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6. Basic Object-Oriented Programming in Java

• The Java Tutorials - What Is an Object?
• The Java Tutorials - What Is a Class?
• The Java Tutorials - What Is a Package?

Hint:

• Class names start with upper case.
• Names for methods, variables, and packages start with lower case.
• Indent nested blocks consistently.

1) Make a new Eclipse project called shapes or something similar. Your eventual goal will be to make a Circle class with various capabilities (see below), and a test routine that makes some circles and tests them out. Put your Circle class and your test routine in two separate classes, like this:

public class Circle {
	public double radius;
	...
}

public class CircleTest {
	public static void main(String[] args) {
		Circle c = new Circle(...);
		...
	}
}

Capabilities:

• Give the Circle class a radius field of type double.
• Give Circle a getArea method that computes the area (Pi * r * r).
• Give Circle a constructor to which you can pass the radius.
• Have the constructor use the this keyword.

Hint: Unless you have previous Java experience, I strongly recommend you build up to the solution in a piecemeal fashion. For example, first make a Circle class with a radiusfield only, and test it out from the main in CircleTest. Then add the getArea method, and test it. Then add in a constructor. Then test it out again. Then change the constructor to use the this keyword, and test yet again.

Hint: The Circle class does not have a main, so you cannot execute it directly. You only directly run the CircleTest class, similar to the following.

public class CircleTest {
	public static void main(String[] args) {
		for (int i = 0; i < 10; i++) {
			double radius = 10 * Math.random();
			Circle c = new Circle(radius);
			System.out.println("Circle " + i);
			System.out.println("Generated radius: " + radius);
			System.out.println("Stored radius:    " + c.radius);
			System.out.println();
		}
	}
}

2) Modify CircleTest class to create an array of 100 circles, each with a random radius. Print out the sum of the areas of the circles. Also print the biggest and smallest areas.

Hint: Remember that in the two-step array allocation process, the following line only makes space for 100 circles (or, more technically, it allocates an array of 100 null Circle pointers), it does not create any circles: Circle[] circles = new Circle[100];

Hint: To actually create the circles, you have to do a loop as shown below.

for(int i = 0; i < circles.length; i++) {
	circles[i] = new Circle(...);
}

Extra

1) Create a Rectangle class that contains width and height fields. Also give it a getArea method. Again, make a few test cases using a RectangleTest class similar to the following.

public class RectangleTest {
	public static void main(String[] args) {
		for (int i = 0; i < 10; i++) {
			double width = 20 * Math.random();
			double height = 10 * Math.random();
			Rectangle r = new Rectangle(width, height);
			System.out.println("Rectangle " + i);
			System.out.println("Width:  " + r.width);
			System.out.println("Height: " + r.height);
			System.out.println("Area:   " + r.getArea());
			System.out.println();
		}
	}
}

2) Create a Square class with width and getArea. Then, give both Square and Circle setArea methods that let you specify a desired area. Make a few test cases using a SquareTest class like as the following.

public class SquareTest {
	public static void main(String[] args) {
		for (int i = 0; i < 10; i++) {
			double width = 10 * Math.random();
			Square s = new Square(width);
			System.out.println("Square " + i);
			System.out.println("Width:  " + s.width);
			System.out.println("Area:   " + s.getArea());
			System.out.println();
		}
		for (int i = 0; i < 5; i++) {
			double area = 100 * Math.random();
			Square s = new Square(0.0);
			s.setArea(area);
			System.out.println("Square " + i);
			System.out.println("Width:          " + s.width);
			System.out.println("Requested area: " + area);
			System.out.println("Actual Area:    " + s.getArea());
			System.out.println();
		}
	}
}

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