11University of Utah – School of Computing

Computer Science 1020 "Object-Oriented Programming"

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Procedural Programming

• What we've done so far:- variables- functions 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 functions• 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 functions• Or "attributes" and "behaviors"

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Example

• Consider our geometry program• What attributes does a rectangle have?

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Rectangle class

class Rectangle {int length;int width;

}

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Rectangle class

• What do you want to do with a rectangle?- Draw it?- Resize it?- Calculate area?- Calculate perimeter?

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Methods

class Rectangle {

int width;

int height;

void draw() {

//do stuff

}

double computePerimeter() {

//do stuff

}

};

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How to draw a rectangle?

void draw() {

}

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How to find a rectangle's perimeter?

double computePerimeter() {

}

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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:

- Rectangle r;

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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

• A struct's variables are public by default.

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"private" also!

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

• Used for internal data

• By default, everything in a class is private

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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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Rectangle class revisited

class Rectangle {

private:

int width;

int height;

public:

void draw() {

//do stuff

}

double computePerimeter() {

//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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Reminder

• Homework 5 is due tomorrow

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Request for Comments

• Please fill out online survey!