The JAVA API

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

See http://docs.oracle.com/javase/8/docs/api/ for the current APIs 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://docs.oracle.com/javase/8/docs/api/ for the current APIs

The package java.lang The class Object The class Class The root class in Java Example methods: clone(), equals(), toString() Subclasses may override these methods The class Class Example methods: getName(), getSuperClass()

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

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.

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 Inputstreams 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 variables import 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); }

Java GUI The awt class allows you to create frames buttons menus and menubars checkboxes text areas scrolling lists

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

Multiple Threaded Example 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); }

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 Many other web resources