Four applications are presented in order of increasing complexity:A trivial date server and client, illustrating simple one-way communication. The server sends data to the client only.A capitalize server and client, illustrating two-way communication, and server-side threads to more efficiently handle multiple connections simultaneously.A two-player tic tac toe game, illustrating a server that needs to keep track of the state of a game, and inform each client of it, so they can each update their own displays.A multi-user chat application, in which a server must broadcast messages to all of its clients.These applications communicate insecurely.None of these applications even try to secure communication. All data is sent between hosts completely in the clear. The goal at this point is to illustrate the most basic applications and how they use transport-level services. In real life, use a secure sockets layer.A Trivial Sequential ServerThis is perhaps the simplest possible server. It listens on port 59090. When a client connects, the server sends the current datetime to the client. The connection socket is created in a try-with-resources block so it is automatically closed at the end of the block. Only after serving the datetime and closing the connection will the server go back to waiting for the next client.DateServer.javaimport java.io.IOException;import java.io.PrintWriter;import java.net.ServerSocket;import java.net.Socket;import java.util.Date;/** * A simple TCP server. When a client connects, it sends the client the current * datetime, then closes the connection. This is arguably the simplest server * you can write. Beware though that a client has to be completely served its * date before the server will be able to handle another client. */public class DateServer public static void main(String[] args) throws IOException try (var listener = new ServerSocket(59090)) System.out.println("The date server is running..."); while (true) try (var socket = listener.accept()) var out = new PrintWriter(socket.getOutputStream(), true); out.println(new Date().toString()); Discussion:
talking toy java
This next server receives lines of text from a client and sends back the lines uppercased. It efficiently handles multiple clients at once: When a client connects, the server spawns a thread, dedicated to just that client, to read, uppercase, and reply. The server can listen for and serve other clients at the same time, so we have true concurrency.CapitalizeServer.javaimport java.io.IOException;import java.io.PrintWriter;import java.net.ServerSocket;import java.net.Socket;import java.util.Scanner;import java.util.concurrent.Executors;/** * A server program which accepts requests from clients to capitalize strings. * When a client connects, a new thread is started to handle it. Receiving * client data, capitalizing it, and sending the response back is all done on * the thread, allowing much greater throughput because more clients can be * handled concurrently. */public class CapitalizeServer /** * Runs the server. When a client connects, the server spawns a new thread to do * the servicing and immediately returns to listening. The application limits * the number of threads via a thread pool (otherwise millions of clients could * cause the server to run out of resources by allocating too many threads). */ public static void main(String[] args) throws Exception try (var listener = new ServerSocket(59898)) System.out.println("The capitalization server is running..."); var pool = Executors.newFixedThreadPool(20); while (true) pool.execute(new Capitalizer(listener.accept())); private static class Capitalizer implements Runnable private Socket socket; Capitalizer(Socket socket) this.socket = socket; @Override public void run() System.out.println("Connected: " + socket); try var in = new Scanner(socket.getInputStream()); var out = new PrintWriter(socket.getOutputStream(), true); while (in.hasNextLine()) out.println(in.nextLine().toUpperCase()); catch (Exception e) System.out.println("Error:" + socket); finally try socket.close(); catch (IOException e) System.out.println("Closed: " + socket); Discussion:
Now for a pretty simple command line client:CapitalizeClient.javaimport java.io.IOException;import java.io.InputStreamReader;import java.io.PrintWriter;import java.net.Socket;import java.util.Scanner;public class CapitalizeClient public static void main(String[] args) throws Exception if (args.length != 1) System.err.println("Pass the server IP as the sole command line argument"); return; try (var socket = new Socket(args[0], 59898)) System.out.println("Enter lines of text then Ctrl+D or Ctrl+C to quit"); var scanner = new Scanner(System.in); var in = new Scanner(socket.getInputStream()); var out = new PrintWriter(socket.getOutputStream(), true); while (scanner.hasNextLine()) out.println(scanner.nextLine()); System.out.println(in.nextLine()); This client repeatedly reads lines from standard input, sends them to the server, and writes server responses. It can be used interactively:
Explanation : Suppose we have a bird that can makeSound(), and we have a plastic toy duck that can squeak(). Now suppose our client changes the requirement and he wants the toyDuck to makeSound than ? Simple solution is that we will just change the implementation class to the new adapter class and tell the client to pass the instance of the bird(which wants to squeak()) to that class. Before : ToyDuck toyDuck = new PlasticToyDuck(); After : ToyDuck toyDuck = new BirdAdapter(sparrow); You can see that by changing just one line the toyDuck can now do Chirp Chirp !! Object Adapter Vs Class Adapter The adapter pattern we have implemented above is called Object Adapter Pattern because the adapter holds an instance of adaptee. There is also another type called Class Adapter Pattern which use inheritance instead of composition but you require multiple inheritance to implement it. Class diagram of Class Adapter Pattern: Here instead of having an adaptee object inside adapter (composition) to make use of its functionality adapter inherits the adaptee. Since multiple inheritance is not supported by many languages including java and is associated with many problems we have not shown implementation using class adapter pattern. Advantages:
When we talk about primitives, it is easy to know which the minimum or maximum value is. But when we are talking about objects (of any kind), Java needs to know how to compare them to know which one is the maximum and the minimum. That's why the Stream interface needs a Comparator for max() and min():
Described as the best talking bird in the world they can mimic the human voice and are able to talk in the same tones and clarity of speech as the voice they are mimicking, even outdoing the African grey parrot.
About a year ago, I packed up my things and moved dimensions. I went from programming full-time in Java to working on the Flex team at Adobe, where I now program in ActionScript 3. I was looking forward to the change because I think that an occasional radical shakeup usually works out well, unless you're talking about babies or champagne bottles.
I will focus on the modern features of ActionScript, sticking to what ActionScript 3 provides. Many of these features may date back to earlier versions, but to keep things simple I will just assume that we are talking about the latest version, since that is the version that most Flash and Flex developers have access to today.
@Ajai C# can afford to add features anyhow because you cannot compare the high volume of java projects, libraries, frameworks out there with C#. Being relatively younger language to Java, it can afford add many more things but java is always concsious of protecting existing codebase which is still a good thing.
Inheritance in java is one of the core concepts of Object-Oriented Programming. Java Inheritance is used when we have is-a relationship between objects. Inheritance in Java is implemented using extends keyword.
Inheritance is widely used in java applications, for example extending the Exception class to create an application-specific Exception class that contains more information such as error codes. For example NullPointerException.
Now I'll take the same behavior and use mock objects. For this code I'm using the jMock library for definingmocks. jMock is a java mock object library. There are other mockobject libraries out there, but this one is an up to date librarywritten by the originators of the technique, so it makes a good one tostart with.
There are a number of mock object libraries out there. Onethat I come across a fair bit is EasyMock, both in its java and .NETversions. EasyMock also enable behavior verification, but hasa couple of differences in style with jMock which are worthdiscussing. Here are the familiar tests again:
The vocabulary for talking about this soon gets messy - allsorts of words are used: stub, mock, fake, dummy. For thisarticle I'm going to follow the vocabulary of Gerard Meszaros's book. It's not what everyone uses, but I think it's agood vocabulary and since it's my essay I get to pick whichwords to use.
Of these kinds of doubles, only mocks insist upon behaviorverification. The other doubles can, and usually do, use stateverification. Mocks actually do behave like other doubles duringthe exercise phase, as they need to make the SUT believe it'stalking with its real collaborators - but mocks differ inthe setup and the verification phases.
In public places, your child might face a wall or something boring. You can also have a small blanket, mat, or carpet square that you can use for time-outs in public. Your child should be quiet, and no one should be talking to your child or giving him attention. 2ff7e9595c
Comments