by Kim Bruce
3/10/98
Java's GUI components include labels, text fields, text areas, buttons, choice buttons, etc. The Abstract Windowing Toolkit (AWT) also includes containers which can include these components. Containers include frames (windows), canvases (which are used to draw on), and panels (which are used to group components). Panels and canvases are contained in frames (windows) while buttons and other components can be placed either directly on frames or in panels inside the frames.
These GUI components are automatically drawn whenever the window they are in is drawn. Thus we will not need to explicitly put in commands to draw them.
These GUI components are handled using Java's event model. (Note: the Java 1.1. event model is quite different from the 1.0 event model. Thus those texts discussing the earlier even model will not be useful to you. Students taking earlier offerings of CSCI 136 used the Java 1.0 model, so be wary of getting them to help you.)
When a user interacts with a component (clicks on a button, types in a field, chooses from a pop-up menu, etc.), an event is generated by the component that you interact with. For each component of your program, the programmer is required to designate one or more objects to "listen" for events from that component. Thus if your program has a button labelled "start" you must assign one or more objects which will be notified when a user clicks on the button. We begin our discussion of GUI programming in Java with a discussion of buttons in more detail.
Note that Chapters 8 and 9 of Core Java go into a great deal of detail on the Java event model and using GUI components. We will be much less complete here. See the text for more detailed instructions on their use.
Button is a class in package java.awt which represents buttons on the screen. The constructor is:
public Button(String label);
which, when executed, creates a button with "label" printed on it.
Generally the button is large enough to display label. There is also a
parameterless constructor that creates an unlabeled button.Buttons respond to a variety of messages. Button, like virtually all other components, supports the method:
public void setBackground(Color buttonColor);
which changes the color of the button. You may recall from an earlier
lab that Color is a predefined class with many constants representing
colors:
Color.black, Color.darkGray, Color.gray, Color.lightGray,
Color.white, Color.blue, Color.cyan, Color.green, Color.yellow, Color.magenta,
Color.red, Color.orange, and Color.pink. You can also create
your own new colors with the Color constructor (see documentation for
the Color class in java.awt).
You add an "ActionListener" to a button with the method:
public void addActionListener(ActionListener listener);
We will discuss how to create ActionListeners below.
We can add a button to a frame or panel by sending the message:
myFrame.add(startButton);
Normally we include such code in the constructor for the frame or
panel. Hence usually we just write this.add(startButton) or simply
add(startButton).Below is some sample code for a class, ButtonDemo, representing a specialization of Frame with two buttons. The class extends Frame, which is part of java.awt. The constructor for Frame takes a String parameter and creates a window with that string in the title bar. The constructor for ButtonDemo calls the superclass constructor, and then sets the size of the new Frame to be 400 x 200. The setLayout command tells the new Frame that new components should be added from left to right across the panel. If it runs out of space new components will be added in new rows, again from left to right. The code for creating buttons and setting their backgrounds should be self-explanatory. The add commands add the buttons to the frame. We explain the ButtonListener class below. It creates objects which perform actions in response to button clicks. Thus we tell the two buttons (with the addActionListener method) to notify myButtonListener when the user clicks on them. Finally, note that we had to import java.awt.* because Frame, Button, and FlowLayout are classes in the java.awt package.
import java.awt.*;
public class ButtonDemo extends Frame
{
protected Button startButton, stopButton;
// Constructor sets features of the frame, creates buttons,
// adds them to the frame, and assigns an object to listen
// to them
public ButtonDemo()
{
super("Button demo"); // calls Frame constructor
// which adds title to window
setSize(400,200); // sets the size of the window
// sets layout so objects added go from left to right
// until fill up row and then go to next row
setLayout(new FlowLayout());
// create two new buttons w/labels start and stop
startButton = new Button("Start");
stopButton = new Button("Stop");
// set backgrounds of buttons
startButton.setBackground(Color.green);
stopButton.setBackground(Color.red);
add(startButton); // add buttons to frame
add(stopButton);
// create an object to listen to both buttons:
ButtonListener myButtonListener = new ButtonListener();
// tell buttons that myButtonListener should be notified
startButton.addActionListener(myButtonListener);
stopButton.addActionListener(myButtonListener);
}
// Trivial main program associated with ButtonDemo
// Simply creates it and shows it.
public static void main(String[] main){
// Create an instance of Buttons
ButtonDemo app = new ButtonDemo();
app.show(); // Show it on the Mac screen
} // Leave this out & you won't see it!
}
The main program associated with the class simply creates an object from the class and tells it to display itself on the screen. Main programs associated with frames almost always do this minimal amount of work.
Objects which implement the ActionListener interface are required to implement a method actionPerformed which takes a parameter of type ActionEvent. When an action occurs to a button, all objects which have been added as ActionListener's for that button are notified by calling their actionPerformed method with a parameter with information on the exact event that occurred. Thus the system automatically creates the ActionEvent object and sends it along to the listener. You are unlikely to ever need to manually create an ActionEvent object in this course.
The most useful method of ActionEvent is
public String getActionCommand();
If
you send this message to an ActionEvent representing a click of a button, it
will return a string corresponding to the label of the button. The class
ButtonListener below implements ActionListener, and hence an object of this
class can be added to a button as a listener. The actionPerformed method is
very simple. It uses getActionCommand to obtain the label of the button
clicked and then depending on what the button was, prints a different message.
(Recall that the same ActionListener was added to each of the two buttons in
ButtonDemo so getActionCommand needs to figure out which one was clicked.)
Note that because this class is so simple (it doesn't even have instance
variables) it needs no explicit constructor. Instead, like all classes it has
a default constructor (with no parameters) that just allocates storage for new
objects constructed. Note that we must import the java.awt.event package
because ActionEvent and the ActionListener interface are both in that
package.
import java.awt.event.*;
public class ButtonListener implements ActionListener{
public void actionPerformed(ActionEvent evt)
{
// Get label of the button clicked in event passed in
String arg = evt.getActionCommand();
if (arg.equals("Start"))
System.out.println("Start button");
else if (arg.equals("Stop"))
System.out.println("Stop button");
}
}
The
following illustrates the frame created when this program is executed. Rather
than printing the message on the output window, this particular applet just
displays it in the status bar.
While the code above for a class which is an extension of Frame and another class implementing ActionListener, it seems a bit heavy to have to create a completely separate class (which goes in a separate file) in order to create a listener for the two buttons.
Two alternatives are possible. One is to let the frame itself be the ActionListener for the button. We can do this by changing the original class header to read:
public class ButtonDemo extends Frame implements ActionListener
We
must also copy the actionPerformed method from class ButtonListener, and
finally, we change the two statements adding the ActionListener to have the
object itself do the listening:
// tell buttons that this object should be notified
startButton.addActionListener(this);
stopButton.addActionListener(this);
This is the style suggested in Core Java for handling action events in simple cases. The advantage is that now we only need one class instead of two. However it is generally good design to separate the code for building and handling GUI components from the code which actually does the work spawned by the action events. Moreover, there is another style which is almost as simple, but more general. It involves the use of what are called "inner classes".
Java 1.1 allows classes to be nested in each other. Often when dealing with events we will have a special class which is so special that it will really only ever be used in conjunction with another class. Rather than making this auxiliary class public, we can stick it inside of the other class. Thus we could include the ButtonListener class inside of ButtonDemo. The code now would have the following structure:
import java.awt.*;
import java.awt.event.*;
public class ButtonDemo extends Frame
{
protected Button startButton, stopButton;
public ButtonDemo()
{
...
// create an object to listen to both buttons:
ButtonListener myButtonListener = new ButtonListener();
// tell buttons that myButtonListener should be notified
startButton.addActionListener(myButtonListener);
stopButton.addActionListener(myButtonListener);
}
public static void main(String[] main){
... // As before
}
protected class ButtonListener implements ActionListener{
public void actionPerformed(ActionEvent evt)
{
... // As before
}
}
}
Notice
that ButtonListener is now declared to be protected, meaning that it can only
be used inside the containing class, ButtonDemo. The method actionPerformed is
still public, however. If it were declared protected, then it would only be
available from within ButtonListener and not within ButtonDemo (and we need to
use it in the constructor for ButtonDemo).These nested classes would now be contained within a single file named ButtonDemo.java. Another advantage of using nested classes is that all of the instance variables (and methods) of the outer class are visible inside the inner class. This can be handy for picking up information from other components of the outer class.
A Label is a very simple component which contains a string. The constructors are
public Label() // creates label with no text
public Label(String text) //create label with text
The methods available are
public String getText() // return label text
public void setText(String s) // sets the label text
However, we rarely change the text in Labels.
public TextField () // creates text field with no text
public TextField (int columns)
// create text field with appropriate # of columns
public TextField (String s) // create text field with s displayed
public TextField (String s, int columns)
// create text field with s displayed & approp. width
Methods
public void setEditable(boolean s)
// if false the TextField is not user editable
public String getText() // return label text
public void setText(String s) // sets the label text
Many other methods are also available for this component (see also the
documentation for its superclass, TextComponent).When the user types into a text field and then hits the return or enter key, it generates an event which can be handled by the same kind of ActionListener used with Buttons. However, in this case there is no useful String to be returned from getActionCommand. Instead we use the method:
public Object getSource();
which
actually comes from the class EventObject, which is an ancestor of ActionEvent.
Thus in order to respond to the user's hitting of the return key while typing
in a TextField myField, we can write:
public void actionPerformed(ActionEvent evt)
{
if(myField == evt.getSource()){
String contents = myField.getText();
System.out.println("The field contained: "+contents);
} else ...
}
Notice
that this use of myField in actionPerformed only makes sense if we either have
an inner class (and hence have access to all of the instance variables of the
outer class) or if the Frame or Panel where myField is defined is the
associated ActionListener.If for some reason you'd like to be notified every time any change is made to the TextField (whether or not the user hit the return key), one can associate a TextListener to the field. See details in the on-line documentation
The constructors are
public TextArea(int rows, int columns)
// create text field with appropriate # rows & columns
public TextArea(String s, int rows, int columns)
// create text field with rows, columns, & displaying s
Methods
public void setEditable(boolean s)
// if false the TextArea is not user editable
public String getText() // return text in TextArea
public void setText(String s) // sets the text
Unlike TextField, hitting return or enter does not generate
an event. See the example Book or Sort (but ignore layout
issues) in the class examples folder.
The constructor is:
public Choice() // create new choice button
The most useful methods are:
public void addItem(String s) // add s to list of choices
public int countItems() // return # choices in list
public String getItem(int index) // return item at index
public String getSelectedItem() // return selected item
public int getSelectedIndex() // return index of selected
When a user selects an item it generates an ActionEvent which can be
handled as usual with an actionPerformed method. One can determine
whether the choice button generated the event by sending the event the
getSource() method, just as we did with TextFields.
Please have your class implementing ActionListener be an inner class as illustrated above. You may wish to download the folder Button Application from the Assignment 5 folder on Cider Press as it is an example which uses the inner class style of handling the buttons.
Your homework should be turned in by Sunday, March 15 by midnight, though I hope you will attempt to finish it today in lab or by Thursday at the latest as I will try to provide the next assignment by Friday.
Because this week's program should be significantly easier than usual, it will count for 10 points rather than the usual 20.