面试题与答案

Swing is a GUI toolkit for Java. It is one part of the Java Foundation Classes (JFC). Swing includes graphical user interface (GUI) widgets such as text boxes, buttons, split-panes, and tables. 

rnSwing widgets provide more sophisticated GUI components than the earlier Abstract Window Toolkit. Since they are written in pure Java, they run the same on all platforms, unlike the AWT which is tied to the underlying platform's windowing system. Swing supports pluggable look and feel not by using the native platform's facilities, but by roughly emulating them. This means you can get any supported look and feel on any platform. The disadvantage of lightweight components is slower execution. The advantage is uniform behavior on all platforms.

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.

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.

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.

A heavyweight component is one that is associated with its own native screen resource (commonly known as a peer).

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").

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.

Changing the state of an object is called an event.

An event handler is a part of a computer program created to tell the program how to act in response to a specific event.

A layout manager is an object that is used to organize components in a container.

Clipping is the process of confining paint operations to a limited area or shape.

The window, Frame and Dialog classes use a border layout as their default layout.

The preferred size of a component is the minimum component size that will allow the component to display normally.

The setLayout() method is used to specify a container's layout.

The Panel and Applet classes use the FlowLayout as their default layout.

setBounds Method of the Component class is used to set the position and size of a component.

invokeAndWait() method in swing is synchronous. It blocks until Runnable task is complete. InvokeLater() method in swing is asynchronous. It posts an action event to the event queue and returns immediately. It will not wait for the task to complete.

Callbacks are invoked by the event dispatch thread. Event dispatch thread blocks processing of other events as long as call back method executes.

Applet is a java program that runs inside a web browser.

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.

getParameters()
Method is used by the applet to recognize the height and width.

If the applet class is not in the same directory, codebase is used.

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

setSecurityManager()
Method is used for setting security in applets.

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.

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.

Because behind every swing component are many Java objects and resources. This takes time to create them in memory. JDK 1.3 from Sun has some improvements which may lead to faster execution of Swing applications.