Exercise 1: The Lamp

In this exercise you will model a lamp as a Java object. Make a Lamp class. This will contain at least one instance variable which will be of type boolean and will hold the state of the lamp: i.e., whether it is on or off. In addition, add methods to do the following things: switch the light on and off, and check its current state, i.e., whether it is on or off.

Next, write a launcher class with a main() method to carry out the following tasks:

• create a lamp object;
• turn it on and off;
• print the the lamp's on/off status to the console (i.e., using System.out.println()).

public class Lamp {
	boolean lampOn = false;

	/**
	 * Switch the light on and off.
	 */
	void toggleLamp() {
		lampOn = !lampOn;
	}

	/**
	 * Check the status of the lamp.
	 */
	boolean getLampState() {
		return lampOn;
	}

	/**
	 * Print out the lamp's status to the console -- this is just for
	 * convenience.
	 */
	void reportStatus() {
		if (getLampState())
			System.out.println("The lamp is on.");
		else
			System.out.println("The lamp is off.");
	}

	public static void main(String[] args) {
		Lamp l = new Lamp();

		// turn the lamp on
		l.toggleLamp();
		l.reportStatus();

		// turn it off again
		l.toggleLamp();
		l.reportStatus();
	}
}

Exercise 2: Counting the oddballs

In this exercise, you are to create an integer array of length 15 and then define a method called fill() that fills the array with random numbers. Once you have done this, define some code that loops through the array, and for each element, prints to the console whether the number is even or odd. For example if the array is {2,5,2,8, ...}, the output will be:

a[0] = even
a[1] = odd
a[2] = even
a[3] = even
...

To get a random number, first add the following statement at the beginning of your file:

import java.util.*;

Then the following statements create a new Random object rn, declare a variable r of type int, and assign it the result of calling nextInt() on the Random object:

Random rn = new Random();
int r = rn.nextInt();

To find out whether a number n is even, use the modulo operator % to test whether it is divisible by 2 with no remainder:

n % 2 == 0

In order to do this exercise, you should wrap your methods in a class called ArrayWrapper, and then call them from from a main() method in the same class. Here is an example skeleton class to do this, but you are not required to use it; feel free to write your own:

public class ArrayWrapper {
        int[] a = new int[15];

        public void fill() {
                // TODO: fill the array with random integers.
        }

        public boolean isEven(int index) {
                // TODO: test whether the value of array a at
                // index is even.
        }

        public static void main(String[] args) {
                ArrayWrapper wrapper = new ArrayWrapper();
                wrapper.fill();

                // TODO: Print out the even numbers in the array.
        }
}

Exercise 3: Flipping the Array

In this exercise you are to once again make an array of integers and fill it with random numbers. As in the previous exercise, create a wrapper class to hold your methods. But this time, once the array is filled you are to print the array as it is, then call a method reverse() that reverses the array and print it again.

For example if the array is {1,4,7,3}, the output will be:

a[0] = 1
a[1] = 4
a[2] = 7
a[3] = 3

reversing...

a[0] = 3
a[1] = 7
a[2] = 4
a[3] = 1

import java.util.*;

public class ArrayWrapper {

	private int[] a = new int[15];

	/**
	 * Fill the array with random integers.
	 */
	public void fill() {
		Random rn = new Random();
		for (int i = 0; i < a.length; i++)
			a[i] = rn.nextInt();
	}

	/**
	 * Test whether an array holds an even number at a given index.
	 * 
	 * @param index
	 *            The index to be checked
	 * @return True if the number is even.
	 */
	public boolean isEven(int index) {
		return ((a[index] % 2) == 0);
	}

	/**
	 * @return The length of the array.
	 */
	public int length() {
		return a.length;
	}

	/**
	 * Reverse the elements of the array.
	 */
	public void reverse() {
		int temp;
		for (int i = 0; i < (a.length / 2); i++) {
			// swap elements around
			temp = a[i];
			a[i] = a[14 - i];
			a[14 - i] = temp;
		}
	}

	public static void main(String[] args) {
		ArrayWrapper wrapper = new ArrayWrapper();
		wrapper.fill();

		String evenOrOdd = "";
		System.out.println("Odd and Evens:");
		for (int i = 0; i < wrapper.length(); i++) {
			if (wrapper.isEven(i))
				evenOrOdd = "Even";
			else
				evenOrOdd = "Odd";
			System.out.printf("a[%d] = %s%n", i, evenOrOdd);
		}

				
		System.out.println("Array:");
		for (int i = 0; i < wrapper.length(); i++)
			System.out.printf("a[%d] = %d%n", i, wrapper.a[i]);
		
		System.out.println("Reversing...");
		
		wrapper.reverse();
		for (int i = 0; i < wrapper.length(); i++)
			System.out.printf("a[%d] = %d%n", i, wrapper.a[i]);
	}
}