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Database - JDBC (java.sql)

Connecting to a Database

This example uses the JDBC-ODBC bridge to connect to a database called ''mydatabase''.

try {

  Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

   

  String url = "jdbc:odbc:mydatabase";

  Connection con = DriverManager.getConnection(

    url, "login", "password");

} catch (ClassNotFoundException e) {

} catch (SQLException e) {

}

Creating a Table

This example creates a table called ''mytable'' with three columns: COL_A which holds strings, COL_B which holds integers, and COL_C which holds floating point numbers.

try {

  Statement stmt = con.createStatement();

   

  stmt.executeUpdate("CREATE TABLE mytable (

    COL_A VARCHAR(100), COL_B INTEGER, COL_C FLOAT)");

} catch (SQLException e) {

}

Entering a New Row into a Table

This example enters a row containing a string, an integer, and a floating point number into the table called ''mytable''.

try {

  Statement stmt = connection.createStatement();

  stmt.executeUpdate("INSERT INTO mytable

    VALUES ('Patrick Chan', 123, 1.23)");

  connection.close();

} catch (SQLException e) {

}

Getting All Rows from a Table

This example retrieves all the rows from a table called ''mytable''. A row in ''mytable'' consists of a string, integer, and floating point number.

try {

  Statement stmt = connection.createStatement();

   

  // Get data using colunm names.

  ResultSet rs = stmt.executeQuery(

    "SELECT * FROM mytable");

  while (rs.next()) {

    String s = rs.getString("COL_A");

    int i = rs.getInt("COL_B");

    float f = rs.getFloat("COL_C");

    process(s, i, f);

  }

   

  // Get data using colunm numbers.

  rs = stmt.executeQuery(

    "SELECT * FROM mytable");

  while (rs.next()) {

    String s = rs.getString(1);

    int i = rs.getInt(2);

    float f = rs.getFloat(3);

    process(s, i, f);

  }

} catch (SQLException e) {

}

Getting Particular Rows from a Table

This example retrieves all rows from a table called ''mytable'' whose column COL_A equals ``Patrick Chan''. A row in ''mytable'' consists of a string, integer, and floating point number.

try {

  Statement stmt = connection.createStatement();

  ResultSet rs = stmt.executeQuery(

    "SELECT * FROM mytable WHERE COL_A = 'Patrick Chan'");

  rs.next();

  String s = rs.getString("COL_A");

  int i = rs.getInt("COL_B");

  float f = rs.getFloat("COL_C");

  process(s, i, f);

} catch (SQLException e) {

}

Updating a Row of Data in a Table

This example updates a row in a table called ``mytable''. In particular, for all rows whose column COL_B equals 123, column COL_A is set to ''John Doe''.

try {

  Statement stmt = connection.createStatement();

  int numUpdated = stmt.executeUpdate(

    "UPDATE mytable SET COL_A = 'John Doe'

    WHERE COL_B = 123");

  connection.close();

} catch (SQLException e) {

}

Using a Prepared Statement

A prepared statement should be used in cases where a particular SQL statement is used frequently. The prepared statement is more expensive to set up but executes faster than a statement. This example demonstrates a prepared statement for getting all rows from a table called ''mytable'' whose column COL_A equals ''Patrick Chan''. This example also demonstrates a prepared statement for updating data in the table. In particular, for all rows whose column COL_B equals 123, column COL_A is set to ''John Doe''.

try {

  // Retrieving rows from the database.

  PreparedStatement stmt = connection.prepareStatement(

    "SELECT * FROM mytable WHERE COL_A = ?");

  int colunm = 1;

  stmt.setString(colunm, "Patrick Chan");

  ResultSet rs = stmt.executeQuery();

   

  // Updating the database.

  stmt = connection.prepareStatement(

    "UPDATE mytable SET COL_A = ? WHERE COL_B = ?");

  colunm = 1;

  stmt.setString(colunm, "John Doe");

  colunm = 2;

  stmt.setInt(colunm, 123);

  int numUpdated = stmt.executeUpdate();

} catch (SQLException e) {

}

 

 

Examplets TM provided by permission of the publisher, Addision-Wesley, and Author Patrick Chan.

 

Handling Events with an Anonymous Class

If an event handler is specific to a component (that is, not shared by other components), there is no need to declare a class to handle the event. The event handler can be implemented using an anonymous inner class. This example demonstrates an anonymous inner class to handle key events for a component.

 

component.addKeyListener(new KeyAdapter() {

  public void keyPressed(KeyEvent evt) {

  }

});

 

Handling Action Events

Action events are fired by subclasses of AbstractButton and includes buttons, checkboxes, and menus.

 

AbstractButton button = new JButton(quot;OK");

button.addActionListener(new MyActionListener());

   

public class MyActionListener

  implements ActionListener {

    public void actionPerformed(ActionEvent evt) {

      // Determine which abstract

      // button fired the event.

      AbstractButton button =

        (AbstractButton)evt.getSource();

   }

}

Handling Key Presses

You can get the key that was pressed either as a key character (which is a Unicode character) or as a key code (a special value representing a particular key on the keyboard).

 

component.addKeyListener(new MyKeyListener());

   

public class MyKeyListener extends KeyAdapter {

  public void keyPressed(KeyEvent evt) {

    // Check for key characters.

    if (evt.getKeyChar() == 'a') {

      process(evt.getKeyChar());

    }

   

    // Check for key codes.

    if (evt.getKeyCode() == KeyEvent.VK_HOME) {

      process(evt.getKeyCode());

    }

  }

}

Handling Mouse Clicks

component.addMouseListener(

  new MyMouseListener());

   

public class MyMouseListener

  extends MouseAdapter {

    public void mouseClicked(MouseEvent evt) {

      if ((evt.getModifiers() &

        InputEvent.BUTTON1_MASK) != 0) {

          processLeft(evt.getPoint());

      }

      if ((evt.getModifiers() &

        InputEvent.BUTTON2_MASK) != 0) {

          processMiddle(evt.getPoint());

      }

      if ((evt.getModifiers() &

        InputEvent.BUTTON3_MASK) != 0) {

          processRight(evt.getPoint());

      }

    }

}

Handling Mouse Motion

component.addMouseMotionListener(

  new MyMouseMotionListener());

   

    public class MyMouseMotionListener

      extends MouseMotionAdapter {

        public void mouseMoved(MouseEvent evt) {

          // Process current position of cursor

          // while all mouse buttons are up.

          process(evt.getPoint());

        }

        public void mouseDragged(MouseEvent evt) {

           // Process current position of cursor

           // while mouse button is pressed.

           process(evt.getPoint());

        }

    }

Detecting Double and Triple Clicks

component.addMouseListener(

  new MyMouseListener());

   

public class MyMouseListener extends MouseAdapter {

  public void mouseClicked(MouseEvent evt) {

    if (evt.getClickCount() == 3) {

      // triple-click

    } else if (evt.getClickCount() == 2) {

      // double-click

    }

  }

}

Handling Focus Changes

component.addFocusListener(

  new MyFocusListener());

   

public class MyFocusListener

  extends FocusAdapter {

    public void focusGained(FocusEvent evt) {

       // The component gained the focus.

    }

    public void focusLost(FocusEvent evt) {

       // The component lost the focus.

    }

}

Files, Streams, I/O (java.io)

Constructing a Path

On Windows, this example creates the path \blash a\blash b. On Unix, the path would be /a/b.

 

String path = File.separator +

  "a" + File.separator + "b";

 

Reading Text from Standard Input

try {

  BufferedReader in = new BufferedReader(

    new InputStreamReader(System.in));

  String str = "";

  while (str != null) {

    System.out.print("> prompt ");

    str = in.readLine();

    process(str);

  }

} catch (IOException e) {

}

 

Reading Text from a File

try {

  BufferedReader in = new BufferedReader(

    new FileReader("infilename"));

  String str;

  while ((str = in.readLine()) != null) {

    process(str);

  }

  in.close();

} catch (IOException e) {

}

 

Writing to a File

If the file does not already exist, it is automatically created.

try {

  BufferedWriter out = new BufferedWriter(

    new FileWriter("outfilename"));

  out.write("aString");

  out.close();

} catch (IOException e) {

}

 

Creating a Directory

(new File("directoryName")).mkdir();

 

Appending to a File

try {

  BufferedWriter out = new BufferedWriter(

    new FileWriter("filename", true));

  out.write("aString");

  out.close();

} catch (IOException e) {

}

Deleting a File

(new File("filename")).delete();

 

Deleting a Directory

(new File("directoryName")).delete();

 

Creating a Temporary File

try {

  // Create temp file.

  File temp = File.createTempFile(

    "pattern", ".suffix");

   

  // Delete temp file when program exits.

  temp.deleteOnExit();

   

  // Write to temp file

  BufferedWriter out = new BufferedWriter(

    new FileWriter(temp));

  out.write("aString");

  out.close();

  } catch (IOException e) {

}

 

Using a Random Access File

try {

  File f = new File("filename");

  RandomAccessFile raf =

    new RandomAccessFile(f, "rw");

   

  // Read a character.

  char ch = raf.readChar();

   

  // Seek to end of file.

  raf.seek(f.length());

   

  // Append to the end.

  raf.writeChars("aString");

  raf.close();

} catch (IOException e) {

}

Serializing an Object

The object to be serialized must implement java.io.Serializable.

try {

  ObjectOutput out = new ObjectOutputStream(

    new FileOutputStream("filename.ser"));

  out.writeObject(object);

  out.close();

} catch (IOException e) {

}

 

Deserializing an Object

This example deserializes a java.awt.Button object.

try {

  ObjectInputStream in = new ObjectInputStream(

    new FileInputStream("filename.ser"));

  AnObject object = (AnObject) in.readObject();

  in.close();

} catch (ClassNotFoundException e) {

} catch (IOException e) {

}

 

Traversing a Directory

public static void traverse(File f) {

  process(f);

   

  if (f.isDirectory()) {

    String[] children = f.list();

    for (int i=0; i<children.length; i++) {

      traverse(new File(f, children[i]));

    }

  }

}

 

Reading UTF-8 Encoded Data

try {

  BufferedReader in = new BufferedReader(

    new InputStreamReader(new FileInputStream(

    "infilename"), "UTF8"));

  String str = in.readLine();

} catch (UnsupportedEncodingException e) {

} catch (IOException e) {

}

 

Writing UTF-8 Encoded Data

try {

  Writer out = new BufferedWriter(

    new OutputStreamWriter(new FileOutputStream(

    "outfilename"), "UTF8"));

  out.write(aString);

  out.close();

} catch (UnsupportedEncodingException e) {

} catch (IOException e) {

}

 

Reading ISO Latin-1 Encoded Data

try {

  BufferedReader in = new BufferedReader(

    new InputStreamReader(new FileInputStream(

    "infilename"), "8859_1"));

  String str = in.readLine();

} catch (UnsupportedEncodingException e) {

} catch (IOException e) {

}

 

Writing ISO Latin-1 Encoded Data

try {

  Writer out = new BufferedWriter(

    new OutputStreamWriter(new FileOutputStream(

    "outfilename"), "8859_1"));

  out.write(aString);

  out.close();

} catch (UnsupportedEncodingException e) {

} catch (IOException e) {

}

Networking (java.net)

Creating a URL

try {

 // With components.

    URL url = new URL("http","hostname", 80, "index.html");

   

// With a single string.

   url = new URL(

   "http://hostname:80/index.html");

    } catch (MalformedURLException e) {

    }

 

Parsing a URL

try {

  URL url = new URL("http://hostname:80/index.html#_top_");

  String protocol = url.getProtocol();    // http

  String host = url.getHost();  // hostname

   int port = url.getPort();    // 80

   String file = url.getFile(); // index.html

   String ref = url.getRef();    // _top_

    } catch (MalformedURLException e) {

    }

 

Reading Text from a URL

try {

  URL url = new URL("http://hostname:80/index.html");

  BufferedReader in = new BufferedReader(

        new InputStreamReader(url.openStream()));

   

  String str;

   while ((str = in.readLine()) != null) {

  process(str);

     }

    in.close();

    } catch (MalformedURLException e) {

    } catch (IOException e) {

    }

 

Resolving a Hostname

 

Creating a Client Socket

try {

  InetAddress addr = InetAddress.getByName("java.sun.com");

  int port = 80;

  Socket sock = new Socket(addr, port);

   } catch (IOException e) {

   }

 

Creating a Server Socket

try {

  int port = 2000;

   ServerSocket srv = new ServerSocket(port);

 

  // Wait for connection from client.

   Socket socket = srv.accept();

   } catch (IOException e) {

   }

 

Reading Text from a Socket

 try {

  BufferedReader rd = new BufferedReader(

  new InputStreamReader(socket.getInputStream()));

   

  String str;

  while ((str = rd.readLine()) != null) {

  process(str);

 

   }

        rd.close();

    } catch (IOException e) {

    }

 

Writing Text to a Socket

try {

 BufferedWriter wr = new BufferedWriter(

   new OutputStreamWriter(socket.getOutputStream()));

      wr.write("aString");

        wr.flush();

    } catch (IOException e) {

    }

 

Sending a Datagram

public static void send(InetAddress dst,

            int port, byte[] outbuf, int len) {

try {

  DatagramPacket request = new DatagramPacket(

                      outbuf, len, dst, port);

    DatagramSocket socket = new DatagramSocket();

      socket.send(request);

     } catch (SocketException e) {

     } catch (IOException e) {

     }

    }

Receiving a Datagram

 try {

  byte[] inbuf = new byte[256]; // default size

  DatagramSocket socket = new DatagramSocket();

  

  // Wait for packet

  DatagramPacket packet = new DatagramPacket(

                          inbuf, inbuf.length);

   socket.receive(packet);

   

  // Data is now in inbuf

   int numBytesReceived = packet.getLength();

   } catch (SocketException e) {

   } catch (IOException e) {

   }

 

Joining a Multicast Group

public void join(String groupName, int port) {

 try {

  MulticastSocket msocket = new MulticastSocket(port);

  group = InetAddress.getByName(groupName);

   msocket.joinGroup(group);

   } catch (IOException e) {

     }

    }

 

Receiving from a Multicast Group

public void read(MulticastSocket msocket,

                     byte[] inbuf) {

try {

 DatagramPacket packet = new DatagramPacket(

                      inbuf, inbuf.length);

   

 // Wait for packet

 msocket.receive(packet);

 

// Data is now in inbuf

int numBytesReceived = packet.getLength();

   } catch (IOException e) {

     }

    }

 

Sending to a Multicast Group

 byte[] outbuf = new byte[1024];

 int port = 1234;

  try {

  DatagramSocket socket = new DatagramSocket();

  InetAddress groupAddr = InetAddress.getByName(

  "228.1.2.3");

   DatagramPacket packet = new DatagramPacket(

           outbuf, outbuf.length, groupAddr, port);

   socket.send(packet);

   } catch (SocketException e) {

   } catch (IOException e) {

   }

Defining and Exporting a Remote Object

1. Define the remote interface.

    import java.rmi.*;

   

 public interface RObject extends Remote {

  void aMethod() throws RemoteException;

    }

 

Looking Up a Remote Object and Invoking a Method

 try {

 // Look up remote object

 RObject robj = (RObject) Naming.lookup(

 "//localhost/RObjectServer");

   

 // Invoke method on remote object

  robj.aMethod();

 } catch (MalformedURLException e) {

 } catch (UnknownHostException e) {

 } catch (NotBoundException e) {

 } catch (RemoteException e) {

 }

 

Passing Parameters to a Remote Method

Arguments to remote methods must be primitive, serializable, or Remote. This example demonstrates the declaration and use of all three parameter types.

 

1. Define the remote interface.

 import java.rmi.*;

   

 public interface RObject extends Remote {

  // This parameter is primitive.

  void primitiveArg(int num) throws RemoteException;

   

 // This parameter implements Serializable.

    void byValueArg(Integer num) throws RemoteException;

   

 // This parameter implements Remote.

    void byRefArg(ArgObject arg) throws RemoteException;

    }

   

    public interface ArgObject extends Remote {

        int aMethod() throws RemoteException;

    }

   

2. Define the remote object implementation.

 import java.rmi.*;

 import java.rmi.server.UnicastRemoteObject;

   

  public class RObjectImpl extends UnicastRemoteObject implements RObject {

   public RObjectImpl() throws RemoteException {

         super();

    }

  public void primitiveArg(int num) throws RemoteException {

    }

  public void byValueArg(Integer num) throws RemoteException {

    }

   public void byRefArg(ArgObject arg) throws RemoteException {

    }

    }

 

3. Compile the remote object implementation.

    > javac RObject.java RObjectImpl.java

 

4. Generate the skeletons and stubs.

    > rmic RObjectImpl

 

5. Create an instance of RObjectImpl and bind it to the RMI Registry.

    try {

        RObject robj = new RObjectImpl();

        Naming.rebind("//localhost/RObjectServer", robj);

    } catch (MalformedURLException e) {

    } catch (UnknownHostException e) {

    } catch (RemoteException e) {

    }

 

6. Look Up the Remote object and pass the parameters.

    try {

        // Look up the remote object

        RObject robj = (RObject) Naming.lookup("//localhost/RObjectServer");

   

        // Pass a primitive value as argument

        robj.primitiveArg(1998);

   

        // Pass a serializable object as argument

        robj.byValueArg(new Integer(9));

   

        // Pass a Remote object as argument

        robj.byRefArg(new ArgObjectImpl());

    } catch (MalformedURLException e) {

    } catch (UnknownHostException e) {

    } catch (NotBoundException e) {

    } catch (RemoteException e) {

    }

Returning Values from a Remote Method

Return values from remote methods must be primitive, serializable, or Remote. This example demonstrates the declaration and use of all three return types. 1. Define the remote interface.

 

import java.rmi.*;

 public interface RObject extends Remote {

 // This return value is primitive.

  int primitiveRet() throws RemoteException;

 

 // This return value implements Serializable.

   Integer byValueRet() throws RemoteException;

   

 // This return value implements Remote.

     ArgObject byRefRet() throws RemoteException;

    }

   

    public interface ArgObject extends Remote {

        int aMethod() throws RemoteException;

    }

 

2. Define the remote object implementation.

import java.rmi.*;

import java.rmi.server.UnicastRemoteObject;

   

public class RObjectImpl extends UnicastRemoteObject

                              implements RObject {

  public RObjectImpl() throws RemoteException {

            super();

     }

 public int primitiveRet() throws RemoteException {

      return 3000;

    }

  public Integer byValueRet() throws RemoteException {

   return new Integer(2000);

   }

   public ArgObject byRefRet() throws RemoteException {

    return new ArgObjectImpl();

   }

  }

 

3. Compile the remote object implementation.

    > javac RObject.java RObjectImpl.java

   

4. Generate the skeletons and stubs.

  > rmic RObjectImpl

 

5. Create an instance of RObjectImpl and bind it to the RMI Registry.

 try {

RObject robj = new RObjectImpl();

   Naming.rebind("//localhost/RObjectServer", robj);

  } catch (MalformedURLException e) {

 } catch (UnknownHostException e) {

  } catch (RemoteException e) {

 }

6. Look Up the Remote object, invoke the methods, and receive the return values.

 try {

 // Look up the remote object

 RObject robj = (RObject) Naming.lookup(

                         "//localhost/RObjectServer");

   

 // Receive the primitive value as return value

 int r1 = robj.primitiveRet();

   

 // Receive the serializable object as return value

 Integer r2 = robj.byValueRet();

   

  // Receive the Remote Object as return value

   ArgObject aobj =  robj.byRefRet();

  } catch (MalformedURLException e) {

  } catch (UnknownHostException e) {

    } catch (NotBoundException e) {

   } catch (RemoteException e) {

    }

 

Throwing an Exception from a Remote Method

 

1. Define the remote interface.

 import java.rmi.*;

   

 public interface RObject extends Remote {

   void aMethod() throws RemoteException;

    }

 

2. Define the remote object implementation.

import java.rmi.*;

import java.rmi.server.UnicastRemoteObject;

   

public class RObjectImpl extends

 UnicastRemoteObject implements RObject {

 public RObjectImpl() throws RemoteException {

    super();

 }

public void aMethod() throws RemoteException {

// The actual exception must be wrapped in

// a RemoteException

   throw new RemoteException(

 "message", new FileNotFoundException("message"));

     }

    }

 

3. Compile the remote object implementation.

> javac RObject.java RObjectImpl.java

 

4. Generate the skeletons and stubs.

> rmic RObjectImpl

5. Create an instance of RObjectImpl and bind it to the RMI Registry.

 try {

 RObject robj = new RObjectImpl();

  Naming.rebind("//localhost/RObjectServer", robj);

    } catch (MalformedURLException e) {

    } catch (UnknownHostException e) {

    } catch (RemoteException e) {

    }

 

6. Look up the Remote object, invoke the method, and catch the exception.

 try {

 // Look up the remote object.

 RObject robj = (RObject) Naming.lookup(

 "//localhost/RObjectServer");

   

 // Invoke the method.

  robj.aMethod();

    } catch (MalformedURLException e) {

    } catch (UnknownHostException e) {

    } catch (NotBoundException e) {

    } catch (RemoteException e) {

 // Get the actual exception that was thrown.

    Throwable realException = e.detail;

    }

 

Strings (java.lang)

Constructing a String

If you are constructing a string with several appends, it may be more efficient to construct it using a StringBuffer and then convert it to an immutable String object.

StringBuffer buf = new StringBuffer("Initial Text");

   

// Modify

int index = 1;

buf.insert(index, "abc");

buf.append("def");

          

 // Convert to string

 String s = buf.toString();

Getting a Substring from a String

int start = 1;

int end = 4;

 String substr = "aString".substring(start, end);   // Str

 

Searching a String

 String string = "aString";

   

 // First occurrence.

 int index = string.indexOf('S');  // 1

 

 // Last occurrence.

 index = string.lastIndexOf('i');  // 4

   

 // Not found.

 index = string.lastIndexOf('z');  // -1

 

Replacing Characters in a String

// Replace all occurrences of 'a' with 'o'

String newString = string.replace('a', 'o');

 

Replacing Substrings in a String

 static String replace(String str,

         String pattern, String replace) {

   int s = 0;

   int e = 0;

 StringBuffer result = new StringBuffer();

   

  while ((e = str.indexOf(pattern, s)) >= 0) {

    result.append(str.substring(s, e));

    result.append(replace);

    s = e+pattern.length();

      }

    result.append(str.substring(s));

    return result.toString();

    }

Converting a String to Upper or Lower Case

 

 // Convert to upper case

 String upper = string.toUpperCase(); 

   

 // Convert to lower case

 String lower = string.toLowerCase(); 

 

Converting a String to a Number

int i = Integer.parseInt("123");

long l = Long.parseLong("123");

float f = Float.parseFloat("123.4");

double d = Double.parseDouble("123.4e10");

 

Converting Unicode to UTF-8

try {

 String string = "\u5639\u563b";

 byte[] utf8 = string.getBytes("UTF8");

 } catch (UnsupportedEncodingException e) {

    }

 

Converting UTF-8 to Unicode

 public static String toUnicode(byte[] utf8buf) {

  try {

    return new String(utf8buf, "UTF8");

    } catch (UnsupportedEncodingException e) {

    }

    return null;

    }       

 

Determining a Character's Unicode Block

char ch = '\u5639';

Character.UnicodeBlock block =

            Character.UnicodeBlock.of(ch);

 

Breaking a String into Words

String aString = "word1 word2 word3";

StringTokenizer parser =

          new StringTokenizer(aString);

while (parser.hasMoreTokens()) {

        processWord(parser.nextToken());

    }

 

 


Author & Copyrights 2004: Narayanaswami Banukumar