Transcript Java GUI Programming

Java GUI Programming 1
Ellen Walker
Hiram College
What is a GUI?
• Graphical User Interface
– Easier (for most people) to use
– More complex to program
Why Java?
• Java compiles for a virtual machine
• “Write once run anywhere”
– One program looks nice on:
• Linux
• Windows
• Macintosh
• Can also generate “applets” to run from
Web pages (not today)
GUI Programs are Event
Driven
• Event
– Signal that something “interesting” has happened
– Example: user presses a button, user moves the
mouse
• Main program reacts to events
while(true){
Wait for an event
React to it
}
Sequential vs Event-Driven
• Sequential
– Programmer is in complete control
– Program causes a sequence of actions
• Event driven
– User is in complete control
– User causes events, events cause
program actions
Major Libraries Used
• Abstract Window Toolkit (java.awt.*) - for
generating graphics
– Rectangle, Polygon, etc.
• AWT events (java.awt.event.*)
– ActionEvent, MouseEvent, etc.
• SWING (javax.swing.*) - for generating units
to interact with (some units generate events)
– Frames (windows)
– Buttons
– Labels & Text information
Some Swing Classes
• JFrame
– Represents window with title & border
• JLabel
– Displays text or an image (cannot be changed)
• JTextField
– Area for the user to enter one line of text
• JTextArea
– Area for the user to enter multiline text
• JButton
– A push button (to connect to an action)
Structure of a GUI Program
• Main method
– Creates an instance of a GUI class
• GUI class (implements ActionListener)
– Constructor configures components
– Action Performer processes events
• Event dispatcher
– The while loop that waits for events and calls the
action performer
– You don’t have to write this.
WindChill Calculator
• Puts up a window with
– 1 JTextArea (not to be edited)
– 3 Jlabels (F temp., wind, windchill)
– 3 JTextFields (one for each label)
– 1 Jbutton (“Run)
• When “Run” is clicked, windchill temp. is
computed & displayed in its field, using
text from F temp & wind fields
WindChill Calculator
Pseudocode
public class Windchill implements
ActionListener {
static final variables for setup info
instance variables for components
window, labels, textfields, etc.
public Windchill() //constructor
public void actionPerformed(ActionEvent
e) //event processor
}
Constructor
• Configure GUI
– Set window size & default close operation
– Specify properties of objects, e.g.
legendArea is not editable
– Register event listener (associate it with
the button that sends the event)
– Arrange components in the GUI
• window.setVisible(true); //after all that!
Registering Event Listeners
• Every object that can send an event needs to
be associated with a Listener object that
processes that event (using its
ActionPerformed method)
• Usually, the code to set this up is in the
Listener’s constructor
• Example (“this” is the ActionListener class):
– runButton.addActionListener(this);
Arranging Interface Objects
• Every JFrame has a LayoutManager that
determines locations of new components
– FlowLayout - each added object goes to the right
of previous object (or below if there isn’t room to
the right)
– GridLayout - objects are arrayed in a specified
number of columns and/or rows
• JFrame’s add method places each interface
component according to the layout
– E.g. window.add(legendArea);
ActionPerformed
• Takes one parameter, an ActionEvent
– One ActionPerformed method can handle
several different actions (using a case
statement)
• In this case:
– Event is when runButton is pressed
– Action is to compute the windchill and
display it in the appropriate text area