CS 051

Fall 2011

Lecture 15

GUI Objects

We have seen several types of GUI objects so far:

• JComboBox -- a drop down menu where we can select different items
• JButtons -- a labeled button that can be pressed to make something happen
• JSlider -- a button that can slide back and forth to select any value in a range
• JPanel -- a "container" that can hold other types of GUI objects. Even other JPanels!

NOTE: the object obtained from our call to getContentPane is also a container, like the
JPanel objects.

To use GUI objects, our code must do several things:

• Import the appropriate library that defines the GUI, usually javax.swing.* or java.awt.*
• Construct and initialize the component. (e.g., button = new JButton("Start Game");
• Add the object to the content pane or to a JPanel that is added to the content pane.
  Then validate the content pane.

Many GUI objects allow us to use something called a listener. A listener lets the program
respond immediately when the user interacts with the GUI object.

We have seen two types of listeners so far:

• ActionListener -- responds to a single action, like a button press or drop down selection
• ChangeListener -- responds to a continuously changing event, like sliding a slider

To use a listener, our code needs to do several things:

• Import the appropriate library that defines the listener, usually javax.swing.event.* or java.awt.event.*
• Make sure the class that uses the listener implements the appropriate listener interface.
• Define the method that the listener interface requires.
• Add the appropriate componenet to the listener (for all of our class examples, we've added
  "this" to the listener).

We have seen different ways to place our GUI objects on the screen to make them look nice:

• BorderLayout -- used by content pane. Objects may be placed in North, South, East, West, or
  center locations.
• FlowLayout -- requires no configuration from user. Tries to place the objects in a nice way.
  JPanel objects default to FlowLayout, but can be told to use other layouts.
• GridLayout -- allows user to specify number of evenly spaced rows and columns for placing objects.

You can find information on all of the above topics in the GUI cheat sheet located on the class web page.

The class example PanelComboBox shows two JComboBoxes added to a JPanel that uses FlowLayout.

The class example BadButtonPanelDrawingProgram shows three JButton objects and one JComboBox added to a
JPanel that uses FlowLayout. The result is unappealing because there is not
enough room to see the labels on the buttons.

The class example ButtonPanelDrawingProgram fixes this problem by adding the three JButtons to a JPanel,
adding the JComboBox to a separate JPanel, then adding both of these JPanel
objects to a third JPanel object that uses GridLayout with two rows and one column. The
effect is much more appealing.

The class example ColorSlider used JSliders attached to ChangeListeners to create a simple program where the user can
select any color by selecting any possible combination of red, green, and blue values, and
displays that color on the screen. Implementing the ChangeListener interface requires that
the stateChanged method be implemented.