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

• Email, 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

– http://www.wam.umd.edu/~userid/hello.html

– 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

Add int + int produces int

Multiply int * float produces float

Compare int < int produces boolean

Compare char == 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