Shapes

This exercise contains multiple Java classes that implement different geometric shapes.

Download files: Rectangle.java, Shape.java, Circle.java, Shapes.java, Square.java
Areas: More advanced classes and objects.

Rectangle.java

1	/**
2	* instructor: Ray Ortigas
3	* Date: Summer 2000
4	* http://www.dgp.toronto.edu/~rayo/tutorials/108/2000summer/
5	*/
6
7	// A rectangle.
8	public class Rectangle extends Shape {
9
10	// The height of this rectangle.

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Shape.java

1	/**
2	* instructor: Ray Ortigas
3	* Date: Summer 2000
4	* http://www.dgp.toronto.edu/~rayo/tutorials/108/2000summer/
5	*/
6
7	// A shape.
8	abstract public class Shape {
9
10	// The x-coordinate of this shape.

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Circle.java

1	/**
2	* instructor: Ray Ortigas
3	* Date: Summer 2000
4	* http://www.dgp.toronto.edu/~rayo/tutorials/108/2000summer/
5	*/
6
7	// A circle.
8	public class Circle extends Shape {
9
10	// The radius of this circle.

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Shapes.java

1	/** instructor: Ray Ortigas
2	* Date: Summer 2000
3	* http://www.dgp.toronto.edu/~rayo/tutorials/108/2000summer/
4	*
5	*
6	* A small program to test the classes in the Shape hierarchy.
7	*
8	* The Shape hierarchy looks like this:
9	*
10	* Shape

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Square.java

1	/**
2	* instructor: Ray Ortigas
3	* Date: Summer 2000
4	* http://www.dgp.toronto.edu/~rayo/tutorials/108/2000summer/
5	*/
6
7	// A square.
8	public class Square extends Rectangle {
9
10	// Constructs a new square with side length 'x'.

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Question 1

This exercise consists of four Java classes and one driver program. Can you tell which files implement the class hierarchy and which file is the driver program?

Question 2

Can we instantiate a Shape object?

Question 3

Suppose we add the following code to Shape.java.

  public Shape() {
     // do nothing
  }

That is, we provide Shape with a constructor. Can we then instantiate a Shape object?

Question 4

If we remove the keyword abstract from the class declaration of Shape, can we then instantiate a Shape object?

Question 5

If we remove the abstract method getArea() from Shape.java, can we then instantiate a Shape object?

Question 6

If we remove the keyword abstract from both the class declaration and the method declaration of getArea(), can we then instantiate a Shape object?

Question 7

If we remove abstract from the class declaration of Shape and remove the abstract method getArea(), can we then instantiate a Shape object?

Question 8

Can you guess what would happen if we change the getArea() method to abstract private?

Question 9

If we add the constructor below to Shape.java

  public Shape(int xVal, int yVal) {
     x = xVal;
     y = yVal;
  }

will the program still compile?

Question 10

If we add the constructor below to Shape.java, will the program compile?

  private Shape() {
     // do nothing
  }

Question 11

If we cannot instantiate Shape objects without changing the code, why does the statement Shape[] a = new Shape[5]; in Shapes.java cause no error?

Question 12

Will the statement below cause any compile or run time error?

Shape r = new Rectangle(10, 10);

Question 13

In Rectangle.java, the toString() method uses the code:

  return "x = " + x + "; y = " + y + "; height = "
      + height + "; width = " + width;

Where are the variables x and y from?

Question 14

The variables x and y belong to the Shape class; however, Rectangle.java can access them directly. Is this a good programming practice?

Question 15

If we change x and y to private and leave everything else unchanged, will Rectangle.java be affected?

Question 16

If we change x and y to private, will Circle.java be affected?

Question 17

What is the output of the code segment below?

  Square squ = new Square(5);
  system.out.println(squ.toString());

Question 18

Can you explain the meaning of ((Rectangle) a[1]) on line 59 in Shapes.java?

Question 19

In the context of Shapes.java, will the code ((Rectangle) a[2]).exclusiveRectangleMethod(), cause any compile or run time error?

Question 20

Can a Square object call the exclusiveRectangleMethod() method?