Worksheet: J2

Worksheets are self-guided activities that reinforce lectures. They are not graded for accuracy, only for completion. They are due, on github by 11:59pm on the day of the lecture.

Create a new repo using all the steps in Lab 0 called yourgitusername-worksheet-J2. Then, add the following file to it:

You may submit a single file in any format; Java (with comments), text, and/or markdown (an .md file). It can be called anything (reasonable). For the screenshot required in the last question, you can submit it as an additional file with any common extension and reasonable name.

Note

Attempt to answer these questions before running the code. This will improve your ability to analyize and reason about code without an IDE or compiler. This skill we be helpful on the exams.

Questions

1. Which of the following is an object and which is a basic type?

   
   int a;
   double b;
   int c[] = {1, 2, 3};
   String s = "Hello World";
   

   Reveal Solution

   • In the first two lines, int and double are basic types.
   • In the last two lines, array and String are objects.

2. Two part question:

   (A) What is a static method in Java?

   (B) Why does the main method need to be a static method?

   public class Hello {
       public static void main(String[] args) {
           System.out.println("hello, world");
       }
   }
   

   Reveal Solution

   • Static methods are called directly, or statically, without the need of an object instance.
   • main methods need to be static because it needs to be called without instantiating an object of the class.

3. What is the output of the following programs?

   /* Program 1 */
   public static void main(final String args[]) {
       String choice = new String("A");
       if (choice == "A") {
           System.out.println("Correct");
       }
       else {
           System.out.println("Wrong");
       }
   }
   

   /* Program 2 */
   public static void main(final String args[]) {
       String choice = new String("A");
       if (choice.equals("A")) {
           System.out.println("Correct");
       }
       else {
           System.out.println("Wrong");
       }
   }
   

   Reveal Solution

   • Program 1’s output is Wrong and Program 2’s output is Correct.
   • Similar to the case in C, String is an object type where String choice will make choice have the address of the string on the heap. Thus, in Program 1, the if statement is comparing a memory address to a string; while in Program 2, the .equals() method will compare the content choice is pointing at to the string "A".

4. Does the below program change the season? Why, or why not?

   static void change_season(String str) {
       str = "Spring";
   }
   
   public static void main(final String args[]) {
       String season = "Winter";
       change_season(season);
       System.out.println("The current season is: " + season);
   }
   

   Reveal Solution

   The program does not change the season, because the original season variable, which is a pointer to
   Winter is reassigned to a new string, Spring, inside the method. That breaks the link with the original season in main.

5. What is the output of the main method below? Please explain.

   public class Point {
       double x = 0;
       double y = 0;
   
       public Point(double x, double y) {
           x = x;
           y = y;
       }
   }
   

   public static void main(final String args[]) {
       Point point = new Point(1, 2);
       System.out.println("X: " + point.x + " Y: " + point.y);
   }
   

   Reveal Solution

   The output of the main method is X: 0.0 Y: 0.0. The reason is in the Point class, the constructor does not pass the parameters x and y onto x and y of the Point class.

6. What principle of OOP does the private declaration for variable and functions achieve? Explain.

   Reveal Solution

   The priciple of Information Hiding is achieved by private declaration of variable and functions. The reason is private variable and functions cannot be accessed by other objects.

7. In the Point class below, how does Java choose between the two constructors.

   public class Point {
   
      private double x, y; 
      
      public Point(double x, double y) {
           this.x = x;
           this.y = y;
      }
   
      public Point(Point other) {
          this.x = other.getX();
          this.y = other.getY();
      }
   
   }
   

   Reveal Solution

   Java chooses between the two constructors by checking the type of the argument passed in. When a new Point is called with two doubles, it will use the constructor public Point(double x, double y); in contrast, when a new Point is called with a Point passed in, it will use the constructor public Point(Point other).

8. 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.

9. 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.

10. 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).

11. 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.

12. 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.

13. 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;
        }
        
    }
    
    

14. 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 works at both marks. Each mark is adding a sponsor to two different objects, so there is not a conflict of overwriting the other. Since F1 inherits from Racecar, it also has access to the addSponsor function even though it was not explicitly defined in the F1 class.

15. 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 with this worksheet (in the file you’re submitting, or as a screenshot/photo/etc).

    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.

    Lab this week

    Review the instructions for Lab 1 before you arrive at your lab on Wednesday, so you can make the best use of your limited time there!