Kousky kódu v Javě

…aneb může se hodit k testu v labu. (Vzhledem k Hnětynkovým podmínkám je třeba si je vytisknout.) Příklady jsou často převzaté z webu Sunu a lehce upravené.

I/O: Čtení slov nebo čísel pomocí scanneru

Zdroj:

http://java.sun.com/docs/books/tutorial/essential/io/scanning.html

Deprecated: Assigning the return value of new by reference is deprecated in /DISK2/WWW/lokiware.info/mff/formatters/highlight/java.php on line 5

import java.io.*;
import java.util.Scanner;
/* ... */
Scanner s = null;
try {
    s = new Scanner(new BufferedReader(
      new FileReader("slova_odd_bilymi_znaky.txt")
    ));

    while (s.hasNext())
        System.out.println(s.next());
} finally {
    if (s != null)
        s.close();
}

Deprecated: Assigning the return value of new by reference is deprecated in /DISK2/WWW/lokiware.info/mff/formatters/highlight/java.php on line 5

import java.io.*;
import java.util.*;
/* ... */
Scanner s   = null;
double  sum = 0;
try {
    s = new Scanner( new BufferedReader(new FileReader("cisla.txt")) );
    s.useLocale(Locale.US);

    while(s.hasNext() ){
        if( s.hasNextDouble() )
            sum += s.nextDouble();
        else
            s.next(); // ignoruje ne-cisla
    }
} finally {
    s.close();
}
System.out.println(sum);

Tip: Pro použití jiných oddělovačů zavolejte s.useDelimiter(regulárnívýraz);, např. s.useDelimiter(",\\s*");, kde \s značí libovolný white space.

Sockety: Jednoduchý klient

Standardní vstup je posílán serveru, a odpovědi serveru posílány na standardní výstup.

Zdroj: http://java.sun.com/docs/books/tutorial/networking/sockets/readingWriting.html

Deprecated: Assigning the return value of new by reference is deprecated in /DISK2/WWW/lokiware.info/mff/formatters/highlight/java.php on line 5

import java.io.*;
import java.net.*;

public class EchoClient {
  public static void main(String[] args) throws IOException {
    static final HOST = "localhost";
    static final PORTNO = 12345;
    Socket echoSocket = null;
    PrintWriter out = null;
    BufferedReader in = null;

    try {
      echoSocket = new Socket(HOST, PORTNO);
      out = new PrintWriter(echoSocket.getOutputStream(), true);
      in = new BufferedReader(new InputStreamReader(
                    echoSocket.getInputStream()  ));
    } catch (UnknownHostException e) {
      System.err.println("Don't know about host");
      System.exit(1);
    } catch (IOException e) {
      System.err.println("Couldn't get I/O for the connection.");
      System.exit(1);
    }

	BufferedReader stdIn = new BufferedReader(
                   new InputStreamReader(System.in));
	String userInput;

	while ((userInput = stdIn.readLine()) != null) {
	  out.println(userInput);
	  System.out.println("echo: " + in.readLine());
	}

	out.close();
	in.close();
	stdIn.close();
	echoSocket.close();
  }
}

Sockety: Jednoduchý server

Inspirováno

tímto příkladem. Opakuje vstup od klienta po řádcích. Umí obsloužit více spojení.

Deprecated: Assigning the return value of new by reference is deprecated in /DISK2/WWW/lokiware.info/mff/formatters/highlight/java.php on line 5

import java.io.*;
import java.net.*;

public class EchoServer extends Thread {
  private static final int PORT = 12345;
  private static ServerSocket serverSocket;
  private Socket        clientSocket;
  private BufferedReader      is;
  private PrintWriter         os;
  
  public EchoServer(Socket cs) {
    clientSocket = cs;
  }
  
  public static void main(String args[]) {
    try {
      serverSocket = new ServerSocket(PORT);
    } catch (IOException e) {
      System.err.println("Cannot create server socket.");
      System.exit(1);
    }    
    
    for(;;){
      try {
        Socket clientSocket = serverSocket.accept();
        new EchoServer(clientSocket).start();
      }catch (IOException e) {
        System.err.println("Cannot accept client connection.");
      }
    }
  }
  
  public void run() {
    try {
      os = new PrintWriter( new OutputStreamWriter(
          clientSocket.getOutputStream())
        ));
      is = new BufferedReader( new InputStreamReader(
          clientSocket.getInputStream()
        ));

      String input = null;
      while((input = is.readLine()) != null){
        os.println(input);
        os.flush();
      }
    }catch(IOException e){
      System.err.println("Error while handling a connection.");
    }finally{
      try {
        if (is != null)
          is.close();
        if (os != null)
          os.close();
        if (clientSocket != null)
          clientSocket.close();
      }catch (IOException e) {
        System.err.println("Error while closing a connection.");
      }
    }
      
  }
}

A ještě úplně jednoduchý server inspirovaný Hnětynkovým příkladem:

Deprecated: Assigning the return value of new by reference is deprecated in /DISK2/WWW/lokiware.info/mff/formatters/highlight/java.php on line 5

import java.net.*;
import java.io.*;

public class UplneBlbyServer {
  public static void main(String[] args) throws IOException {
    ServerSocket s = new ServerSocket(12345);
    try {
      Socket socket = s.accept();
      try {
        BufferedReader in = new BufferedReader(new InputStreamReader(
            socket.getInputStream()
          ));
        PrintWriter out = new PrintWriter(new OutputStreamWriter(
            socket.getOutputStream()
          ), true /* autoFlush */);
		String s = null;
        while(  (s = in.readLine()) != null )
          out.println(str);
      } finally {
        socket.close();
      }
    } finally {
      s.close();
    }
  }
}

URL: čtení a zápis

Zdroj:

http://java.sun.com/docs/books/tutorial/networking/urls/index.html

Nejjednodušší čtení URL:

Deprecated: Assigning the return value of new by reference is deprecated in /DISK2/WWW/lokiware.info/mff/formatters/highlight/java.php on line 5

import java.net.*;
import java.io.*;

public class URLReader {
    public static void main(String[] args) throws Exception {
	URL yahoo = new URL("http://www.yahoo.com/");
	BufferedReader in = new BufferedReader(
	  new InputStreamReader( yahoo.openStream()) );

	String inputLine;
	while ((inputLine = in.readLine()) != null)
	    System.out.println(inputLine);
	in.close();
    }
}

Složitější čtení pomocí URLConnection (můžeme zapisovat, číst hlavičky):

Deprecated: Assigning the return value of new by reference is deprecated in /DISK2/WWW/lokiware.info/mff/formatters/highlight/java.php on line 5

import java.net.*;
import java.io.*;

public class URLConnectionReader {
    public static void main(String[] args) throws Exception {
        URL yahoo = new URL("http://www.yahoo.com/");
        URLConnection yc = yahoo.openConnection();    // (!) URLConnection
        /* tady můžeme udělat třeba tohle:
         * con.connect();
         * System.out.println("Content-Type: "+con.getContentType());
         * System.out.println("Content-Length: "+con.getContentLength());
         */
        BufferedReader in = new BufferedReader(
                                new InputStreamReader(
                                yc.getInputStream()));
        String inputLine;
        while ((inputLine = in.readLine()) != null) 
            System.out.println(inputLine);
        in.close();
    }
}

I/O: Jednoduchá serializace

Zdroj:

http://java.sun.com/developer/technicalArticles/Programming/serialization/

Serializovatelná třída musí implementovat interface java.io.Serializable, který ovšem neobsahuje žádné metody. V nejjednodušším případě to může vypadat takhle:

Deprecated: Assigning the return value of new by reference is deprecated in /DISK2/WWW/lokiware.info/mff/formatters/highlight/java.php on line 5

import java.io.*;
/* ... */

/* Deklarace serializovatelné třídy, např.: */
public class PersistentTime implements Serializable{ /* ... */ }

/* Zápis: */
PersistentTime     time = new PersistentTime();
FileOutputStream   fos  = null;
ObjectOutputStream out  = null;
try{
  fos = new FileOutputStream("time.ser");
  out = new ObjectOutputStream(fos);
  out.writeObject(time);
  out.close();
}catch(IOException ex){
  ex.printStackTrace();
}

/* Čtení: */
PersistentTime    time = null;
FileInputStream   fis  = null;
ObjectInputStream in   = null;

try{
  fis  = new FileInputStream("time.ser");
  in   = new ObjectInputStream(fis);
  time = (PersistentTime)in.readObject();
  in.close();
}catch(IOException ex){
  ex.printStackTrace();
}catch(ClassNotFoundException ex){
  ex.printStackTrace();
}

Swing: Hello World

Zdroj:

http://java.sun.com/docs/books/tutorial/uiswing/start/compile.html

Deprecated: Assigning the return value of new by reference is deprecated in /DISK2/WWW/lokiware.info/mff/formatters/highlight/java.php on line 5

import javax.swing.*;        

public class HelloWorldSwing {
    /**
     * Create the GUI and show it. For thread safety, this method
     * should be invoked from the event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("HelloWorldSwing");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Add the ubiquitous "Hello World" label.
        JLabel label = new JLabel("Hello World");
        frame.getContentPane().add(label);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

Pozn.: Součástí výše uvedených příkladů z webu Sunu je toto upozornění.

Matfiz: Java/KouskyKódu

Kousky kódu v Javě

I/O: Čtení slov nebo čísel pomocí scanneru

Sockety: Jednoduchý klient

Sockety: Jednoduchý server

URL: čtení a zápis

I/O: Jednoduchá serializace

Swing: Hello World