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

1. io package as Java’s basic I/O systemcontinue’d

2. 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); } }

3. //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]); }}}

4. HERE IS A SAMPLE RUN C:\JBUILDER8\JDK1.4\bin\javaw-classpath"C:\WINDOWS\jbproject\untitled40\classes;C:\...........JBUILDER8\JDK1.4\JRE\classes;C:\JBUILDER8\jdk1.4\lib\tools.jar" untitled40.Editor Enter lines of text. Enter ‘Stop’ to quit. 13 20 46 -43 -346789 stop Here is your file: 13 20 46 -43 -346789

5. 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.

6. 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'); }

7. 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 PrintWriterstream. • 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, booleanflushOnNewline)

8. PrintWriter(OutputStream outputStream, booleanflushOnNewline) • 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:

9. 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);

10. //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 -7 4.5E-7

11. 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.*;

13. 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.

14. 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.

15. 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.

16. 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

17. 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 handlerclass 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... }

19. Handling Action Events • Action events are fired by subclasses ofAbstractButton and includes buttons, checkboxes, and menus. AbstractButton button = new JButton("OK"); button.addActionListener(newMyActionListener()); public class MyActionListener implements ActionListener { public void actionPerformed(ActionEvent evt) { // Determine which abstract button fired the event. AbstractButton button = (AbstractButton)evt.getSource(); } }

21. 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); }

22. 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); } } } }