Lab 12: Graphical User Interfaces (GUI's) in Java
For most of the relevant Information on how to build GUIs in Java, see the following reference: Java GUI Programming Primer
The content below discusses material not explicitly covered in the above reference.
Few people code GUI's by hand these days--it's just too complicated. Typically, one would use a "GUI builder" to drag-and-drop GUI components onto a design canvas, greatly simplifying and speeding up the GUI creation process. But it is very helpful to know the basics of how to build a GUI by hand because one often needs to go in an manually tweak the GUI code even when using a sophisticated GUI builder tool.
Basic structure of a simple GUI application:
import java.awt.*; import javax.swing.*; import java.awt.event.*; class MyGUIApp extends JFrame { // Use fields for GUI components that need to be referenced outside of the GUI initialization process (initGUI) /** * Constructor for the GUI. */ public MyGUIApp() { // initialize fields here. Surrounding your code in a protective try-catch is recommended. setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // Causes the application to end if the GUI frame is closed. initGUI(); // initialize the GUI components } /** * Actually start the GUI */ public void start() { // Do any last second initializations, if needed. setVisible(true); // Make the GUI visible } /** * Initialize the GUI components */ private void initGUI() { Container contentPane = getContentPane(); // all GUI components are added to the frame's ContentPane. // You can also just call the accessor each time as well and not use a local variable. // instantiate, initialize and connect all the GUI's components here. // Don't forget to add them to the container that holds them, e.g. the ContentPane. } /** * Main method that starts the app. * In general, this would not be in this class but rather in a separate "controller" class * with slightly different coding (as required for a true Model-View-Controller system). * We put it here for now, for simplicity's sake only. */ public static void main(String[] args) { // initialize and start the GUI on the GUI thread. SwingUtilities.invokeLater(new Runnable() { public void run() { MyGUIApp view = new MyGUIApp(); // instantiate the GUI but don't show it yet. // Model classes would be instantiated here plus any other required objects. view.start(); // Start the application by making the GUI visible. } }); } }
Some typical GUI initializations:
Set title and size of the frame:
setTitle("My Incredible GUI App"); // set the frame's title setSize(400, 300); // set frame size to 400x300
Panels in main frame:
JPanel controlPnl = new JPanel(); controlPnl.setBackground(Color.BLUE); // Set panel color to blue contentPane.add(controlPnl, BorderLayout.NORTH); // Fixed-size panel for control components, e.g. buttons, textfields, etc, at the top of the frame. JPanel displayPnl = new JPanel(); contentPane.add(displayPnl, BorderLayout.CENTER); // Adjustable-size panel for display components, e.g. text areas, etc, in the middle of the frame.
A Button with a listener on the control panel:
JButton runBtn = new JButton("Run!"); runBtn.addActionListener(new ActionListener() { /** * This method runs when the button is clicked. */ public void actionPerformed(ActionEvent evt) { // Do stuff here. Generally, delegate to a non-GUI object (the "model"). } }); controlPnl.add(runBtn);
A Label and a TextField with a listener on the control panel:
JLabel infoLbl = new JLabel(); infoLbl.setText("Enter text here:"); controlPnl.add(infoLbl); final JTextField infoTF = new JTextField("default text"); // don't need "final" below if this line was defining a field instead of a local variable. infoTF.setPreferredSize(new java.awt.Dimension(75, 23)); // set default text field size to 75x23 pixels. infoTF.addActionListener(new ActionListener() { /** * This method runs when "Enter" is pressed after entering text into the text field. */ public void actionPerformed(ActionEvent evt) { String text = infoTF.getText(); // the text of the text field. // Do stuff here. Generally, delegate to a non-GUI object (the "model"). } }); controlPnl.add(infoTF);
A text area with scroll bars in the center of the frame:
JScrollPane scrollPn = new JScrollPane(); contentPane.add(scrollPn, BorderLayout.CENTER); // No center panel needed. ScrollPane added directly to frame. JTextArea displayTA = new JTextArea("initial text"); scrollPn.setViewportView(displayTA); // put the text area in the scroll pane
SerialVersionUID compiler warning:
A very common compiler warning to receive is the following for your frame class and other customized GUI components:
The serializable class XXX does not declare a static final serialVersionUID field of type long
If you do, add the following field to the class:
private static final long serialVersionUID = 42L; // Use a random, unique integer value here
For more information, see this web page .