Methods for classes (lập TRÌNH cơ bản SLIDE)

Method Templatepublic class CoffeeReceipt { private String kind; private double pricePerPound; private double weight; public CoffeeReceiptString kind, double pricePerPound, double weight

Section 02

Methods for classes

Basic Programming Course

Week 3

design of a good program, but it isn't enough We also need functions that

compute results from some given data In an object-oriented language, functions are implemented as METHODS Designing methods proceeds in the fashion as

designing functions, which we are familiar with from How to Design Programs

At a Specialty Coffee Seller

Coffee example

coffee seller from a receipt that includes the kind of coffee, the unit price , and

Property or field

Method Data type

Class

Method Template

public class CoffeeReceipt {

private String kind;

private double pricePerPound;

private double weight;

public CoffeeReceipt(String kind, double pricePerPound, double weight) {

}

Add a contract,

a purpose statement

METHOD SIGNATURE

Method Implementation

public class CoffeeReceipt {

private String kind;

private double pricePerPound;

private double weight;

public CoffeeReceipt(String kind, double pricePerPound, double weight) {

this.kind = kind;

this.pricePerPound = pricePerPound;

this.weight = weight;

}

// to compute the total cost of this coffee purchase [ in cents]

public double sellingCost() {

returnthis pricePerPound * this weight;

}

}

public class TestCoffeeReceipt extends TestCase {

public void testConstructor() {

new CoffeeReceipt("Hawaiian Kona", 15.95, 100);

new CoffeeReceipt("Ethiopian", 8.00, 1000);

new CoffeeReceipt("Colombian Supreme ", 9.50, 1700);

}

public void testSellingCost() {

CoffeeReceipt hk = new CoffeeReceipt("Hawaiian Kona", 15.95, 100);

New Requirement

design

Class Diagram

• Q: Develop some examples for weighsMore()

// should produce → false

2 new CoffeeReceipt("Ethiopian", 8.00, 1000 ) weighsMore ( 1000 )

3 new CoffeeReceipt(“Columbian“,

9.50, 200 ) weighsMore ( 100 )

Q: Write a template for weighsMore() and implement it

weighsMore() Template

public class CoffeeReceipt {

private String kind;

private double pricePerPound;

private double weight;

weighsMore() Implementation

public class CoffeeReceipt {

private String kind;

private double pricePerPound;

private double weight;

…

public boolean weighsMore( double amount ) {

return this. weight > amount ;

}

}

• Q: Write a unit test for weighsMore()

Unit Test for weighsMore()

public class CoffeeReceiptTest extends TestCase {

…

public void testWeighsMore() {

Assert.assert False (new CoffeeReceipt("Hawaiian Kona“, 15.95, 100).weighsMore(200));

Assert.assert False (new CoffeeReceipt("Ethiopian", 8.00, 1000).weighsMore(1000)); Assert.assert True (new CoffeeReceipt(“Columbian“,

9.50, 200).weighsMore(100));

}

}

Another New Requirement

than some other coffee (by comparing the price per pound)

• Q: Supplement the class diagram to include this requirement to the current design

Class Diagram

• Q: Develop some examples for isCheaperThan()

1 new CoffeeReceipt("Hawaiian Kona“, 15.95, 100)

.isCheaperThan(new CoffeeReceipt("Ethiopian", 20.00, 100)) // should produce → true

2 new CoffeeReceipt("Hawaiian Kona“, 15.95 , 200)

.isCheaperThan(new CoffeeReceipt("Ethiopian", 15.95 , 100)) // should produce → false

3 new CoffeeReceipt("Hawaiian Kona“, 15.95 , 300)

.isCheaperThan(new CoffeeReceipt("Ethiopian", 8.00 , 100)) // should produce → false

Q: Write a template for isCheaperThan() and implement it

public class CoffeeReceipt {

private String kind;

private double pricePerPound;

private double weight;

…

public boolean isCheaperThan( CoffeeReceipt that ) {

return this. pricePerPound < that pricePerPound ;

}

}

• Q: Write a unit test for isCheaperThan()

Unit Test for IsCheaperThan()

public class CoffeeReceiptTest extends TestCase {

…

public void testIsCheaperThan() {

Assert.assertTrue(new CoffeeReceipt("Hawaiian Kona“, 15.95, 100)

}

Another New Requirement

less than some other coffee receipt

Class Diagram

1. new CoffeeReceipt("Hawaiian Kona", 15.95, 100)

.costsLessThan(new CoffeeReceipt("Ethiopian“, 8.00, 1000))// should produce

→ false

public class CoffeeReceipt {

private String kind;

private double pricePerPound;

private double weight;

…

public boolean costsLessThan( CoffeeReceipt that ) {

return this sellingCost() < that sellingCost() ;

}

}

Unit Test for costsLessThan()

public class CoffeeReceiptTest extends TestCase {

…

public void testCostsLessThan() {

CoffeeReceipt hk = new CoffeeReceipt("Hawaiian Kona", 15.95, 100); CoffeeReceipt e = new CoffeeReceipt("Ethiopian", 8.00, 1000);

CoffeeReceipt cs = new CoffeeReceipt("Colombian Supreme ", 9.50, 1700);

Java data type

we represent primitive forms of information Here we use four of them: int,

double, boolean, and String

Character type

Java represent a Unicode character (2 bytes)

Name size Range Default value

char 2 bytes ‘\u0000’… ’\uffff’ null

Q: Distinguish char type and String type ?

A: char c = “t’;

String s = “tin hoc”;

Boolean type

Exercises

Exercise 3.1.2 (HTDCH)

• Study this class definition:

public class Image {

private int width; // pixels

private int height; // pixels

private String fileName;

private String quality; // low, medium, or high

public Image(int width, int height, String fileName, String quality) {

• And here are some examples

new Image(10, 5, "small.gif", "low");

new Image(120, 200, "med.gif", "low");

new Image(1000, 1200, "large.gif", "high");

Exercise 3.1.2 (HTDCH)(cont.)

Develop the following methods for this class:

some other image.

Solutions

Extended Exercises (Lab hours)

Exercise 3.1.6

• Modify the CoffeeReceipt class so that sellingCost() takes into account bulk discounts:

Develop a program that computes the cost of selling bulk coffee at a specialty coffee seller from a receipt that includes the kind of coffee, the unit price, and the total amount (weight) sold If the sale is for less than 5,000 pounds, there is no discount For sales of 5,000 pounds

to 20,000 pounds, the seller grants a discount of 10% For sales of 20,000 pounds or more, the discount is 25% .

Don't forget to adapt the examples, too.

Exercise 3.1.7

• Design the method sizeString() for the Image class It produces one of three strings, depending on the number of pixels in the image:

• “small” for images with 10,000 pixels or fewer;

• “medium” for images with between 10,001 and 1,000,000 pixels;

• “large” for images that are even larger than that.

Prepare for next week

How to design class hierarchy

• III Fun Methods       

& Do Exercis e

Solution 3.1.2: isPortrait()

• Method implementation

public boolean isPortrait() {

return this.width < this.height;

}

• Unit testing

public void testIsPortrait() {

Assert.assertFalse(new Image(5, 10, "small.gif", "low“).isPortrait()); Assert.assertFalse(new Image(120, 200, "med.gif", "low").isPortrait()); Assert.assertTrue(new Image(1200, 1000, "large.gif", "high").isPortrait());

}

Solution 3.1.2: size()

• Method implementation

public int size() {

return this.height * this.width;

}

• Unit testing

public void testSize() {

Assert.assertEquals(new Image(5, 10, "small.gif", "low").size(), 50); Assert.assertEquals(new Image(120, 200, "med.gif", "low").size(),

24000);

Assert.assertEquals(new Image(1200, 1000, "large.gif", "high").size(),

1200000);

}

Solution 3.1.2: isLarger()

• Method implementation

public boolean isLarger(Image that) {

return this.size() > that.size();

}

• Unit testing

public void testIsLarger() {

Image img1 = new Image(5, 10, "small.gif", "low");

Image img2 = new Image(120, 200, "med.gif", "low");

Image img3 = new Image(1200, 1000, "large.gif", "high");

Assert.assertFalse(img1.isLarger(img2));

Assert.assertTrue(img3.isLarger(img2));