Worksheet: J2

Please submit your answers to the questions as comments in a J2.md markdown file you’ll be writing in this lab. To render your file, create a github repo and upload your file there – it can be viewed in your web browser.

Grading rubric and submission

When you are done, submit your J2.md file to BB.

You will be graded on the following:

Questions

1. How are private and public visibility modifiers handled when fields/methods with these different visibilities are inherited from a parent class?

   From a grandparent class?

   How do classes outside the class inheritance hierarchy see (or not see) fields/methods with these different modifiers?

   Reveal Solution

   Any class anywhere can see public fields/methods. Any child classes in the inheritance hierarchy can see anything protected, but other classes outside the hierarchy cannot see these items. Only the class itself can see anything private.

2. For the below questions, when the class Point is referenced, we are talking about the below class, which you can assume is fully implemented and working as described:

   public class Point {
      private double x,y; //the x,y fields
      public Point(double x,double y); //construct a point from an x,y
      public Point(Point other); //construct a point from another point
      public double getX(); //return the x component
      public double getY(); //return the y component
      public double setXY(double x, double y); //sets the x and y fields
      public String toString(); //return the string representation
      private double sum_x_y(); // Returns the sum of X and Y
   }
   
   

   Say we want to make a class that extends Point with a method that can reflect a point across the X and Y axis:

   public class CustomPoint extends Point {
       public void reflect(); // Reflects point
   }
   

   Which of the following implementations achieves this?

       // Option 1
       public void reflect() {
           x = -x;
           y = -y;
       }
   
       // Option 2
       public void reflect() {
           this.x = -this.x;
           this.y = -this.y;
       }
   
       // Option 3
       public void reflect() {
           this = Point(-x,-y);
       }
       
       // Option 4
       public void reflect() {
           double x = -this.getX();
           double y =-this.getY();
           this.setXY(x,y);
       }
       
       // Option 5
       public void reflect() {
           x = -this.getX();
           y = -this.getY();
       }
   

   Explain why.

   Reveal Solution

   Option 4 achieves the goal of reflecting the point. None of the other options compiles because x and y in CustomPoint is inheritted from Point and cannot be read/written to directly in CustomPoint since they are declared private in Point.

3. If we add this constructor to CustomPoint:

       public CustomPoint() {
           setXY(10, 10); // Line 1
           super(0, 0); // Line 2
       }
   

   …and then run this program, what is the output?

       public static void main(final String args[]) {
           CustomPoint p = new CustomPoint();
           System.out.println(p.toString());
       }
   

   Reveal Solution

   The program does not compile because the constructor call on Line 2 must be called before anything else inside the constructor for CustomPoint.

4. What if we switch line 1 and 2 in the previous question?

   Reveal Solution

   Now the program compiles and the output is (10.0, 10.0).

5. If we want to override sum_x_y in our custom point, but first reflect the point before returning the sum, which of the following implementations are valid? (Note: assume that reflect has a valid implementation)

       //Option 1
       public double sum_x_y() {
           this.reflect();
           return super.sum_x_y();
       }
   
       //Option 2
       public double sum_x_y() {
           this.reflect();
           return this.getX() + this.getY();
       }
   
       //Option 3
       public double custom_sum_x_y() {
           this.reflect();
           return super.sum_x_y();
       }
   
       //Option 4
       public double custom_sum_x_y() {
           this.reflect();
           return this.getX() + this.getY();
       }
   
   

   Explain your answer.

   Reveal Solution

   Option 2 is valid. Option 1 and 3 does not compile because sum_x_y() is a private method that cannot be accessed by CustomPoint class. Option 4 has the correct implementation of the method but does not override sum_x_y as it has a different function name.

6. What is the point of the protected modifier? Why do we have it and how does it work in terms of inheritance?

   Reveal Solution

   The protected modifier allows all child (and grand-child, etc) classes to see the variable or method within the inheritance hierachy. Otherwise, like private, other classes cannot see these items. It’s meant to obviate the need for child classes to use public getters and setters to access these items, especially when we don’t actually want them to be fully public to the rest of the world.

7. Consider the following class

   
   public class Racecar {
   
       private int number; 
       private Driver driver; //assume implemented properly
       protected String sponsor = null;
       public Racecar(int n, Driver d) {
           number = n;
           driver = d;
       }
   
       public String toString() {
           return "Car #" + number + " Driver: " + driver;
       }
       
       protected addSponsor(String sp) {
           sponsor = sp;
       }
   }
   

   Suppose we want to extend this to a FormulaOne class which has a make, e.g., Mercedes. Complete the constructor and toString() method.

   
   public class FormulaOne extends Racecar {
       private String make;
   
       //TODO
   }
   

   Reveal Solution

   
   public class FormulaOne extends Racecar {
   
       private String make;
       public FormulaOne( int n, Driver name, String car_make) {
           super(n, name);
           make = car_make;
   
       }
   
       @Override
       public String toString() {
           return super.toString() +" Make: " + this.make;
       }
       
   }
   
   

8. Using the Racecar and FormulaOne classes above, if we had a main method in a different class than either of those,

   
   public static void main(String args[]) {
   
   
      Racecar r = new Racecar(/* ... some args .. */);
      r.addSponsor("Home Depot"); //<--A
   
      FormulaOne f1 = new FormulaOne(/* ... some args .. */);
      f1.addSponsor("Home Depot"); //<--B
        
   }
   

   Does the code work at mark A or mark B or neither? Explain.

   Reveal Solution

   The code does not work; addSponsor is a protected method, so this class outside of the inheritance hierarchy of RaceCar->FormulaOne would not have access to it since it’s not public.

9. Consider the UML diagram from the notes. Expand this to include an intern. An intern is like an employee, has a manager, unit, but has an expiration on their employment. How does this fit into the UML diagram?

   Include your UML diagram and explanation below in this markdown file.

   Reveal Solution

   The intern should have Employee as its parent class, with an additional Date field called employmentExpiration.

   Additional type left as an exercise to the reader.

10. Give an example of a case where it would make sense to have a public static field.

    Reveal Solution

    Something like a mathematical constant, such as pi, could be declared as such and then called on something like the math class: Math.pi without the need to create an object. (In this instance we would also probably want to declare the field final, which means its value cannot be modified after it is initialized.)