Java

OOP GUIDE / WORK

RECTANGLE CLASS 1 SOLUTIONS

TASK – CODE ANALYSIS QUESTIONS

Consider the following class and answer the questions at the bottom:

public class Rectangle

{

public double x, y; //x and y of top left corner

public double w, h; //width and height

public String colour; //ie: "pink" or "blue"

public String toString()

{

String s = "Top left corner: (" + x + "," + y + "), ";

s = s + "Width: " + w + ", Height: " + h + ", ";

s = s + "Colour: " + colour;

return s;

}

public double area()

{

return w * h;

}

}

a) What is the name of the class?

Answer: Rectangle

b) What is the name of the file that this code gets stored into?

Answer: Rectangle.java

c) How many instance variables do this class have?

Answer: 5

d) How many datafields do this class have?

Answer: 5 (datafield is simply a different name for instance variable – they are the same thing)

e) How many constructors does this class have?

Answer: 0

f) How many instance methods does this class have?

Answer: 2 (toString() and area())

g) What type of data does the toString() method return?

Answer: String

h) Which instance variables does the toString() method use?

Answer: All five instance variables (x, y, w, h and colour)

i) What type of data does the area() method return?

Answer: double

j) Which instance variables does the area() method use?

Answer: w and h

k) Write the code needed to create a rectangle that has (x,y) = (4,9), a width of 6, a height of 3 and a “red” colour.

Answer:

Since there is no constructor, we have to create the object with the default constructor and then set each instance variable individually. (Painful!!!)

Rectangle r = new Rectangle();

r.x = 4;

r.y = 9;

r.w = 6;

r.h = 3;

r.colour = "red";

l) Where would your place the above code to construct the rectangle?

Answer:

It can be placed in any other class. For now, we would most likely place it inside of a main function that is inside a class named something like RectangleTester.

m) What is the statement needed to output to screen the area of the rectangle that you created above?

Answer:

System.out.println(r.area());