to simultaneously generate bytecode for the Java VM and transpiled JavaScript source for browser environments.

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. An instance of Wrapped can be created only by code inside the class Wrapper.

Given this code: The argument list must be changed to final int[] vals.

The code would reliably return the correct sum of the numbers if the body of the method were changed to this: The code reliably returns the correct sum of the numbers as it is. Question 4. The program immediately prints submitted.

The code would reliably return the correct sum of the numbers if the type of total were changed to Integer. The code would reliably return the correct sum of the numbers if the call to. The program pauses ive seconds. The program throws a RejectedExecutionException because there are no available threads at line n2. Compilation fails due to an error at line n1. Compilation fails due to an error at line n3.

The code would reliably return the correct sum of the numbers if the type of total were changed to LongAdder and if the lambda in peek were changed accordingly. Given that the current working directory of the following program contains this tree: Question 5. The correct answers are options A and D. Immutable objects such as Integer and String are special. Option C builds on this question of inality. This is an important point about inal-ness for variables.

Java 8 changed the rules for those method-local variables. This one is interesting. Given that restriction. As mentioned.

It turns out that the compiler has had the ability since the beginning to determine if the rules of final are properly adhered to. Java 8 changed the requirements a little. This means that option B would be correct for Java 7. A method-local variable ceases to exist when the method returns. A final variable that is of reference type cannot be changed to point at a diferent object.

The irst decision is probably to determine whether the code compiles or whether the change outlined in option B is necessary. The preix can be explicit as it is here. Option C is not a valid construction of anything. Wrapped is a nested class. Assigning a new value to vals—that is. Option E is just made-up syntax. The bottom line for this question. The general approach is that there must be some kind of preix for the new operation.

While acknowledging that there are. All the other syntax oferings are incorrect for the purpose at hand. Such a method contradicts normal Java style conventions. Java uses the term nested class to describe classes deined inside other classes. This is why option D turns out to be the required syntax.

On the other hand. If myWrapper were a variable of type Wrapper. Option B would work if Wrapped were a nested class. That said. The code can be added. This is a question with less depth and complexity than many. That is. The terminology sometimes can be confusing. By the same token. What matters. The correct answer is option D. An ExecutionException arises if the job itself throws an exception. Option E is just wrong. The whole stream. The variable total is a method-local variable.

For both these reasons. A common recommendation is that stream processing. Integer objects are immutable. Option B looks tempting. If shared state will be modiied.

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The unchecked exception is Cancellation Exception. An InterruptedException results if the get method. In that situation. Such an approach is a great way to ensure that concurrency problems do not arise. This is why option A is incorrect.

The correct answer is option B. That is unusual in the actual certiication exam. Both would be functionally correct. This question investigates the concurrent mode of stream processing and the rules that ensure it runs correctly and in a thread-safe fashion. The correct answer is option E. This successfully sidesteps the problem of a final variable. It could have been set to parallel by the caller. Option D might seem tempting.

This question tests one speciic piece of understanding. Option C is unworkable. The checked exceptions are InterruptedException and Execution Exception. The code does not compile. Presumably the idea is that if the mutation operation.

The starting point is considered to be a directory. The irst two take 5 seconds to complete. The next two arguments are the depth of the search and a BiPredicate that ilters. The timing suggested by option E is testing to see if you know that the ExecutorService has a job queue. Why are they wrong? Option A suggests that compilation fails at line n1. Option C tries to get you to believe that if an Executor Service is created with a ixed thread count of two. After the pause.

The depth parameter. Two jobs start executing promptly. It takes a starting directory as its irst argument.

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The submit calls for all three jobs to complete without noticeable delay. The class java. Those who have spent time playing with an API are likely to use it more luently.

Option D. The find method along with a close cousin. The find method is clearly modeled on the UNIX find utility. The class also gained quite a few additional methods with Java 8. If it did not. You can submit more jobs.

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They know what is available. Nothing in the question tells you that the arguments are a Path and a BasicFileAttributes. The irst oicial meeting was held in January and was initiated with these words: He wrote several Java certiication guides and is currently a freelance educator who teaches at many large companies.

Until this year. Now you know that the correct answer is option B. This is called for every path element that is seen to determine whether to include that element in the output. This actually raises another observation that the question ignores. Members of the JUG pay a yearly fee. In such situations. The JUG hosts an annual Java developer conference. Javagruppen has for many years organized programming for kids and has joined the Devoxx4Kids initiative.

Stream objects implement AutoCloseable. JDK IO. From that.

Today the group consists of about 80 paying members. Oracle Corporation. Unless you speciically tell us that your correspondence is private.

While it will have no inluence on our decision whether to publish your article or letter. Redwood Shores. Comments and article proposals should be sent to our editor. We also are interested in proposals for articles on Java utilities either open source or those bundled with the JDK. CA Java Magazine Uploaded by Groulot. Flag for inappropriate content. Related titles. Jump to Page.I strongly believe that the abundance of Java style guides is itself a limitation.

Fetching latest commit…. One of the main problems you can face when you start a new IoT project with Java is that the versions for the upm and mraa libraries included in Intel System Studio IoT Edition must match the versions of these libraries installed in the version of Yocto Poky Linux that is running on the Intel Galileo Gen 2 board. The photoresistor is a variable resistor whose resistance value changes based on the ambient light intensity.

Mayo Norcross. Ramesh Gavva. Similar APIs are available for other collection types.

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