Java Swing interview questions

1) Can a class be it’s own event handler? Explain how to implement this.

Ans: Sure. an example could be a class that extends Jbutton and implements ActionListener. In the actionPerformed method, put the code to perform when the button is pressed.

2) Why does JComponent have add() and remove() methods but Component does not?

Ans: because JComponent is a subclass of Container, and can contain other components and jcomponents.

3) How would you create a button with rounded edges?

Ans: there’s 2 ways. The first thing is to know that a JButton’s edges are drawn by a Border. so you can override the Button’s paintComponent(Graphics) method and draw a circle or rounded rectangle (whatever), and turn off the border. Or you can create a custom border that draws a circle or rounded rectangle around any component and set the button’s border to it.

4) If I wanted to use a SolarisUI for just a JTabbedPane, and the Metal UI for everything else, how would I do that?

Ans: in the UIDefaults table, override the entry for tabbed pane and put in the SolarisUI delegate.

5) What is the difference between the ‘Font’ and ‘FontMetrics’ class?

Ans: The Font Class is used to render ‘glyphs’ - the characters you see on the screen. FontMetrics encapsulates information about a specific font on a specific Graphics object. (width of the characters, ascent, descent)

6) What class is at the top of the AWT event hierarchy?

Ans: java.awt.AWTEvent. if they say java.awt.Event, they haven’t dealt with swing or AWT in a while.

7) Explain how to render an HTML page using only Swing.

Ans: Use a JEditorPane or JTextPane and set it with an HTMLEditorKit, then load the text into the pane.

8) How would you detect a keypress in a JComboBox?

Ans: This is a trick. most people would say ‘add a KeyListener to the JComboBox’ - but the right answer is ‘add a KeyListener to the JComboBox’s editor component.’

9) Why should the implementation of any Swing callback (like a listener) execute quickly?

Ans: Because callbacks are invoked by the event dispatch thread which will be blocked processing other events for as long as your method takes to execute.

10) In what context should the value of Swing components be updated directly?

Ans: Swing components should be updated directly only in the context of callback methods invoked from the event dispatch thread. Any other context is not thread safe.

11) Why would you use SwingUtilities.invokeAndWait or SwingUtilities.invokeLater?

Ans: I want to update a Swing component but I’m not in a callback. If I want the update to happen immediately (perhaps for a progress bar component) then I’d use invokeAndWait. If I don’t care when the update occurs, I’d use invokeLater.

12) If your UI seems to freeze periodically, what might be a likely reason?

Ans: A callback implementation like ActionListener.actionPerformed or MouseListener.mouseClicked is taking a long time to execute thereby blocking the event dispatch thread from processing other UI events.

13) Which Swing methods are thread-safe?

Ans: The only thread-safe methods are repaint(), revalidate(), and invalidate()

14) Why won’t the JVM terminate when I close all the application windows?

Ans: The AWT event dispatcher thread is not a daemon thread. You must explicitly call System.exit to terminate the JVM.