Java Swing Question and Answer

Question Set 1 :

1) What is Event-Driven-Thread (EDT) in Swing?

Event-Driven-Thread or EDT is a special thread in Swing and AWT. Event-Driven Thread is used to draw graphics and listen for events in Swing. You will get a bonus point if you able to highlight that time consuming operations like connecting to database, opening a file or connecting to network should not be done on EDT thread because it could lead to freezing GUI because of blocking and time consuming nature of these operations instead they should be done on separate thread and EDT can just be used to spawn those thread on a button click or mouse click.

2) Does Swing is thread safe? What do you mean by swing is not thread-safe?

This swing interview questions is getting very popular now days. Though it’s pretty basic many developer doesn't understand thread-safety issue in Swing. Since Swing components are not thread-safe it means you can not update these components in any thread other than Event-Driven-Thread. If you do so you will get unexpected behavior. Some time interviewer will also ask what are thread-safe methods in swing which can be safely called from any thread only few like repaint() and revalidate().

3) What are differences between Swing and AWT?

One of the classic java swing interview questions and mostly asked on phone interviews. There is couple of differences between swing and AWT:

1) AWT component are considered to be heavyweight while Swing component are lightweights

2) Swing has plug-gable look and feel.

3) AWT is platform depended same GUI will look different on different platform while Swing is developed in Java and is platform dependent.

4) Why Swing components are called lightweight component?

Another popular java swing interview question, I guess the oldest that is generally comes as follow-up of earlier question based on your answer provided. AWT components are associated with  native screen resource and called heavyweight component while Swing components is uses the screen resource of an ancestor instead of having there own and that's why called lightweight or lighter component.

5) What is difference between invokeAndWait and invokeLater?

This swing interview question is asked differently at different point. some time interviewer ask how do you update swing component from a thread other than EDT, for such kind of scenario we use SwingUtilities.invokeAndWait(Runnable r) and SwingUtilities.invokeLetter(Runnable r) though there are quite a few differences between these two, major one is invokeAndWait is a blocking call and wait until GUI updates while invokeLater is a non blocking asynchronous call. In my opinion these question has its own value and every swing developer should be familiar with these questions or concept not just for interview point of view but on application perspective.

6) Write code for JTable with custom cell editor and custom cell renderer?

Now comes harder part of swing interviews, questions asked in this part of Swing interview is mostly about writing code and checking developer’s capability of API familiarity, key concept of swing code etc.

JTable is one of favorite topic of all Swing interviews and most popular questions on swing interviews are from JTable why? Because here interviewer will directly asked you to write code another reason is JTable heavily used in all Electronic trading GUI. GUI used for online stock trading uses JTable to show data in tabular format so an in depth knowledge of JTable is required to work on online trading GUI developed in Swing. While this question is just an example questions around JTable are mostly centered around updating table, how do you handle large volume of data in table, using customize cell renderer and editor, sorting table data based on any column etc. so just make sure you have done quite a few handsone exercise on JTable before appearing for any java swing interview in IB.

7) Write code to print following layout (mainly focused on GridBag layout)?

After JTable second favorite topic of swing interviewer is GridBagLayout. GridBagLayout in swing is most powerful but at same time most complex layout and a clear cut experience and expertise around GridBagLayout is desired for developing Swing GUI for trading systems. No matter whether you are developing GUI for equities trading, futures or options trading or forex trading you always required GridBagLayout. Swing interview question on GridBagLayout will be mostly on writing code for a particular layout just like an example shown here. In which six buttons A, B, C, D, E and F are organized in certain fashion.

8) How do you handle opening of database, file or network connection on a click of button?

This one is one of the easy java swing interview question. Interviewer is interested on whether you know the basic principle of Java GUI development or not. answer is you should not do this operation in EDT thread instead  spawn a new thread from actionListener or button and disable the button until operation gets completed to avoid resubmitting request. Only condition is that your GUI should always be responsive no matter what happens on network connection or database connection because these operations usually take time.

9) Prediction of output of code?

This is another category of Swing interview questions asked in IB whether they will give you code and asked what would be the output , how will the GUI look like. This type of question is based upon how well you understand and visualize the code. Whether you are familiar with default layout manager of various component classes or not e.g. default layout of JFrame is BorderLayout. So do some practice of these kinds of java Swing interview questions as well?

10) Question around JList like Creating a Jlist component such which should contain all the asset classes like stocks, futures, options and derivatives etc in form of String. But constraint is JList should always be sorted in Ascending order except that all assets which begins with "Electronic trading” appears on top?

This is an excellent java swing interview questions which is based on real world task and focus on JList component, Sorting and customizing JList model. For sorting you can use comparable in Java and by customizing the JList model you can display content as requested. Once you successfully answer this question there may be some followup on customizing the color of cell etc which can be done by customizing renderer.

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

A: 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.

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

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

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

A: 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.

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

A: in the UIDefaults table, override the entry for tabbed pane and put in the SolarisUI delegate. (I don’t know it offhand, but I think it’s "com.sun.ui.motiflookandfeel.MotifTabbedPaneUI" - anything simiar is a good answer.)

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

A: 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)

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

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

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

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

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

A: 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.’

19) Why should the implementation of any Swing callback (like a listener) execute quickly?   A: 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.

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

A: 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?

21) Why would you use SwingUtilities.invokeAndWait or SwingUtilities.invokeLater?              A: 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.

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

A: 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.

23) Which Swing methods are thread-safe?                                                              

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

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

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

25 What is the difference between a Choice and a List?
A: A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.

26 what interface is extended by AWT event listeners?
A: All AWT event listeners extend the java.util.EventListener interface.

27 what is a layout manager?
A: A layout manager is an object that is used to organize components in a container.

28 Which Component subclass is used for drawing and painting?
A: Canvas

29 what is the difference between a Scrollbar and a Scroll Pane?
A: A Scrollbar is a Component, but not a Container. A Scroll Pane is a Container. A Scroll Pane handles its own events and performs its own scrolling.

30 Which Swing methods are thread-safe?
A: The only thread-safe methods are repaint (), revalidate (), and invalidate ()

31 which containers use a border Layout as their default layout?
A: The Window, Frame and Dialog classes use a border layout as their default layout

32 what is the preferred size of a component?
A: The preferred size of a component is the minimum component size that will allow the component to display normally

33 which containers use a Flow Layout as their default layout?
A: The Panel and Applet classes use the Flow Layout as their default layout

34 what is the immediate super class of the Applet class?
A: Panel

35 Name three Component subclasses that support painting?
A: The Canvas, Frame, Panel, and Applet classes support painting

36 what is the immediate super class of the Dialog class?
A: Window

37 what is clipping?
A: Clipping is the process of confining paint operations to a limited area or shape.

38 what is the difference between a Menu Item and a Check box MenuItem?
A: The Checkbox Menu Item class extends the MenuItem class to support a menu item that may be checked or unchecked.

39 what class is the top of the AWT event hierarchy?
A: The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy

40 In which package are most of the AWT events that support the event-delegation model defined?
A: Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package. The AWTEvent class is defined in the java.awt package.

41 which class is the immediate super class of the Menu Component class?
A: Object

42 which containers may have a Menu Bar?
A: Frame

43 what is the relationship between the Canvas class and the Graphics class?
A: A Canvas object provides access to a Graphics object via its paint () method.

44 How are the elements of a Border Layout organized?
A: The elements of a Border Layout are organized at the borders (North, South, East, and West) and the center of a container.

Question Set 2 :

1. What is JFC?
A: JFC stands for Java Foundation Classes. The Java Foundation Classes (JFC) are a set of Java class libraries provided as part of Java 2 Platform, Standard Edition (J2SE) to support building graphics user interface (GUI) and graphics functionality for client applications that will run on popular platforms such as Microsoft Windows, Linux, and Mac OSX.

2. What is AWT?
A: AWT stands for Abstract Window Toolkit. AWT enables programmers to develop Java applications with GUI components, such as windows, and buttons. The Java Virtual Machine (JVM) is responsible for translating the AWT calls into the appropriate calls to the host operating system.

3. What are the differences between Swing and AWT?
A: AWT is heavy-weight components, but Swing is light-weight components. AWT is OS dependent because it uses native components, But Swing components are OS independent. We can change the look and feel in Swing which is not possible in AWT. Swing takes less memory compared to AWT. For drawing AWT uses screen rendering where Swing uses double buffering.

4. What are heavyweight components?
A: A heavyweight component is one that is associated with its own native screen resource (commonly known as a peer).

5. What is lightweight component?
A: A lightweight component is one that “borrows” the screen resource of an ancestor (which means it has no native resource of its own — so it’s “lighter”).

6. What is double buffering?
A: Double buffering is the process of use of two buffers rather than one to temporarily hold data being moved to and from an I/O device. Double buffering increases data transfer speed because one buffer can be filled while the other is being emptied.

7. What is an event?
A: Changing the state of an object is called an event.

8. What is an event handler ?
A: An event handler is a part of a computer program created to tell the program how to act in response to a specific event.

9. What is a layout manager?
A: A layout manager is an object that is used to organize components in a container.

10. What is clipping?
A: Clipping is the process of confining paint operations to a limited area or shape.

11. Which containers use a border Layout as their default layout?
A: The window, Frame and Dialog classes use a border layout as their default layout.

12. What is the preferred size of a component?
A: The preferred size of a component is the minimum component size that will allow the component to display normally.

13. What method is used to specify a container’s layout?
A: The setLayout() method is used to specify a container’s layout.

14. Which containers use a FlowLayout as their default layout?
A: The Panel and Applet classes use the FlowLayout as their default layout.

15. Which method of the Component class is used to set the position and size of a component?
A: setBounds

16. What is the difference between invokeAndWait() and invokeLater()?
invokeAndWait is synchronous. It blocks until Runnable task is complete. InvokeLater is asynchronous. It posts an action event to the event queue and returns immediately. It will not wait for the task to complete

17. Why should any swing call back implementation execute quickly?
Callbacks are invoked by the event dispatch thread. Event dispatch thread blocks processing of other events as long as call back method executes.

18. What is an applet?
Applet is a java program that runs inside a web browser.

19. What is the difference between applications and applets?
Application must be run explicitly within Java Virtual Machine whereas applet loads and runs itself automatically in a java-enabled browser. Application starts execution with its main method whereas applet starts execution with its init method. Application can run with or without graphical user interface whereas applet must run within a graphical user interface. In order to run an applet we need a java enabled web browser or an appletviewer.

20. Which method is used by the applet to recognize the height and width?
getParameters().

21. When we should go for codebase in applet?
If the applet class is not in the same directory, codebase is used.

22. What is the lifecycle of an applet?
init( ) method – called when an applet is first loaded
start( ) method – called each time an applet is started
paint( ) method – called when the applet is minimized or maximized
stop( ) method – called when the browser moves off the applet’s page
destroy( ) method – called when the browser is finished with the applet

23. Which method is used for setting security in applets?
setSecurityManager

24. What is an event and what are the models available for event handling?
Changing the state of an object is called an event. An event is an event object that describes a state of change. In other words, event occurs when an action is generated, like pressing a key on keyboard, clicking mouse, etc. There different types of models for handling events are event-inheritance model and event-delegation model

25. What are the advantages of the event-delegation model over the event-inheritance model?
Event-delegation model has two advantages over event-inheritance model. a)Event delegation model enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component’s design and its use. b)It performs much better in applications where many events are generated. This performance improvement is due to event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance.

26. What is source and listener?
A source is an object that generates an event. This occurs when the internal state of that object changes in some way. A listener is an object that is notified when an event occurs. It has two major requirements. First, it must have been registered with a source to receive notifications about specific event. Second, it must implement necessary methods to receive and process these notifications.

27. What are controls and what are different types of controls in AWT?
Controls are components that allow a user to interact with your application. AWT supports the following types of controls: Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, and Text Components. These controls are subclasses of Component.

28. What is the difference between choice and list?
A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices and only one item may be selected from a choice. A List may be displayed in such a way that several list items are visible and it supports the selection of one or more list items.

29. What is the difference between scrollbar and scrollpane?
A Scrollbar is a Component, but not a Container whereas Scrollpane is a Container and handles its own events and perform its own scrolling.

30. What is a layout manager and what are different types of layout managers available?
A layout manager is an object that is used to organize components in a container. The different layouts are available are FlowLayout, BorderLayout, CardLayout, GridLayout , GridBagLayout, Boxlayout and SpringLayout

31. How are the elements of different layouts organized?
The elements of a FlowLayout are organized in a top to bottom, left to right fashion. The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container. The elements of a CardLayout are stacked, on top of the other, like a deck of cards. The elements of a GridLayout are of equal size and are laid out using the square of a grid. The elements of a GridBagLayout are organized according to a grid. However, the elements are of different size and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes. It is the most flexible layout.

32. What are types of applets?
There are two different types of applets. Trusted Applets and Untrusted applets. Trusted Applets are applets with predefined security and Untrusted Applets are applets without any security.

33. What are the restrictions imposed by a Security Manager on Applets?
Applets cannot read or write files on the client machine that’s executing it. They cannot load libraries or access native libraries. They cannot make network connections except to the host that it came from. They cannot start any program on the client machine. They cannot read certain system properties. Windows that an applet brings up look different than windows that an application brings up.

34. What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.

35. What is the relationship between an event-listener interface and an event-adapter class?
An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface.

36. How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.

37. What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

38. What interface is extended by AWT event listeners?
All AWT event listeners extend the java.util.EventListener interface.

39. What is Canvas ?
Canvas is a Component subclass which is used for drawing and painting. Canvas is a rectangular area where the application can draw or trap input events.

40. What is default Look-and-Feel of a Swing Component?
Java Look-and-Feel.

41. What are the features of JFC?
Pluggable Look-and-Feel, Accessibility API, Java 2D API, Drag and Drop Support.

42. What does x mean in javax.swing?
Extension of java.

43. What are invisible components?
They are light weight components that perform no painting, but can take space in the GUI. This is mainly used for layout management.

44. What is the default layout for a ContentPane in JFC?
BorderLayout.

45. What does Realized mean?
Realized mean that the component has been painted on screen or that is ready to be painted. Realization can take place by invoking any of these methods. setVisible(true), show() or pack().

46. What is difference between Swing and JSF?
The key difference is that JSF runs on server. It needs a server like Tomcat or WebLogic or WebSphere. It displays HTML to the client. But Swing program is a stand alone application.

47. Why does JComponent class have add() and remove() methods but Component class does not?
JComponent is a subclass of Container and can contain other components and JComponents.

48. What method is used to specify a container’s layout?
The setLayout() method is used to specify a container’s layout.

49. What is the difference between AWT and SWT?
SWT (Standard Widget Toolkit) is a completely independent Graphical User Interface (GUI) toolkit from IBM. They created it for the creation of Eclipse Integrated Development Environment (IDE). AWT is from Sun Microsystems.

50. What is the difference between JFC & WFC?
JFC supports robust and portable user interfaces. The Swing classes are robust, compatible with AWT, and provide you with a great deal of control over a user interface. Since source code is available, it is relatively easy to extend the JFC to do exactly what you need it to do. But the number of third-party controls written for Swing is still relatively small.
WFC runs only on the Windows (32-bit) user interface, and uses Microsoft extensions to Java for event handling and ActiveX integration. Because ActiveX components are available to WFC programs, there are theoretically more controls available for WFC than for JFC. In practice, however, most ActiveX vendors do not actively support WFC, so the number of controls available for WFC is probably smaller than for JFC. The WFC programming model is closely aligned with the Windows platform.

51. What is a convertor?
Converter is an application that converts distance measurements between metric and U.S units.

52. What is the difference between a Canvas and a Scroll Pane?
Canvas is a component. ScrollPane is a container. Canvas is a rectangular area where the application can draw or trap input events. ScrollPane implements horizontal and vertical scrolling.

53. What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.

54. What is the difference between a MenuItem and a CheckboxMenuItem?
The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or unchecked.

55. Which is the super class of all event classes?
The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.

56. How the Canvas class and the Graphics class are related?
A Canvas object provides access to a Graphics object via its paint() method.

57. What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a menu bar. A window can be modal.

58. What is the relationship between clipping and repainting?
When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.

59. What advantage do Java’s layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java’s layout managers aren’t tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.

60. When should the method invokeLater() be used?
This method is used to ensure that Swing components are updated through the event-dispatching thread.