11University of Utah – School of Computing

Computer Science 1021 "Object-Oriented Programming"

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Procedural Programming

• What we've done so far:- variables- methods that operate on those variables

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Large-Scale Programming

• Hard to write BIG programs using procedural programming techniques

• WHY?

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Problem #1

• Global Variables!

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Problem #2

• Limits of human comprehension

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Problem #3

• Brittle dependencies

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Conceptual Leap

• 1. Dividing a large job into small methods• 2. Dividing several large jobs (or

concepts) into multiple classes!

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Analogy

• Toy blocks

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Classes

• Have variables and methods• Or "attributes" and "behaviors"

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Example

• Modify our circle drawing program• What attributes does a circle have?

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Circle class

public class Circle {int x, y;int radius;Color color;

}

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Circle class

• What do you want to do with a circle?- Draw it?- Move it?- Resize it?- Calculate area?- Calculate circumference?

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Methods

class Circle {

int x, y;

int radius;

Color circle;

void draw(Graphics g) {

//do stuff

}

double computeArea() {

//do stuff

}

}

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How to draw a circle?

void draw(Graphics g) {

}

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How to find the circle's area?

double computeArea() {

}

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Classes vs. Objects

• A class is just the "definition" or the code• An object is when you make a variable

whose type is a class• Example:

- JPanel p;- p = new JPanel();

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Block example revisited

• Wrap up all attributes and methods inside a class

• Easier to manage!

• Safer too? Why?

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Car example

• You can drive a car without knowing how it works internally

• "public" user interface:- steering wheel- gas/brake pedals- ignition

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Car example

• You can drive a car without knowing how it works internally

• "private" inner workings- don't know, don't care

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"public" explained

• public methods and variables- Other classes can use them

• So far, this is all we've used

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"private" also!

• private methods and variables- Other classes may NOT access them

• Used for internal data

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A General Rule

• private variables- they are our internal data

• public methods- they are how other classes use our class

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Circle class revisited

class Circle {

private int x, y;

private int radius;

private Color circle;

public void draw(Graphics g) {

//do stuff

}

public double computeArea() {

//do stuff

}

}

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Why public and private?

• Secrecy? No.• Simplicity? Yes.• Order? Yes.• Maintainability? Yes.

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Examples

• Math.random()• JPanel.add()

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Tomorrow

• Work on "Circle" example together

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Reminder

• Homework 5 is due tomorrow, not Wednesday, due to Fourth of July

• Office hours today from 9:45 to noon

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Request for Comments

• Please fill out online survey!