Examples

We have now seen how to perform operations on variables, and how to get user input. Let’s practice with two complete example programs.

Area of a rectangle

Suppose we want to ask the user for the length and width of a rectangle, and then print out the area of the rectangle.

Here are the steps we will need:

• Declare all the variables for the program. This includes the length, width, and area of the rectangle.
• Set up a Scanner since we will need user input.
• Ask the user to enter the width. Read what was typed into the width variable.
• Ask the user to enter the length. Read what was typed into the length variable.
• Assign area to be the width times the length (which is how the area of a rectangle is computed)
• Print the area to the screen

Here is the complete program:

import java.util.*;
public class Rectangle
{
    public static void main(String[] args)
    {
        Scanner s = new Scanner(System.in);
        int length;
        int width;
        int area;
        System.out.print("Enter the width: ");
        width = s.nextInt();
        System.out.print("Enter the length: ");
        length = s.nextInt();
        area = length*width;
        System.out.printf("The area is: %d%n", area);
    }
}

Temperature calculator

Next, suppose we want to ask the user for a temperature in Celsius, and then print out the result in Fahrenheit. Here are the steps we will need:

• Declare all the variables for the program. This includes the temperature in Fahrenheit (f) and the temperature in Celsius (c).
• Set up a Scanner since we will need user input.
• Ask the user to enter the temperature in Celsius. Read what was typed into the c variable.
• Assign f to be 9/5*c + 32 (that is the formula for converting from Celsius to Fahrenheit)
• Print the temperature in Fahrenheit (f) to the screen

Here is the complete program:

import java.util.*;
public class Temperature
{
    public static void main(String[] args)
    {
        Scanner s = new Scanner(System.in);
        double f;
        double c;
        System.out.print("Temperature in Celsius: ");
        c = s.nextDouble();
        f = (9.0/5.0)*c + 32;
        System.out.printf("In Fahrenheit: %f%n", f);
    }
}

Note that we changed the conversion formula to be 9.0/5.0*c + 32 instead of 9/5*c + 32. Can you think why that would be? (Remember integer division – if we divide two whole numbers, the decimal portion of the result is cut off. Since 9 and 5 are both whole numbers, 9/5 is just 1, the whole number portion. However, 9.0 and 5.0 have decimals, so 9.0/5.0 is 1.8, as we want.)