UMBC CMSC 331 Java The JAVA API. UMBC CMSC 331 Java 2 A Tour of the Java API An API User’s Guide, in HTML, is bundled with Java Much of the “learning

UMBC CMSC 331 Java

The JAVA The JAVA APIAPI

2UMBC CMSC 331 Java

A Tour of the Java API

• An API User’s Guide, in HTML, is bundled with Java

• Much of the “learning curve” is in the API• Let’s look at some packages

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The Java API

• java.applet– Applet class

• java.awt– Windows, buttons, mouse,

etc.

• java.awt.image– image processing

• java.awt.peer– GUI toolkit

• java.io– System.out.print

• java.lang– length method for arrays;

exceptions

• java.net– sockets

• java.util– System.getProperty

See http://java.sun.com/j2se/1.3/docs/api/for the current APIs

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The package java.lang

• The class Object– The root class in Java

– Example methods: clone(), equals(), toString()

– Subclasses may override these methods

• The class Class– Example methods: getName(), getSuperClass()

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Observing an object’s class

void printClassName (Object obj) { System.out.println("The class of " + obj + " is " + obj.getClass().getName());}

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Strings in Java• Strings are nt a primitive data type, but represented

as objects.• Many methods defined in class java.lang:

– Several constructors– Lots of methods: concat(), equals(), indexOf(), length()– strings are concatenated with +

• Strings are constants (immutable) – You can concatenate two strings to produce a new,

longer string, using the + operator, but you cannot add, delete, insert into, or delete from any particular string.

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StringBuffers in Java

• Several methods defined in class java.lang:– Constructors can specify length or initial value– append(), insertf(), length(), toString()


Java.lang.system

• Printstreams– System.out.err(“Error message”);

• Inputstreams– System.in.read(inputCharacter)

– System.in.read(inputBuffer, 0, maxChars)


The Cloneable Interface

• A class implements the cloneable interface by overriding the Object method clone()

• For example, we could add a clone() method to the FIFO class, if we wanted to be able to make copies of queues.


The class java.util

• Interface to host OS• Some basic functions and data structures

– BitSet, Dictionary (the superclass of Hashtable), Stack, Vector

– Random number generation


System Properties

• System properties are like UNIX environment variables, but platform independent

• The API class java.util has methods for accessing the system properties

// determine environment variablesimport java.util.*;class envSnoop { public static void main ( String args[] ) { Properties p; String s; p = System.getProperties(); p.list(System.out); s = System.getProperty("user.name"); System.out.println("user.name="+s); s = System.getProperty("user.home"); System.out.println("user.home="+s); }}

C:\UMBC\331\java>java envSnoop

-- listing properties --

java.specification.name=Java Platform API Specification

awt.toolkit=sun.awt.windows.WToolkit

java.version=1.2

java.awt.graphicsenv=sun.awt.Win32GraphicsEnvironment

user.timezone=America/New_York

java.specification.version=1.2

java.vm.vendor=Sun Microsystems Inc.

user.home=C:\WINDOWS

java.vm.specification.version=1.0

os.arch=x86

java.awt.fonts=

java.vendor.url=http://java.sun.com/

user.region=US

file.encoding.pkg=sun.io

java.home=C:\JDK1.2\JRE

java.class.path=C:\Program Files\PhotoDeluxe 2.0\Adob...

line.separator=

java.ext.dirs=C:\JDK1.2\JRE\lib\ext

java.io.tmpdir=C:\WINDOWS\TEMP\

os.name=Windows 95

java.vendor=Sun Microsystems Inc.

java.awt.printerjob=sun.awt.windows.WPrinterJob

java.library.path=C:\JDK1.2\BIN;.;C:\WINDOWS\SYSTEM;C:\...

java.vm.specification.vendor=Sun Microsystems Inc.

sun.io.unicode.encoding=UnicodeLittle

file.encoding=Cp1252

java.specification.vendor=Sun Microsystems Inc.

user.language=en

user.name=nicholas

java.vendor.url.bug=http://java.sun.com/cgi-bin/bugreport...

java.vm.name=Classic VM

java.class.version=46.0

java.vm.specification.name=Java Virtual Machine Specification

sun.boot.library.path=C:\JDK1.2\JRE\bin

os.version=4.10

java.vm.version=1.2

java.vm.info=build JDK-1.2-V, native threads, symcjit

java.compiler=symcjit

path.separator=;

file.separator=\

user.dir=C:\UMBC\331\java

sun.boot.class.path=C:\JDK1.2\JRE\lib\rt.jar;C:\JDK1.2\JR...

user.name=nicholas

user.home=C:\WINDOWS

C:\UMBC\331\java>


Java GUI

• The awt class allows you to create– frames

– buttons

– menus and menubars

– checkboxes

– text areas

– scrolling lists


GUI Examples

• “Hello World”, revisited• The Scribble example• Both examples adapted from Flanagan’s “Java in a

Nutshell”, first edition

import java.applet.*;import java.awt.*;

public class SecondApplet extends Applet { static final String message = "Hello World"; private Font font;

// One-time initialization for the applet public void init() { font = new Font("Helvetica", Font.BOLD, 48); }

// Draw the applet whenever necessary. // Do some fancy graphics. public void paint(Graphics g) { // The pink oval g.setColor(Color.pink); g.fillOval(10, 10, 330, 100);

// The red outline. java doesn't support wide lines, so we// try to simulate a 4-pixel wide line by drawing four ovals g.setColor(Color.red); g.drawOval(10,10, 330, 100); g.drawOval(9, 9, 332, 102); g.drawOval(8, 8, 334, 104); g.drawOval(7, 7, 336, 106);

// The text g.setColor(Color.black); g.setFont(font); g.drawString(message, 40, 75); }}


Scribble

• Graphics objects can be added to applets, e.g. buttons and menus

• Events, such as mouse clicks, are handled

import java.applet.*;import java.awt.*;

public class Scribble extends Applet { private int last_x = 0; private int last_y = 0; private Color current_color = Color.black; private Button clear_button; private Choice color_choices; // Called to initialize the applet. public void init() {

// Set the background colorthis.setBackground(Color.white);

// Create a button and add it to the applet.// Also, set the button's colorsclear_button = new Button("Clear");clear_button.setForeground(Color.black);clear_button.setBackground(Color.lightGray);this.add(clear_button);

// Create a menu of colors and add it to the applet.// Also set the menu's colors and add a label.color_choices = new Choice();color_choices.addItem("black");color_choices.addItem("red");color_choices.addItem("yellow");color_choices.setForeground(Color.black);color_choices.setBackground(Color.lightGray);this.add(new Label("Color:"));this.add(color_choices);

}

// Called when the user clicks the mouse to start a scribble public boolean mouseDown(Event e, int x, int y) { last_x = x; last_y = y; return true; }

// Called when the user scribbles with the mouse button down public boolean mouseDrag(Event e, int x, int y) { Graphics g = this.getGraphics(); g.setColor(current_color); g.drawLine(last_x, last_y, x, y); last_x = x; last_y = y; return true; }

// Called when the user clicks the button or chooses a color public boolean action(Event event, Object arg) { // If the Clear button was clicked, handle it if (event.target == clear_button) { Graphics g = this.getGraphics(); Rectangle r = this.bounds(); g.setColor(this.getBackground()); g.fillRect(r.x, r.y, r.width, r.height); return true;

} // Otherwise if a color was chosen, handle that

if (event.target == color_choices) { if (arg.equals("black")) current_color=Color.black; else if (arg.equals("red")) current_color=Color.red; else if (arg.equals("yellow")) current_color=Color.yellow; return true;}

// Otherwise let the superclass handle the event else return super.action(event, arg); }}


Java.net

• Defines several useful objects:– URLs

– Internet Addresses

– Sockets

– Datagrams• packets

• sockets


Manipulating URLs

• A Web server can provide lots of information about a file being sent back in response to an HTTP request.

• A URL can be read– into a string, using the getContent()

– or opened as a DataInputStream, and read using readLine()

import java.net.*;import java.io.*;import java.util.*;

public class GetURLInfo {

// Create a URL from the specified address, open a connection // to it, and then display information about the URL. public static void main(String[] args) throws MalformedURLException, IOException { URL url = new URL(args[0]); URLConnection connection = url.openConnection(); printinfo(connection); }

public static void printinfo(URLConnection u) throws IOException { // Display the URL address, and information about it. System.out.println(u.getURL().toExternalForm() + ":"); System.out.println(" Content Type: " + u.getContentType()); System.out.println(" Content Length: " + u.getContentLength()); System.out.println(" Last Modified: " + new Date(u.getLastModified())); System.out.println(" Expiration: " + u.getExpiration()); System.out.println(" Content Encoding: " + u.getContentEncoding()); // Read and print out the first five lines of the URL. System.out.println("First five lines:"); DataInputStream in = new DataInputStream(u.getInputStream()); for(int i = 0; i < 5; i++) { String line = in.readLine(); if (line == null) break; System.out.println(" " + line); } }}


UDP Examples

• UDP - Unreliable datagram packet• Examples from Flanagan

– Send a UDP packet

– Receive a UDP packet

// This example is from the book _Java in a Nutshell_ by // David Flanagan.// Written by David Flanagan. // Copyright (c) 1996 O'Reilly & Associates.// You may study, use, modify, and distribute this example for // any purpose. This example is provided WITHOUT WARRANTY // either expressed or implied.

import java.io.*;import java.net.*;

// This class sends the specified text as a datagram to// port 6010 of the specified host.public class UDPSend { static final int port = 6010; public static void main(String args[]) throws Exception { if (args.length != 2) { System.out.println( "Usage: java UDPSend <hostname> <message>"); System.exit(0); }

// Get the internet address of the specified host InetAddress address = InetAddress.getByName(args[0]); // Convert the message to an array of bytes int msglen = args[1].length(); byte[] message = new byte[msglen]; args[1].getBytes(0, msglen, message, 0);

// Initilize the packet with data and address DatagramPacket packet = new DatagramPacket(message, msglen, address, port); // Create a socket, and send the packet through it. DatagramSocket socket = new DatagramSocket(); socket.send(packet); }}

import java.io.*;import java.net.*;

// This program waits to receive datagrams sent to port 6010. // When it receives one, it displays the sending host and port, // and prints the contents of the datagram as a string.public class UDPReceive { static final int port = 6010; public static void main(String args[]) throws Exception { byte[] buffer = new byte[1024]; String s; // Create a packet with an empty buffer to receive data DatagramPacket packet = new DatagramPacket(buffer, buffer.length); // Create a socket to listen on the port. DatagramSocket socket = new DatagramSocket(port);

for(;;) { // Wait to receive a datagram socket.receive(packet); // Convert the contents to a string s = new String(buffer, 0, 0, packet.getLength()); // And display them System.out.println("UDPReceive: received from " + packet.getAddress().getHostName() + ":" + packet.getPort() + ": " + s); } // close for } // close main} // close class UDPReceive


Using Threads

• Thread objects are the basis for multi-threaded programming.

• Multi-threaded programming allows a single program to conduct concurrently running threads that perform different tasks.

• Defined in java.lang.Thread


The Runnable Interface

• A class that implements this interface can provide the “body” of a thread

• The run() method needs to be specified


Using a Single Thread

• The simpleThread class implements the Runnable interface– Implements run() method for Runnable interface

– Implements init(), start() and stop(), extending Applet class

– Can’t just extend Thread class, since Thread.stop() is final and can’t be overriden (why?)

// Simple Thread Example, based on "Neon Sign"

public class OneThread extends java.applet.Applet implements Runnable { Thread kicker;

public void init() { kicker = null; } public void start() { if(kicker == null) { // If no thread is started yet kicker=new Thread(this); // then create one kicker.start(); // and start it } }

public void stop() { kicker.stop(); kicker = null; }

public void run() { long sleepTime; System.out.println("Hello!"); sleepTime = 5000;

while(true) { // Loop forever // The sleep method below might be interrupted // and cause an exception, so catch it. try { // Wait for some period of time Thread.sleep( sleepTime ); sleepTime = (sleepTime == 3000 ? 4000 : 3000 ); } catch (Exception e){ return; } System.out.println("Hello again after "+sleepTime+ " milliseconds"); } }}


Neon Sign Example

• Start a thread• Load two GIF images• Repeat forever

– Display first image

– Sleep a random period of time

– Display the other image

// Blinking Neon Light by Mattias Flodin

import java.awt.*;

public class BlinkItem extends java.applet.Applet implements Runnable { Image imPic[]; // Array that holds the two images int iPicIndex=0; // Keeps track of which image is displayed Thread kicker;

public void init() { // *Always* resize, in case the HTML author forgot to // include WIDTH and HEIGHT tags resize(512,243); }

public void Paint(Graphics g) { update(g); }

// Using the update method will get rid of some flickering public void update(Graphics g) { // Display an error message if something // unexpected has happened to the images if(imPic[iPicIndex]==null) g.drawString("Error when loading picture", 0, 172);

// Draw the current image g.drawImage(imPic[iPicIndex],0,0, this); }

public void start() { if(kicker == null) { // If no thread is started yet kicker=new Thread(this); // then create one kicker.start(); // and start it } } public void stop() { kicker=null; } public void run() { imPic=new Image[2]; // Dimension the image array // Load the two images in our 'animation' imPic[0]=getImage(getCodeBase(), "images/Homepage1.gif"); imPic[1]=getImage(getCodeBase(), "images/Homepage2.gif");

for(;;) { // Loop forever repaint(); // Redraw the window iPicIndex=iPicIndex==0 ? 1 : 0; // Switch image // The sleep method below might be interrupted // and cause an InterruptedException, so we'll // have to catch it. try { // Wait for a random amount of time Thread.sleep( (int) (Math.random()*500)); } catch (InterruptedException e){} } }}


Multiple Threads

• An applet that creates a pool of simple threads• Each thread has an ID number, and may handle

different tasks• Note separation of applet and thread classes


public class ThreadExample extends java.applet.Applet {

public void init() { simpleThread[] threadPool = new simpleThread[2]; threadPool[0] = new simpleThread(1); threadPool[1] = new simpleThread(2); }}

Multiple Threaded Example

public class simpleThread implements Runnable {

Thread thisThread; int thisThreadID; public simpleThread (int i) { thisThreadID = i; this.start(); } public void start() { if(thisThread == null) { // If thread isn’t started thisThread = new Thread(this); // create one thisThread.start(); // and start it } }

public void stop() { thisThread.stop(); thisThread = null; }

public void run() { long sleepTime; System.out.println("Hello from simpleThread "+ thisThreadID); while(true) { // The sleep method below might be interrupted // and cause an exception, so catch it. try { // Wait for a random amount of time sleepTime = (long) ( Math.random()*4 ); Thread.sleep( sleepTime*1000 ); } catch (Exception e) { return; } System.out.println("Hello again from simpleThread "+ thisThreadID+" after "+sleepTime+" seconds"); } } }


Other Ideas

• Patterns as an addition to the StringBuffer class (ala Perl)

• One could easily write a better browser– intelligent pre-fetching of URLs

– other ideas?


Where to get more information

• There are now many decent Java books, e.g. Dietel and Dietel, and the second edition of Flanagan

• Many other web resources

Documents

UMBC CMSC 331 Java The JAVA API. UMBC CMSC 331 Java 2 A Tour of the Java API An API User’s Guide, in HTML, is bundled with Java Much of the “learning