Questions

• Reading?
• Quiz?
• Homework?

Tracing an object

Look at tracing using the Product class we wrote earlier

• Open the Product project
• Here is the ProductTester and the Product classes
  
  

  public class ProductTester { public static void main(String[] args) { Product thing = new Product("iPad", 450.00); thing.reducePrice(10); thing.increasePrice(10); double myPrice = thing.getPrice(); System.out.println(myPrice); String name = thing.getName(); System.out.println(name); } }

  public class Product { private String name; private double price; public Product(String theName, double thePrice) { name = theName; price = thePrice; } public String getName() { return name; } public double getPrice() { return price; } public void reducePrice(double percent) { double amountToReduce = price * percent /100; price = price - amountToReduce; } public void increasePrice(double percent) { double amountToIncrease = price * percent /100; price = price + amountToIncrease; } }

• Trace through what happens when we run the tester.
• This is called "playing computer"

Activity - Instance variables

Record your participation in Piazza clicker question Lesson6 Q1

This class has a problem. What is it?

• Create a project and copy in both the Circle class and the CircleTester class.
• Run the project
• Look at the actual output and expected output
• Trace through the code to find the problem

Circle

public class Circle
{
    private double radius;
    private double area;

    public Circle(double theRadius)
    {
        radius = theRadius;
        area = radius * radius * Math.PI;
    }

    public double area()
    {
        return area;
    }
    
    public void setRadius(double newRadius)
    {
        radius = newRadius;
    }
}
    

CircleTester

public class CircleTester
{
    public static void main(String[] args)
    {
        Circle c = new Circle(1);
        System.out.println(c.area());
        System.out.println("Expected: 3.141592653589793");
        
        c.setRadius(10);
        System.out.println(c.area());
        System.out.println("Expected: 314.1592653589793");
    }
}

 

So what is the problem?

Activity - Local variables

Record your participation in Piazza clicker question Lesson6 Q2

What are the local variables of the Product class increasePrice method?

    public void increasePrice(double percent)
    {
       double amountToIncrease = price * percent / 100;
       price = price + amountToIncrease;
    } 

Click all that apply.

1. percent
2. amountToIncrease
3. price
4. there are no local variables

Activity - Initializing local variables

Record your participation in Piazza clicker question Lesson6 Q3

What are local variables initialized to by the compiler? Check all that apply.

1. objects to null
2. Strings to the empty string ("")
3. numbers to 0
4. local variables are not initilized for you. You have to do it yourself or you will get a compile-time error

 

Activity - Initializing instance variables

Record your participation in Piazza clicker question Lesson6 Q4

What are instance variables initialized to by the compiler? Check all that apply.

1. objects to null
2. Strings to the empty string ("")
3. numbers to 0
4. instance variables are not initilized for you. You have to do it yourself or you will get a compile-time error

 

The implicit and explicit parameters

• Look at this method

      public void increasePrice(double percent)
      {
          double amountToIncrease = price * percent / 100;
          price = price + amountToIncrease;
      }

• Look at this code to call the method

  thing.increasePrice(10);

• What is the explicit parameter?
• What is the implicit parameter?
• Whose price is changed?
• The implicit parameter is passed to the method as the this reference
  • When used in the class, this means this object
• We could say

      this.price = this.price + amountToIncrease;

Activity - The this Reference

Record your participation in Piazza clicker question Lesson6 Q5

What is wrong with this code?

    private double price;
    private String name;  
    
    public Product(String name, double price )
    {
        price = price;
        name = name;  
    }

Activity - Write a Class

Record your participation in Piazza clicker question Lesson6 Q6

Write a Student class. A Student has a name and a total score (sum of all the points earned). It has a method to add a score to the total score, a method to get the total score and a methods to get and set the name.

• What does the class need to remember? What are the instance variables?
• Define the instance variables in your code. Call them name and totalScore
• Write the constructor. It takes one parameter: the name. Initialize both instance varaibles
• Write a method to add a new score to the total score(call it addScore)
• Write a method to get the total score (call it getTotalScore)
• Write a method to set the name
• Write a method to get the name

Here is a tester for the Student class. Copy it into your project. Does your code pass?