GRAPHICAL USER INTERFACE (GUI) AND OBJECT-ORIENTED DESIGN (OOD) - Java Programming: From Problem Analysis to Program Design - page 348

The buttons calculateB and exitB can be placed into the container pane by using the
method add . The following statements add these buttons to the pane :

pane.add(calculateB);
pane.add(exitB);

Now you have two more objects in the container, so you need to modify the
GridLayout to accommodate five rows and two columns, and then add all the compo-
nents. The following statements create the required grid layout and add the labels, text
fields, and buttons to the container pane :

pane.setLayout( new GridLayout(5,2)); //specify the layout

pane.add(lengthL); //add the label lengthL

pane.add(lengthTF); //add the text field lengthTF

pane.add(widthL); //add the label widthL

pane.add(widthTF); //add the text field widthTF

pane.add(areaL); //add the label areaL

pane.add(areaTF); //add the text field areaTF

pane.add(perimeterL); //add the label perimeterL

pane.add(perimeterTF); //add the text field perimeterTF

pane.add(calculateB); //add the button calculateB

pane.add(exitB); //add the button exitB

Notice that the preceding add statements place the components from left to right and
from top to bottom.

HANDLING AN EVENT
You have now learned how to create a window, how to create a container, and how to
create labels, text fields, and buttons.
Now that you can create a button, such as calculateB , you need to specify how such
a button should behave when you click it. For example, when you click the button
calculateB , you want the program to calculate the area and perimeter of the
rectangle and display these values in their respective text fields. Similarly, when you
click the button exitB , the program should terminate.
Clicking a JButton creates an event, known as an action event, which sends a
message to another object, known as an action listener. When the listener receives
the message, it performs some action. Sending a message or an event to a listener
object simply means that some method in the listener object is invoked with the
event as the argument. This invocation happens automatically; you will not see the
code corresponding to the method invocation. However, you must specify two
things: