Programming

• What is a program?

– A set of instructions

– Understood by a computer

What does a program look like?

• An algorithm is a set of instructions designed to accomplish a specific goal

• For a temperature f in Fahrenheit• Subtract 32 from f.• Divide f by 1.8.• Display the value of f.• [Converts to Celsius]

• Don’t need to understand the goal to follow instructions

Programming Languages

• A language is a tool with which we tell a computer an algorithm

• Compilers translate from one computer language to another

• Each language has own advantages and disadvantages

The Java Language

• Widely Used

• Can be compiled on many computer systems

• Object Oriented (we’ll learn what this means later)

Event Driven Programming

• Old programs would start with all input at once and run until completion

• Event: An action, a mouse click, an item selected

• Using events made easy in Java

A First Program

import objectdraw.*;import java.awt.*;

public class TouchyWindow extends WindowController {public void onMousePress ( Location point ) {

new Text("I’m touched", 40, 50, canvas );}

public void onMouseRelease( Location point ) {canvas.clear();

}}

What do all these words mean?

• Let’s break the program apart

Some of the words

• Class– A set, collection, group, or configuration

containing members regarded as having certain attributes or traits in common. (American Heritage Dictionary)

– Very similar meaning in Java

Software Libraries

• Reuse code already written

• extends– Builds upon an already written class

• import– Allows the use of instructions written

elsewhere

Parts of a Class

• Body– Enclosed in the curly braces found on the line

“public class…”

• Methods:public void onMousePress ( Location point ) {

new Text( "I’m touched", 40, 50, canvas );

}

The Life of a Program

• Write the Program

• Translate the program into a language the computer understands

• Run the Program

Integrated Development Environments

• Eclipse– Professional Programming Tool

• TouchyWindow in Eclipse

Integrated Development Environments

• BlueJ– Designed to teach Java

• TouchyWindow in BlueJ

Integrated Development Environments

• Eclipse and BlueJ:

– Available on Windows, MacOS, Unix

– Free and downloadable

More Programming

• Normal vs Graphics Coordinate Systems

Graphical Objects

new Text ( "I’m Touched", 40, 50, canvas );• new an instruction that tells we want to

construct a new object• Text the object we want to construct• (…) comma delineated “parameters” that tell

how to construct the object• ; semicolons are important too!

– every command semicolon terminated

Graphical Objects

new Text ( "I’m Touched", 40, 50, canvas );

Events

public void onMousePress( Location point )– Will run the code when mouse button is

pressed

• onMouseRelease:– Runs code when the buttons released

• Many more mouse event handling methods

The begin Method

public void begin(){

…

}

• begin method– Executed (run) exactly once for each program– Executed early in program’s life

Graphical Objects

new Line( 100, 150, 200, 0 canvas);

Graphical Objects

• New FilledRect( 10, 20, 30, 40, canvas );– Creates a 30x40 rectangle– Upper left corner at (10,20)

• new FilledOval( 10, 20, 30, 40, canvas);– Imagine a 30x40 rectangle at (10,20)– Draws the largest oval that can fit inside the

imaginary rectangle

Mistakes? Impossible!

• Sadly, computers read what is written, not the intention.

• Errors will often generate complaints from the IDE.

• Some complaints are more informative than others