Due

Purpose

Introduction

The applet given below changes its background or foreground (text) color when you click one of the buttons. There are three new topics that must be introduced before you can write this applet.

Event Listeners

Different components fire different kinds of events. Buttons, for example, fire ActionEvents. An object that listens for ActionEvents is called an ActionListener. When an action listener "hears" an event, it executes its actionPerformed method. The actionPerformed method should then contain the statements you want executed when the user clicks a button.

In order to set this up in Java, we need to do several things.

• We need to tell the compiler we are going to use events by importing the java.awt.event package.
• We need to tell the compiler which objects can act as listeners by adding "implements XXXListener" to the end of the line that gives the name of the class defining those listeners (replace XXX with the kind of event).
• Components need to be told what objects listen for events sent from them. To tell a component about a listener, one must use an addXXXListener method. The parameter to addXXXListener is the listener.
• The listener classes must have the appropriate method written for them. That method will take an XXXEvent as its parameter.

if statements

if (condition)
{
   // statements to execute if condition is true
}
else
{
  // statements to execute if condition is false
}

condition must be an expression that results in a boolean value, for example, one that uses the comparison operators <, >, ==, <=, >= and !=. The else case is optional. The braces are also optional if there is only a single statement in each case.

When execution gets to the beginning of the if, the condition is evaluated. If the condition is true, then the statements after the if are executed and the statements following the else are skipped. If the condition is false, then the statements after the if are skipped and the statements after the else are executed.

For example, assume that grade was declared as an int and warning was declared as a String. Then the following piece of code sets the String to "Failure" if the value stored in grade is less than 60. Otherwise, it sets the String to "None".

if (grade < 60)
{
  warning = "Failure";
}
else
{
  warning = "None";
}

The Math class and random numbers

Methods that don't have anything to do with the state of an object can be declared as static methods. Mathematical functions are a good example of these: in order to compute a sine or cosine, you don't need to know anything about objects. Java supplies a Math class that contains a lot of these functions. To call a static function one gives the name of the class where the name of an object would normally go. For example, Math.sin(0.0) will return the sine of 0 radians.

Math supplies a method called random that returns a random double in the range [0.0, 1.0) (the '[' means that the lower endpoint is included; ')' means that the upper endpoint is not). We can use this method to obtain random int in any desired range by multiplying the result of Math.random() to scale it to the desired range and then truncating it with the (int) type conversion operator. For example, (int)(Math.random() * 10) gives a random int between 0 and 9 inclusive (Math.random() gives a number in [0.0, 1.0), multiplying by 10 gives a number in [0.0, 10.0), and truncating gives an integer in [0, 9]).

Assignment

• Create an applet that contains two Buttons. The default FlowLayout will be just fine. You should declare the Buttons as instance variables instead of local variables in init.
• Declare the Lab4 class as an ActionListener. Be sure to import the java.awt.event package. You will have to write an actionPerformed method for your class. actionPerformed takes an ActionEvent object as its parameter and returns void. You can leave the body of actionPerformed empty for now. Make sure that your code compiles without errors at this point.
• Tell the Buttons that they should send their events to the applet by passing this to their addActionListener methods. (Methods are written from the point of view of the recipient of messages. If the recipient needs to tell something else about itself [here the applet needs to tell the components about itself], it can get its address from this). At this point you should put a System.out.println statement in actionPerformed. When you run the applet, you should see the result of the System.out.println every time you click one of the buttons.
• Create a randomly selected color in actionPerformed. To do this, you will need to use Math.random to generate three random integers in the range 0 to 255 inclusive. You can then create a Color object, passing those three numbers to its constructor. Use setBackground to change the background of the applet to that color. If you run your applet at this point, the background color should change every time you click either button. In the final step you will change things so that background or foreground color changes depending on which button you clicked.
• Add an if statement so that you call setBackground if the "Background" button was clicked to tell the applet to change its background, and you call setForegound twice if the "Foreground" button was clicked to tell the buttons to change their foreground colors. There are two ways to determine which button was clicked. Both use the ActionEvent object that is passed into actionPerformed: 1) use the getSource method to ask the ActionEvent object which component fired it and use == to comare the result to one of your buttons; 2) use the getActionCommand method to ask the ActionEvent method what text was on the button that fired it and use the equals method to compare that to "Background" or "Foreground". After adding this code, your applet should work just like the one given above.

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