CS2030S-Problem Set 1 Basics of Object-Oriented Programming Solved

1.    Consider the following two classes:

class P { private int x;

void changeSelf() { x = 1;

}

void changeAnother(P p) {

p.x = 1;

}

}

class Q { void changeAnother(P p) {

p.x = 1;

}

}

(a)    Which line(s) above violate the private access modifier of x?

(b)   What does this say about the concept of an “abstraction barrier”?

2.    Consider the following definition of a Vector2D class:

class Vector2D { private double x; private double y;

Vector2D(double x, double y) { this.x = x; this.y = y;

}

void add(Vector2D v) { this.x = this.x + v.x; this.y = this.y + v.y;

// line A

}

}

(a)    Suppose that the following program fragment is executed in JShell, show the content of the stack and the heap when the execution reaches the line labelled A above.

Vector2D v1 = new Vector2D(1, 1); Vector2D v2 = new Vector2D(2, 2); v1.add(v2);

Label your variables and the values they hold clearly. You can use arrows to indicate object references.

(b)   Suppose that the representation of x and y have been changed to a double array:

class Vector2D { private double[] coord2D;

...

}

i.      What changes do you need for the other parts of class Vector2D?

ii.    Would the program fragment in 2a above be valid?

Show the content of the stack and the heap when the execution reaches the line labelled A again.

3. Below is our familiar Point classes augmented with a toString method.

class Point { private final double x; private final double y;

Point(double x, double y) { this.x = x; this.y = y;

}

double distanceTo(Point otherpoint) { double dispX = this.x - otherpoint.x; double dispY = this.y - otherpoint.y; return Math.sqrt(dispX * dispX + dispY * dispY);

}

@Override

public String toString() { return "(" + this.x + ", " + this.y + ")";

}

}

The toString method provides a way to output an object in a more meaningful way, rather than just a reference value. To illustrate using JShell,

jshell> /open Point.java

jshell> Point p = new Point(0, 0) p ==> (0.0, 0.0)

You are also given the Circle class.

class Circle { private final Point centre; private final double radius;

Circle(Point centre, double radius) { this.centre = centre; this.radius = radius;

}

boolean contains(Point point) { return centre.distanceTo(point) <= radius;

}

@Override

public String toString() { return "Circle centred at " + this.centre +

" with radius " + this.radius; }

}

We can define an array of five points as follows:

jshell> Point[] points = new Point[]{new Point(0,0), new Point(0,-1),

...> new Point(1,0), new Point(0,1), new Point(-1,0)}; points ==> Point[5] { (0.0, 0.0), (0.0, -1.0), (1.0, 0.0), (0.0, 1.0), (-1.0, 0.0) }

(a)    Within JShell, define a method countCoverage that takes in a Circle object, and an array of Point objects. This method will return the number of points contained within the circle.

(b)   Write single line tests in JShell to test the correctness of the method. For example,a circle centred at the origin with radius 1.0 contains all five points; a circle centred at (0.0,−1.0) with radius 1.0 contains two points.