Programming Class 9 LBSC 690 Information Technology

Agenda Questions Programming –The mythical person-month –History of programming –Object oriented programming –Programming exercise in Java

Software Software models aspects of reality –Input and output represent the state of the world –Software describes how the two are related Examples –Ballistic computations –Alta Vista –Microsoft Word

Programming Languages Used to specify every detail of the model Special purpose –Able to specify an entire class of models Spreadsheets (Excell, Quatro Pro,...) Databases (Access, Paradox,...) General purpose –Able to specify any possible model Pascal, C, Java,...

The Mythical Person-Month If it would take one person three months, why does it take four people SIX months? Four causes –It wouldn’t have taken 3 months anyhow! –Partitioning strategy –Training time –Communications effort

How long will it take? Rules of thumb –1/3 specification –1/6 coding –1/2 test planning, testing, and fixing! Add time for coding to learn as you go, but don’t take time away from the other parts! –Reread the section on “gutless estimating” if you are tempted

Training Time Simple Example –Full time person = 2,000 hours/year –Part time person = 288 hours per year –No training -> 7 part time people –With training -> 10 part time people Learning the specification takes lots of time –Learning organizational “rules” takes longer

Communications Sort of like continuous training –Who needs to know what I just learned? Can be minimized by good partitioning –Limit the number of interfaces Can be facilitated by computers –Asynchronous communication techniques , BBS, voice mail

History of Programming Machine code –Zeroes and Ones Assembly language –“Assembler” changes names to machine code High-level languages –“Compiler” translates math to machine code Independent of machine “architecture” –FORTRAN, COBOL

History of Programming Structured (Modular) Programming –Group instructions into meaningful abstractions –C, Pascal Object-oriented programming –Group “data” and “methods” into “objects” –C++, Java

Your First Java Program Log in to WAM cd ~/../pub cp /users/rba/pub/java/*. –DONT FORGET THE DOT AT THE END!!! tap java javac HelloWorld.java java HelloWorld

What You Just Did First you got some Java program files –Each.java file specifies one “class” Then you “compiled” one using javac –This produced HelloWorld.class Then you ran it using java –.class files are a “bytecode” representation An “interpreter” (“java”) is needed to run them –You don’t specify.class when you run it

Java Bytecode “Machine language” depends on the machine –Programs compiled for a Sun won’t work on a PC Java claims “write once, run anywhere” –Without recompiling Traditional interpreters are slow –Because they must read every character every time Java compiles to a standardized bytecode –Web browsers include a bytecode interpreter

Changing the Program Use pico to edit HelloWorld.java –Change “Hello, world!” to something different –Be careful not to change anything else! Use javac to compile it –This produces the bytecode in the.class file Use java to run it –It should print whatever you told it to –It won’t work if you include “.class” in the name

Using Java with the Web Web browsers render HTML –But HTML lacks control and data structures Newer browsers can interpret java bytecode –Java applets are programs designed for the web This allows programs to be “rented” –They still execute on the client machine –But some classes can be obtained on the web

Java Applet Example Change HelloWorldApplet.java –Make it say whatever you want javac HelloWorldApplet.java Fire up Netscape or Internet Explorer – –You should get whatever you told it to say You can also change hello.html –Then select “reload” to reload the web page

Object Models Represent things in the world as “objects” –The simplest objects are “variables” Represent actions with “methods” –The simplest methods are “operations” “Classes” group objects with methods –Classes model aspects of reality Objects are instances of classes

A Simple Example Variables: Height, Weight, Shoe size Operations: Multiply, Divide Method: Shoe size=4*Weight/Height Class: Person Object: George

Instances Classes model kinds of things –“person” is an example of a class A Class may be instantiated –I am an instance of person Object-Oriented Programming –Define simple classes with variables & operations –Define more complex classes using simple objects –Invoke a method in some class to start things

Data Types int –Like integers, but there is a biggest and smallest float –Like real numbers, but there are a finite number char –Any character in any language (Unicode) boolean –True or false

Arrays Lists of elements, each of the same data type –For example, the number of days in each month Each element is assigned an integer index –The index is used to refer to the element (x[4]) –In Java, the index numbers start at zero An string acts like an array of characters

Some Basic Operations Negate- int produces int Addint + int produces int Multiplyint * float produces float Compareint < int produces boolean Comparechar == char produces boolean

Statements Simple assignment statements –numberOfBirds = numberOfHawks + numberOfOrioles; –note difference with comparison == Statements that invoke a method –airForceOne = new Airplane(“747”); This is called a “constructor method” –Altitude = airForceOne.readAltitude(); Return a value from a method –return numberOfBirds;

Making Methods Three ways to combine statements: –Sequential {… ; …; …; …} –Conditional if (i= =3) then {…} else {…} try {…} catch (exception e) {…} –Loop do {…} while (i<5) for (i=0;i<10;i++) {…}

A Full-Featured Example TestDate computes Julian dates –The number of days since January 1 You type in the year, month, and day –The program expects these to be integers It prints out the Julian date –And it knows about leap years

Testing the Date Program javac *.java –This will compile everything java TestDate.java –TestDate contains a magic incantation to start it Answer the questions –If you make a mistake, it should complain Try lots of possibilities –February 29, 1900 is particularly interesting

Examining the Classes TestDate exercises methods in the Date class –It looks a lot like HelloWorld –Don’t mess with it - it works! TermInput reads input from the terminal –Comments at the top describe what it does –The rest is very grungy - don’t mess with it! But don’t forget to compile it!

A Full-Featured Example ValInt is a class, but it contains only data –Public data can be seen by methods in other classes Date has all the really interesting stuff in it. –This is what you will modify for homework –Use “pico” or “more” to read it

Date.java Braces and semicolons indicate sequential –Notice how layout is done for readability –Java does not care where line breaks happen But not in the middle of a word! if (boolean) {;;;} is a conditional do {;;;} while (boolean) is a loop –for (;;) {;;;} is another kind of loop

Date.java Comments explain what each part does –// indicates a comment to the end of the line Lots of attention to error handling –People WILL make mistakes –Many are easily corrected

The Key Ideas Java is a general purpose language –Sequential, conditional, and iteration Java is object oriented –Combine objects and methods to make classes Java can be used to make applets –Which can be run by web browsers