Graphics in Java

CS 21b

The paint() Method Method in a visual component that

specifies what that component looks like

Particularly important for applets, panels, and frames

Takes a Graphics object as a parameter signature: void paint(Graphics g) Graphics class contains drawing methods

Exampleimport java.awt.*;

import java.applet.Applet;

public class HelloAgain extends Applet {

public void paint(Graphics g) {

g.drawString(“Hello”,50,50);

}

}

The Graphics class Methods: drawing commands such as

drawOval(int x, int y, int width, int height) setColor(Color c) fillOval(int x, int y, int width, int height)

For a complete listing of available methods, javap java.awt.Graphics

About paint() Describes “current” picture Not cumulative

if variables are used as arguments to the drawing commands, and the variables are updated, the drawing changes

repaint() method called to explicitly update

drawing called automatically every few seconds

Animation Example paint() defined as follows:

public void paint(Graphics g) {

g.drawString(“Hello”,x, 50);

}

init() with code that updates x:x += 5;

repaint();

effect: String “moves” to the right

Listeners in Java

CS 21b

Listeners ActionListener

used for button and text field events refer to discussion on event models

for examples WindowListener

when windows are closed, minimized, etc

Listeners, continued

MouseListener when the mouse is clicked, pressed,

or released MouseMotionListener

when the mouse is moved or dragged

Listeners are Interfaces

A listener imposes on the implementing class that particular methods be defined

To find out what methods need to be implemented, use javap e.g., javap

java.awt.event.MouseListener

Associating Listenersto Visual Objects Use addxyzListener() methods

associate listener objects to visual components called on the visual object that needs

listening to takes a listener object as a parameter

Effect: a method on the listener object is invoked

when an event occurs on the visual object

WindowListener Methods void WindowActivated(WindowEvent) void WindowClosed(WindowEvent) void WindowClosing(WindowEvent) void WindowDeactivated(WindowEvent) void WindowDeiconified(WindowEvent) void WindowIconified(WindowEvent) void WindowOpened(WindowEvent)

WindowAdapter Class that implements

WindowListener and provides defaults definitions for the methods

Default definition: { } When creating a listener,

instantiate the adapter and then override the methods of interest (using anonymous classes)

Example:Closing a Frame

// in the constructor ...

addWindowListener(new WindowAdapter() {

public void windowClosing(WindowEvent e) {

System.exit(0);

}

// no need to define the other methods

});

MouseListener Methods

void MouseClicked(MouseEvent) void MouseEntered(MouseEvent) void MouseExited(MouseEvent) void MousePressed(MouseEvent) void MouseReleased(MouseEvent)

MouseMotionListener Methods

void MouseMoved(MouseEvent) void MouseDragged(MouseEvent)

Mouse Event

Represents the state of the mouse pointer when the event occurs

Most useful methods: getX() and getY() returns the coordinates of the mouse

Mouse Adapter Classes MouseAdapter

implements MouseListener {} definitions for all methods

MouseMotionAdapter implements MouseMotionListener {} definitions for all methods

Example:Dragging a Circle In paint(),

g.drawCircle(x, y, 10, 10); Define:

MousePressed(): compare mouse pointer coordinates with x and y, and check if circle was “selected”

MouseDragged(): update x and y so that the circle “moves” with the mouse

The Applet as Listener Make the applet implement

MouseListener and MouseMotionListener

In init(), addMouseListener(this); addMouseMotionListener(this);

define all seven mouse methods as methods of the applet

Third-Party Listeners and Adapters

In init(), create separate MouseListener and MouseMotionListener objects instantiate the corresponding

adapters and then, using anonymous classes, define only the methods you care to define

associate these listeners to the applet

Exceptions

CS 21b

Definition Exception: something unexpected

that can occur in the execution of a program e.g., divide by zero or attempt to

open a file that does not exist Java provides a way to handle

exceptions that are thrown: the try-catch statement

Try-catch Statement Syntax:

try { … } catch(Exception e) { ... } Example:

try {

System.out.println(5/x);

} catch(Exception e) {

System.out.println(“/ by zero”);

}

Breaking out of the try Blocktry { statement1; statement2; // if exception occurs here, // statement3 will be skipped statement3;} catch(Exception e) { statement4; // executed after exception

occurs}

Why Use Try-catch ? Alternative: if-statement

will be complex and hard to read if there are several exceptions

what if the exception occurs within a loop ? (will need to worry about breaking out of the loop)

Using try-catch is a more robust and structured way of handling exceptions

Exception Classes

Classes that extend the Exception class

Allows the programmer to be more specific about the exception: try { … } catch (ArithmeticException

e) { … } Useful in a try-catch chain

Try-catch Chaintry { …

}

catch(SomeException se) { …

}

catch(AnotherException ae) { …

}

catch(YetAnotherException yae) { …

}

…

Files

CS 21b

File Unit of secondary storage Stores a sequence of bytes/characters

Stream operations: read from stream, write to

stream Associated with a filename Often organized under a directory

hierarchy

Input/Output Classesin Java I/O viewed as a stream of bytes

parent classes: InputStream, OutputStream

As a stream of (Unicode) characters parent classes: Reader, Writer

Need to import java.io.*;

* An application employing files will use a subclass of one of the above classes

Text Files To create a text file, use PrintStream

f = new PrintStream(new FileOutputStream(“filename.txt”));

To write to the text file use print methods f.println(…); // use like System.out

Make sure to close the file before exiting the program f.close(); // ensures contents are updated

Text Files, continued To read from text files, use either

DataInputStream or BufferedReader f = new

DataInputStream( FileInputStream(“filename.txt”));

f = new BufferedReader(new FileReader(“filename.txt”));

Use read methods to read from file s = f.readLine(); // reads a string

Exceptions File operations throw exceptions so

make sure statements are enclosed in a try-catch statement

Exceptions thrown: IOException Common (specific) exception:

FileNotFoundException

Reading a File from the Web Use URL class from java.net To open ,

wpage = new URL(address); f = new BufferedReader(new

InputStreamReader(wpage.openStream())));

* address is a String specifying the webpage address (e.g., “http://www.admu.edu.ph”)

Binary Files Text files are often sufficient but

sometimes we want to store objects as they are (not their text forms) in a file

Use ObjectOutputStream and ObjectInputStream operations: writeObject() and readObject() common technique: store objects in a

Vector and then save the Vector in the file

java.io.* Summary There is a host of classes under this

package that serve a variety of purposes

Hints: use “javap java.io.classname” to find out

available constructors and methods you often need to use FileInputStream,

FileOutputStream, FileReader, and FileWriter to associate a name to the file

Threads

CS 21b

Threads in Java Definition

unit of program control that represents an execution sequence

Java supports multi-threaded execution

* Programs we have written up to this point have been on a single thread of control

The Thread class Thread creation

instantiate a class that extends Thread instantiate the class Thread but

provide a run() method Thread execution

let t refer to a thread object t.start() causes the thread to execute

its run() method “in the background”

Example 1: PrintThread PrintThread class extends Thread Attributes: name & timedelay Constructor sets name and

timedelay run method prints name twice but

with a time delay in between the print statements use Thread method sleep(timedelay)

run() Methodfor PrintThreadpublic class PrintThread extends Thread {...

public void run() {System.out.println(name);try { sleep(timedelay); }catch(Exception e) {}System.out.println(name);

}…}

Using PrintThreadPrintThread t1,t2,t3;t1 = new PrintThread("tic",5000);t2 = new PrintThread("tac",1000);t3 = new PrintThread("toe",3000);t1.start();t2.start();t3.start();

// expected output ? last output line should be tic

Example 2: MovingCircle MovingCircle class extends Applet Attributes: xpos & size (of circle)

provide initial values paint() method: draws the circle Thread created inside init()

run() method has a loop that continuously updates xpos & size and then repaints

A button executes the thread (t.start())

MovingCircle class (attributes & paint())// sample animation using Threadspublic class MovingCircle extends Applet {

int xpos = 5; // these variables will be updatedint size = 10; // on a different thread

...public void paint(Graphics g) {

g.drawOval(xpos,50,size,size);}

…}

MovingCircle class(thread code)t = new Thread() {

public void run() {while (xpos < 80) {try {sleep(1000);} catch(Exception e) {}xpos +=5;size += 3;repaint(); // forces paint() to be re-executed}

}}; // this statement placed inside the init() method

The Runnable Interface Runnable is an interface that contains

the run() method One of the constructors for Thread:

public Thread(Runnable r)

Can create a thread by giving it an argument (object) that implements run() alternative to extending thread and then

overriding run()

Networking

CS 21b

Client/Server Computing Communication over the network often

occurs between a client and a server A server listens for connection requests

and then responds to messages A client establishes a connection to a

server and then sends messages TCP/IP: abstract layer that simplifies

the above activities

Hosts, Ports, and Sockets Computers on the network are

(uniquely) specified by a host name or an IP address

Communication between hosts occurs through their ports each port allows several connections

Socket connection handle that facilitates

communication over the network

Networking in Java java.net package

import java.net.*; Most important classes

Socket ServerSocket URL (discussed earlier)

The Socket class Constructor requires a host and

port that the client intends to connect to

Useful Socket methods InputStream getInputStream() OutputStream getOutputStream()

Use file/stream interfaces to carry out the communication

The ServerSocket class Constructor requires a port

number that the server wishes to listen from

accept() method returns a Socket object once a connection from a client has been established blocks (hangs) until the connection

occurs

On the Server Program ... Create a ServerSocket object

specify port Invoke accept() on that object Obtain Socket object

returned by accept() Obtain I/O streams from the socket

carry out communication using these streams

On the Client Program ... Create Socket object

specify host and port of server Obtain I/O streams from the socket

carry out communication using these streams

* execute the server before the client

Allowing Multiple Connections to a Server Use Threads Have a loop that continuously calls

accept() main thread

Create and start a thread whenever accept() returns a socket facilitate communication on the socket

using a separate thread