Questions

• Reading?
• Homework?
• Quiz?

Concept - Creating a Product Class

• We want to define a Product class that represents various types of Product objects
• We want to be able to:
  • create the object
  • decrease the price
  • increase the price
  • get the cost of the Product
  • get the name of the Product
• An application that used our class might have code like this:

  Product thing = new Product(......); //create the object
  thing.reducePrice(percent);  //decrease the price by the given percent
  thing.increasePrice(percent);  //increase the price the given percent
  double price = thing.getPrice(); //get the price
  String name = thing.getName(); //get the name

Activity - Starting the Product class

Record your participation in Piazza clicker question Lesson4 Q1

Let's write the class together

• What does the class need to remember? (What are its instance variables?)
• Create a project and a new class Product
• Start the class by writing the class header and defining the instance variables
• The code so far

  public class Product
  {
      private String name;
      private double price;
  }
      

• Always make your instance variables private
• Now create a constructor that takes name and price as parameters in that order
• We need to implement the methods for our Product class
• Accessors: Retrieve or compute a result
  • getPrice
  • getName
• Muators: Update the instance variables in some way
  • reducePrice
    (note to instructor: use a local variable: amountToReduce)
  • increasePrice
    (note to instructor: use a local variable: amountToIncrease)
• Provide javadoc for every:
  • public constructor
  • public method
  • parameter
  • return values
  • And the class itself
• Finally: Write a program to test our class

Activity - Accessors and Mutators

Record your participation in Piazza clicker question Lesson4 Q2

What is wrong with this sequence of statements?

Product toaster = new Product("Toaster", 39.99); 
System.out.println(toaster.reducePrice(10));

1. There is nothing wrong
2. reducePrice is an accessor method and can not be used as input to the println method
3. reducePrice is an mutator method and can not be used as input to the println method
4. Something else is wrong

Activity - Instance variables

Record your participation in Piazza clicker question Lesson4 Q3

• This class has a problem. What is it?

  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;
      }
  }
      

• Look at the tester. What is printed? Trace the output

  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?
• Trace the code

Activity - Local variables

Record your participation in Piazza clicker question Lesson4 Q4

What are the local variables of the Product class increasePrice method? Click all that apply.

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

Activity - Initializing instance variables

Record your participation in Piazza clicker question Lesson4 Q5

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 this reference

• 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 of the method?
• What is the implicit parameter of the method?
• Whose price is changed?
• We could say

      this.price = this.price + amountToIncrease

Activity - The this Reference

Record your participation in Piazza clicker question Lesson4 Q6

What is wrong with this code for the constructor of the Product class?

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

Activity - Write a Class

Record your participation in Piazza clicker question Lesson4 Q7

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?
• Write the constructor
• Write a method to add a new score to the total score
• Write a method to get the total score
• Write a method to set the name
• Write a method to get the name

Activity - private instance variables

Record your participation in Piazza clicker question Lesson4 Q8

In the previous activity, why should the total score variable be private? What might happen if it were public?