OOP&M - laboratory lectures1 JPN – GUI and Applets “… I am sure ‘it wont fail as much as the one before …” - Bill Gates about W98 -

OOP&M - laboratory lectures2 JPN – Java and the Net Clients and Servers One of the reasons of the interest in Java is its delivery mechanism. Java applets do not reach the user as a fixed and closed product into a floppy or CD. Nor they are downloaded only once from the net to be used over and over again. Instead, they are embedded within a web page and downloaded anew with each loading of the web page. “Java applets are not only on the Internet, but are creatures of the net as well” - Arnow/Weiss -

OOP&M - laboratory lectures3 JPN – Java and the Net Internet Communication Communication on the internet can be defined as the transfer of data from one machine to another. Into this definition there are many different concepts that fit: - -Remote logins -Download of web pages and applets -Chat rooms -… Information is break into small pieces called packets and transferred in that form

OOP&M - laboratory lectures4 JPN – Java and the Net Internet Communication The machine that is sending the information is almost never connected directly to the receiver, so these packets are actually transferred from one machine to another through the internet until they arrive at their destination The machines responsible of this transfer of data are called routers and are dedicated only to this purpose. Therefore they use a special software and have special form (and are quite expensive … go to and check it out) The transfer of packets is not secure, they can get lost in the way. The connection between machines is not good enough, and the machines themselves have limited memory … if packets arrive at rate higher than machine’s process rate, they could be lost

OOP&M - laboratory lectures5 JPN – Java and the Net Internet Communication Network A Computer 1 Computer 2 Computer 3 Network B Computer 4 Computer 5 Computer 6 R2 R6 R4 R5 R1 R3 Computer 7 Computer 8Computer 9 Computer 10 Network C

OOP&M - laboratory lectures6 JPN – Java and the Net Internet Communication Network A Computer 1 Computer 2 Computer 3 Network B Computer 4 Computer 5 Computer 6 R2 R4 R5 R1 R3 Internet Communication is: –One way –Broken into packets –Unreliable (packets can get lost) –Transmitted over many hops

OOP&M - laboratory lectures7 JPN – Java and the Net Internet Communication Network A Computer 1 Computer 2 Computer 3 Network B Computer 4 Computer 5 Computer 6 R2 R4 R5 R1 R3 With TCP it appears to be: –Bidirectional –A continuous Stream –Reliable –Direct

OOP&M - laboratory lectures8 JPN – Java and the Net Internet Communication Every router must be connected, at least, to another router, so that there will always be a path of routers between any couple of machines residing in different networks. The fact of the looses in terms of packets makes the network a not useful tool for almost any kind of applications. But there is a set of tools, which are the communication protocols, in order to make possible, the use of the net a communication medium. First, the “Internet Protocol” is running in all the machines at the net. Our applications use the “Transfer Control Protocol”. Both observe how the packets are transferred through the network and forces the sender to repeat if any loose of data is detected. THIS EMULATES A DIRECT STREAM OF DATA BETWEEN TWO M.

OOP&M - laboratory lectures9 JPN – input / output to programs Java program keyboard monitor files network InputStream System.out FileOutputStream Java program FileInputStream System.in InputStream OutputStream

OOP&M - laboratory lectures10 JPN – input/output … an overview Java program InputStream bytes of data ewsc24rfds53Hej20Hur20m76ar20du20?rsf Input source We must build a bridge between the Java Program and the input source!!

OOP&M - laboratory lectures11 Stages JPN – input … as example Construct a reference to an InputStream object, and with it Construct an InputStreamReader object, and with it Construct a BufferedReader object This references could be of two types FileInputStreamReader for files BufferedInputStream for keyboard and the network Note that: InputStream inputs sequences of bytes into the program InputStreamReader models the stream of input as characters, but doesn’t recognize ends of line a.o. BufferedReader has got a collection of methods that allow us to work in a similar way as we did with PrintStream

OOP&M - laboratory lectures12 JPN – a board with keys, the example Java provides a predefined object to represent the stream of input that comes from the keyboard. System.in is a reference to this object contained into the BufferedInputStream class Unlike System.out, which refers to a PrintStream object and therefore can be used right away to write Strings to the monitor, System.in, a reference to a BufferedInputStream object, cannot be readily used to read Strings keyboard [existing] InputStream new InputStreamReader new BufferedReader keyb isr System.in

OOP&M - laboratory lectures13 the constructor for InputStreamReader accepts the keyboard (a BufferedInputStream reference) as its argument new InputStreamReader ( System.in ) Java provides System.in contained into the BufferedInputStream class the constructor for BufferedReader accepts a InputStreamReader reference as its argument new BufferedReader ( ISR ) JPN – a board with keys, the example

OOP&M - laboratory lectures14 The total declaration would be: InputStreamReader isr; BufferedReader keyb; isr = new InputStreamReader(System.in); keyb = new BufferedReader(isr); And now, for reading a line: InputStreamReader isr; BufferedReader keyb; String inputline; isr = new InputStreamReader(System.in); keyb = new BufferedReader(isr); inputline = keyb.readLine(); *.readLine() is a method from the BufferedReader class JPN – a board with keys, the example

OOP&M - laboratory lectures15 Example: a program that writes the “plural” of a word import java.io.*; /* * This program writes the plural of the word typed in the * keyboard … it just adds an “s” */ class plural { public static void main(String[] arg) throws Exception{ InputStreamReader isr; BufferedReader keyb; String inputline; isr = new InputStreamReader(System.in); keyb = new BufferedReader(isr); inputline = keyb.readLine(); System.out.print(inputline); System.out.println(“s”); } JPN – a board with keys, the example

OOP&M - laboratory lectures16 JPN – come IN PUT files Obtaining input from disk files is only a little bit more complicated than from the keyboard. Our starting point must be to find some sort of InputStream object. As we said before, both BufferedInputStream and FileInputStream belong to that class new FileInputStream new InputStreamReader new BufferedReader bsr isr System.in is already defined into the class BufferedInputStream, the problem is that the FileInputStream class does not have defined objects like that. Therefore we need to first define an object that makes reference to a file. fsr files

OOP&M - laboratory lectures17 JPN – come IN PUT files the constructor for InputStreamReader accepts a stream from a file (a FileInputStream reference) as its argument new InputStreamReader ( FileInputStream ) the constructor for FileInputStream accepts a file (a File reference) as its argument the constructor for BufferedReader accepts a InputStreamReader reference as its argument new BufferedReader ( ISR ) new FileInputStream ( file )

OOP&M - laboratory lectures18 JPN – come IN PUT files The total declaration would be: File f; FileInputStream fsr; InputStreamReader isr; BufferedReader bsr; f = new File(“Big_parties_many_potatis.txt”); fsr = new FileInputStream(f); isr = new InputStreamReader(fsr); bsr = new BufferedReader(isr);

OOP&M - laboratory lectures19 JPN – come IN PUT files Let’s read a couple of lines from the file and print them: File f; FileInputStream fsr; InputStreamReader isr; BufferedReader bsr; String inputline; f = new File(“Big_parties_many_potatis.txt”); fsr = new FileInputStream(f); isr = new InputStreamReader(fsr); bsr = new BufferedReader(isr); inputline = bsr.readLine(); System.out.println(inputline); inputline = bsr.readLine(); System.out.println(inputline);

OOP&M - laboratory lectures20 JPN – Streams from the internet When we are try to read information from the Web The Net as other communication media for our programs can be read using Streams There is a difference between what we saw until now and what will happen with the Net: files are in HTML code The constructors are: Strings; URL u; InputStream ins; InputStreamReader isr; BufferedReader link; u = new URL(" ins = u.openStream(); isr = new InputStreamReader(ins); link = new BufferedReader(isr); s = link.readLine(); As you see here, the structure is similar to the one that we saw for the files. The difference is at the constructor for the InputStream object, which is special!! But the method for reading the information is the same: *.readline()

OOP&M - laboratory lectures21 import java.net.*; import java.io.*; class MyFirstBrowser { public static void main(String[] arg) throws Exception { URL uNet; InputStream insNet; InputStreamReader isrNet; BufferedReader linkNet; String s; uNet = new URL(" insNet = uNet.openStream(); isrNet = new InputStreamReader(insNet); linkNet = new BufferedReader(isrNet); s = linkNet.readLine(); System.out.println(s); s = linkNet.readLine(); System.out.println(s); System.out.println("connection closed"); } Example – Just reading a file JPN – Streams from the internet

OOP&M - laboratory lectures22 Features needed: Interpret HTML TAGs underline a text if it is a link ( ) jump line with show the name of the images ( ) try to show tables (...) think how to ask to the user for the next jump to do... Interpret scripting languages JavaScript Visual Basic Script Perl Calls external applications Office Java Virtual Machine JPN – internet The Browser is much more than only that

OOP&M - laboratory lectures23 JPN – Java and the Net Sockets A TCP connection between two programs in many ways is like a phone connection. Data can move in both directions simultaneously. But, before the communication act takes place, the connection must be established  one party must call the other Both parties need something beside, like in a phone call, both speakers have a telephone. The analogy between the telephone and the programming world is the socket. It is to the communication between computers like the telephone between two persons In Java we model the telephone with an object of the Socket class. A socket is created for making a single TCP connection

OOP&M - laboratory lectures24 JPN – Java and the Net Sockets The arguments to the constructor of the socket are the machine to contact and the port where the communication will take place. The port could be understood as the extension to the phone number in a large phone system new Socket(“ 80); The socket provides the methods: *.getInputStream() and *.getOutputStream(), from whose return values BufferedReader objects and PrintStream objects can be constructed. These objects use the TCP connection as their input source or output target

OOP&M - laboratory lectures25 JPN – Java and the Net Sockets The typical example would look like: import java.io.*; import java.net.*; Socket s = new Socket(hostname, portnumber); BufferedReader br = new BufferedReader( new InputStreamReader( s.getInputStream())); PrintStream ps = new PrintStream( s.getOutputStream()); String line = br.readLine(); //reads from the host ps.println(“hej”); //writes to the host

OOP&M - laboratory lectures26 JPN – Java and the Net Client-Server Computing Sockets and TCP/IP connections allow programs to communicate throw the internet, but they don’t determine any particular style of communication or relationship between the programs. There is a widely used design approach for Internet applications - the so called client-server model – In this model, an application consist of one program running as server, and another as a client Server: a program that provides a service Client: a program that requests a service

OOP&M - laboratory lectures27 JPN – Java and the Net Client-Server Computing Client an server converse, what means that they follow some communication rules. From the point of view of the client, what happens is: 1.Creates a socket object with the machine address and port to the server 2.Sends a message to the server on the TCP connection associated to the socket 3.Waits for response from the server that will be: More information needed Requested action performed The requested data Each communication is established (1) only once, but the conversation can be repeated (2-3) many times

OOP&M - laboratory lectures28 JPN – Java and the Net Client-Server Protocols Each client-server application has its own rules (or protocol) governing the conversation between client and server The web clients and servers use one protocol called HyperText Transport Protocol (HTTP) clients and servers use a different protocol, called Simple Mail Transfer Protocol (SMTP) if we send mail or POP if we receive (there are other variants, like IMAP, a.o.)