Worksheet: J2

Worksheets are self-guided activities that reinforce lectures. They are due Thursdays the week they are assigned.

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:

Item	Points
Answers are completed (for content)	100

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!

Questions

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";

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");
    }
}

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");
    }
}

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);
}

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);
}

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

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();
   }

}

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.

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());
    }

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

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.

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

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
}

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.

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.