Java Coding Question And Answers

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) { }

Java Coding Question And Answers

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) {
}

Java Coding Question And Answers

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);
socket.receive(packet); // Wait for 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) {
}

Java Coding Question And Answers

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) {
}

Java Coding Question And Answers

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) { }

Java Coding Question And Answers

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");
char ch = raf.readChar();// Read a character.
raf.seek(f.length());// Seek to end of file.
raf.writeChars("aString"); // Append to the end.
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 {
traverse(new File(f, children[i]));
}
}
}
}

Java Coding Question And Answers

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) {
}

Java Coding Question And Answers

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(“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());
}
}

Java Coding Question And Answers

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) {}

Java Coding Question And Answers

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 column 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 column 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) {}

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