Methods for classes (lập TRÌNH cơ bản SLIDE)

Basic Programming Course

Section 02

Methods for classes
Week 3


Methods

•

Designing a good data representation for a problem is the first step toward the
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

•

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

Examples

1.
2.
3.

100 pounds of Hawaiian Kona at $15.95/pound
→ $1,595.00
1,000 pounds of Ethiopian coffee at $8.00/pound
→ $8,000.00
1,700 pounds of Colombian Supreme at $9.50/pound
→ 16,150.00


Class Diagram
Class

Data type

CoffeeReceipt
- String kind
- double pricePerPound
- double weight

Property or field

+ ??? sellingCost(???)
Method

Method Template
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 ???? sellingCost(???) {
… this.kind …

Add a contract,
a purpose statement

… this.pricePerPound …
… this.weight …
}
}

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() {
return this.pricePerPound * this.weight;
}
}


Testing
Instance of class
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);
Assert.assertEquals(hk.sellingCost(), 1595.00, 0.001); //0.00

CoffeeReceipt e = new CoffeeReceipt("Ethiopian", 8.00, 1000);
Assert.assertEquals(e.sellingCost(), 8000.00, 0.001);

CoffeeReceipt cs = new CoffeeReceipt("Colombian Supreme ", 9.50, 1700);
Assert.assertEquals(cs.sellingCost(), 16150.00, 0.001);
}
}


New Requirement

•

Requirement: The coffee shop owner may wish to find out whether a coffee sale
involved more than a certain number of pounds.

•

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


Class Diagram
CoffeeReceipt
- String kind
- double pricePerPound
- double weight
+ double sellingCost()
+ ???? weighsMore(???)

•

Q: Develop some examples for weighsMore()

Examples

1.

new CoffeeReceipt("Hawaiian Kona“, 15.95, 100).weighsMore(200)

// should produce → false
2. new CoffeeReceipt("Ethiopian", 8.00, 1000).weighsMore(1000)
// should produce →false
3. new CoffeeReceipt(“Columbian“,
9.50, 200).weighsMore(100)
// should produce → true

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


weighsMore() Template
public class CoffeeReceipt {
private String kind;
private double pricePerPound;
private double weight;

…

public ???? weighsMore(???) {
… this.kind …
… this.pricePerPound …
… this.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.assertFalse(new CoffeeReceipt("Hawaiian Kona“, 15.95, 100).weighsMore(200));
Assert.assertFalse(new CoffeeReceipt("Ethiopian", 8.00, 1000).weighsMore(1000));
Assert.assertTrue(new CoffeeReceipt(“Columbian“,

9.50, 200).weighsMore(100));
}
}


Class Diagram

CoffeeReceipt
- String kind
- double pricePerPound
- double weight
+ double sellingCost()
+ boolean weighsMore(double amount)


Another New Requirement

•

Requirement: A coffee shop customers want to determine whether one coffee is cheaper
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

CoffeeReceipt

- String kind
- double pricePerPound
- double weight
+ double sellingCost()
+ boolean weighsMore(double amount)
+ boolean isCheaperThan(CoffeeReceipt that)

•

Q: Develop some examples for isCheaperThan()


Examples
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


IsCheaperThan()
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)
.isCheaperThan(new CoffeeReceipt("Ethiopian", 20.00, 100));

Assert.assertFalse(new CoffeeReceipt("Hawaiian Kona“, 15.95, 200)
.isCheaperThan(new CoffeeReceipt("Ethiopian", 15.95, 100)) ;

Assert.assertFalse(new CoffeeReceipt("Hawaiian Kona“, 15.95, 300)
.isCheaperThan(new CoffeeReceipt("Ethiopian", 8.00, 100));
}
}


Another New Requirement

•

The coffee shop customers want to determine whether one coffee receipt costs
less than some other coffee receipt


Class Diagram

CoffeeReceipt
- String kind
- double pricePerPound
- double weight
+ double sellingCost()
+ boolean weighsMore(double amount)
+ boolean isCheaperThan(CoffeeReceipt that)
+ boolean costsLessThan(CoffeeReceipt that)


Examples

1.

new CoffeeReceipt("Hawaiian Kona", 15.95, 100)
.costsLessThan(new CoffeeReceipt("Ethiopian“, 8.00, 1000))
// should produce
→ true

2.

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

3.

new CoffeeReceipt(" Colombian Supreme", 9.50, 1700)
.costsLessThan(new CoffeeReceipt("Ethiopian“, 8.00, 1000))
// should produce
→ false


costsLessThan()
public class CoffeeReceipt {
private String kind;
private double pricePerPound;
private double weight;

…

public boolean costsLessThan(CoffeeReceipt that) {

return this.sellingCost() < that.sellingCost();
}
}