io package as Java’s basic I/O system continue’d

import java.io.*; public class Read { public static void main ( String args[]) throws IOException { BufferedReader br= new BufferedReader (new InputStreamReader(System.in)); String str; System.out.println("Enter your name"); str = br.readLine(); System.out.println("Your name is " + str); }

//An editor example. import java.io.*; class Editor { public static void main (String args[]) throws IOException { //create a BufferedReader using System. in BufferedReader br= new BufferedReader (new InputStreamReader(System.in)); String str[] = new String [100]; System.out.println (“Enter lines of text.”); System.out.println (“Enter ‘Stop’ to quit.”); for (int i= 0 ; i<100 ;i++) { str[i] = br.readLine(); if (str[i].equals (“stop”)) break; } System.out.println(“\nHere is your file:”); //display the lines for (int=0; i<0 ;i++) { if (str[i].equals (“stop”)) break; System.out.println(str[i]); } } }

HERE IS A SAMPLE RUN C:\JBUILDER8\JDK1.4\bin\javaw- classpath"C:\WINDOWS\jbproject\untitled40\classes;C:\ JBUILDER 8\JDK1.4\JRE\classes;C:\JBUILDER8\jdk1.4\lib\tools.jar" untitled40.Editor Enter lines of text. Enter ‘Stop’ to quit stop Here is your file:

Writing Console Output  Console output is most easily achieved with print() and println()  These methods are defined by the class PrintStream  which is the type of the object referenced by System.out  Even though System. out is a byte stream, using it for simple program output is acceptable. Shortly;  PrintSteam is an output stream derived from OutputStream,  And it implements the low-level method write()  write() can be used to write to the console.  The simples form of write() defined by PrintStream is: void write (int byteval) throws IOException  write method writes to the file the byte specified by byteval.  byteval is declared as an integer, only the low order eight bits are written.

Demonstrating the System.out.write() method package untitled41; import java.io.*; //This is a short example that uses write() to output character “1” followed by a new line to the screen class WriteDemo { public static void main (String args[]) { int a; a='1'; System.out.write(a); System.out.write('\n'); }

The PrintWriter Class  Using System.out to write to the console is possible under Java. But:  Its use is recommended mostly for debugging purposes or for sample programs such as we studied in this course  For real world programs, the recommended method of writing to the console is through a PrintWriter stream.  PrintWriter is one of the character-based classes.  Using this class for console output makes easier to internationalize the program PrintWriter defines several constructors. The one is: PrintWriter(OutputStream outputStream, boolean flushOnNewline)

PrintWriter(OutputStream outputStream, boolean flushOnNewline)  outputStream is an object type OutputStream  flushOnNewLine controls whether Java flushes the output stream every time a newline (‘\’) character is output.  If flushOnNewline is true, flushing automatically takes place.  If false, flushing is not automatic  PrintWriter supports the print() and println() methods for all types including Object. Finally:

 We can use these methods in the same way as they have been used with System.out.  If an argument is not a simple type, the PrintWriter methods call the object’s toString() method and then print the result  To write to the console by using a PrintWriter, specify System.out for the output stream and flush the stream after each newline. For example:  The fallowing line of code creates a PrintWriter that is connected to console output: PrintWriter pw= new PrintWriter(System.out, true);

//Demonstration of the running of PrintWriter method import java.io.*; public class PrintWriterDemo { Public static void main (String args [] ) { PrintWriter pw= new PrintWriter(System.out, true); pw.println (“This is a string”); int i=-7; pw.println(i); double d=4.5e-7; pw.println(d); } } Since no there is no advantage to use PrintWriter in the simple programs., We can use System.out to write simple text output to the console. The output is: untitled42.PrintWriterDemo This is a string E-7

Using URLs in Applets Network Access  To access a remote URL we need to establish a network connection.  Java offers several ways of doing this URL connections Sockets In any case we need to use the java.net package in addition to applet, GUI-elements and event handling. import java.applet.* import java.awt.*; import java.awt.event.*; import java.net.*;

Event Handling  Events are supported by java.awt.event package  Event model defines standard and consistent mechanism to generate and process events  A source generates an event and sends it to one or more listeners.  In this scheme, the listener simply waits until it receives an event  Once received, the listener processes the event and then returns  Every time the user types a character or pushes a mouse button, an event occurs.  Any object can be notified of the event.  All it has to do is implement the appropriate interface and be registered as an event listener on the appropriate event source.

 Events can be generated as a consequence of a person interacting with elements in a graphical user interface.  In addition to pressing a button, other activities to be generated are:  entering a character via the keyboard,  selecting an item in a list,  clicking the mouse.  Events may also occur that are not directly caused by interactions with a user interface  For example, an event may be generated when a timer expires, a counter exceeds a value, a software or hardware failure occurs, an operation is completed.

Event Listeners  A listener is an object that is notified when an event occurs  It has two major requirements:  It must have been registered with one or more sources to receive notifications about specific types of events  It must implement methods to receive and process these notifications.

Event Classes  The classes that represent events are core of Java’s event handling mechanism.  The root of java event class hierarchy is EventObject, which is in java.util.  EventObject is the superclass for all events.  Its one constructor is: EventObject (Object src) src is the object that generates this event.  EventObject contains two methods: getSource(): returns the source of the event toString(): returns the string equivalent of the event. The class AWTEvent defined within the java.awt package is: a superclass of all AWT events that are handled by the delegation event model. a subclass of EventObject

How to Implement an Event Handler Every event handler requires three bits of code:  In the declaration for the event handler class, code that specifies that the class either implements a listener interface or extends a class that implements a listener interface. public class MyClass implements ActionListener {  Code that registers an instance of the event handler class as a listener upon one or more components aComponent.addActionListener(instanceOfMyclass)  Code that implements the methods in the listener interface. For example: public void actionPerformed(ActionEvent e) {...//code that reacts to the action... }

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(); }

import java.applet.*; import java.awt.*; import java.awt.event.*; import java.net.*; public class URLApplet extends Applet implements ActionListener { private TextField URLText = null ; private Button URLButton = null; public URLApplet() { } public void init() { add(new Label ("URL")); URLText = new TextField (30); add (URLText); URLButton = new Button ("Go"); add (URLButton); URLButton.addActionListener(this); }

public void actionPerformed (ActionEvent e) { String actionCommand = e.getActionCommand(); if (e.getSource() instanceof Button) if (actionCommand.equals ("Go")) { System.out.println ("Go"); try { AppletContext context = getAppletContext(); URL url = new URL(URLText.getText()); context.showDocument(url); } catch(Exception ex) { showStatus ("Error "+ex); }