1. The JAVA API

2. 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

3. 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 for the current APIs

4. 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()

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

6. 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.

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

8. Java.lang.system Printstreams Inputstreams
System.out.err(“Error message”);
Inputstreams
System.in.read(inputCharacter)
System.in.read(inputBuffer, 0, maxChars)

9. 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.

10. 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

11. System Properties
System properties are like UNIX environment variables, but platform independent
The API class java.util has methods for accessing the system properties

12. // 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); }

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

14. Java.net Defines several useful objects: URLs Internet Addresses
Sockets
Datagrams
packets
sockets

15. 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()

16. 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); }

17. 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); }

18. UDP Examples UDP - Unreliable datagram packet Examples from Flanagan
Send a UDP packet
Receive a UDP packet

19. // 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); }

20. // 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); }

21. 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);

22. 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

23. 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

24. The Runnable Interface
A class that implements this interface can provide the “body” of a thread
The run() method needs to be specified

25. 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?)

26. // 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; }

27. 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"); }

28. 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

29. // 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);

30. 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){}

31. 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

32. 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); }

33. 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; }

34. 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");

35. 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?

36. Where to get more information
There are now many decent Java books
Many other web resources