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) {
}
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());
}