Chapter 61 Programming Exercises

Exercise 1 --- Color Panels

Modify the final version of the percentFat program so that each panel has a different background color. You can do this using the setBackground() method with each panel. Try various colors such as Color.red, Color.yellow, and Color.green

Exercise 2 -- Error Checking

Modify the final version of the percentFat program so that when the user clicks on the doit button, the program checks if there is text in each text field before attempting to use parseInt(). If either or both fields are empty, the program does nothing.

You will need to check if a String is empty by using userIn.equals("").

Improved version: when the button is clicked, check the text fields as above, but now when an empty field is detected, change the background color of the field's panel to red (to show that it is in error.) Set the background of non-empty fields to white (so previous errors are cleared.) Alternatively, you can use setText() with the empty textfield to write an error message into it.

Exercise 3 -- Enabling a Button

This will be more work than the previous exercises. Modify the fat calculator so it works as follows: Start out the program with the doIt button disabled. This is done by using doit.setEnabled( false ); This method will dim the image of the button and disable it from responding to user clicks. Enable the button using doit.setEnabled( true ) only after both text fields contain text.

To do this each text field needs to register the object as a listener. Modify actionPerformed( ActionEvent evt ) so that it can handle events coming from the text fields as well as from the button. Use getText() inside actionPerformed() to check if both text fields contain text. If they do, enable the Button.

A disadvantage of this version is that the user must hit the "enter" key after putting text into a text field.

Exercise 4 -- Floating Point Input

Modify the percentage fat application so that it accepts input text that contains a decimal point. For example, the user might enter "6.5" for grams of fat and "230.9" as calories. To do this, review chapter 11 on floating point input. You will have to do something like:

tempDouble = Double.valueOf( userIn  ) ;
value      = tempDouble.doubleValue() ;

Exercise 5 -- Ideal Gas Law

The formula that relates the pressure of a gas (P) to its volume (V) and its temperature (T) is:

PV = nRT

P is the pressure in atmospheres
V is the volume in liters
n is the number of moles
R is the universal gas constant of 0.082057
T is the temperature in degrees Kelvin

Exercise 6 -- Quadradic Formula

The solutions for the equation

ax2 + bx + c = 0

       
x = ( -b + Sqrt( b2 - 4ac ) ) / 2a,   and

x = ( -b - Sqrt( b2 - 4ac ) ) / 2a