Examples of graphics programming

CS 284 (CSA), Spring 2005

GUI concepts

• One of Java's main design goals is platform independence: A (byte-compiled) Java program should run on any correct JVM interpreter, no matter what computer that interpreter is running on. (The combination of hardware and software for a particular computing system is called a platform.)

• GUIs or graphics user interfaces, provide graphics components to their users that (hopefully!) visually explain the activities available for a user. The following are some examples of components in Java:

  • Button and JButton
  • TextField and JTextField
  • Applet and JApplet
  • Choice and JComboBox
  • Frame and JFrame
  • Window and JWindow
  • Panel and JPanel
  • Label and JLabel
  • ScrollPane and JScrollPane

• Top-level containers include applets, frames, windows, and dialog boxes.

• Containers are components that can contain other components. For example, a JApplet can contain JButtons, JTextFields, etc. On the other hand, Button and JButton are not a containers.

• A layout manager is an object that determines the arrangement of components within a container. Java provides numerous layout managers, including:

  • BorderLayout, in which components can be added to the "north," "south," "east," "west," or "center" of a container;
  • FlowLayout, in which components are added from left to right then from top to bottom (similar to the order we write in); and
  • GridBagLayout, which allows for rectangular arrangements of components organized into rows and columns with "gutter" spacing between, etc.

Java's AWT graphics

• AWT (Abstract Window Toolkit) graphics is available for Java version 1.1 and later. In AWT, each component in a Java program has a counterpart in the underlying graphics platform.

• Example: If running on a Linux platform, every Java Button component has a corresponding X-windows Button object (which is what actually appears on the screen).

  If running on a Windows platform, a Windows Button is used instead; likewise for a Macintosh.

• The platform-specific counterpart of a Java component is sometimes called that component's native peer.

• Observe that the "look and feel" of the GUI is determined by the graphics platform when AWT is used. For example, on an older Macintosh system, several parallel horizontal lines appear in the title bar of each window.

• Advantages: Reuse the code already written for the native peers.

• Disadvantages:

  • Native peers for different systems behave a little differently.
  • AWT must focus on the "least common denominator" for all systems for consistency, which limits capabilities of Java components and/or inhibits taking advantage of special features that may be available on one system and not another.
  • Large Java applications become harder to maintain as the native graphics systems diverge.

• Conclusion: AWT works fine for small applets, but breaks down for large, ambitious applications (e.g., bn.com).

Java Swing

• The Swing graphics system appeared in Java 1.2. In Swing, only top-level components have native peers; all other components are drawn on those top-level components.

• Components with native peers are often refered to as heavyweight components, having the "overhead" of their native peers when they are used. Components that are merely drawn on heavyweight components are called lightweight components.

• The lightweight components are those with a J at the beginnings of their names, e.g., JButton, JLabel, etc.

• Swing takes advantage of the Model/View/Control (MVC) model for behavior of a graphics system. Consider the example of a scroll pane:

  • The data model include the length of the "thumb" and its maximum and minimum values.

  • The view is the way that scroll pane appears visually on the screen.

  • The control includes the ability of a user to drag the thumb.

  For another example, consider the Scribble applet.

  • The state variables lastX and lastY are part of the model.

  • The scribbling region and the JButton and JComboBox menu make up the view.

  • Each callback is part of the control.

  The control determines data model values and may cause changes in the view (e.g., requesting a repaint() to overwrite any scribbling); the model values determine what the view can draw; and the view may provide for control via user actions (e.g., selecting a new color or clearing the drawing area).

• Since Swing lightweight components are implemented entirely in Java, Swing can separate model from view and control and provide these to the programmer. Swing combines view and control in an object called a UI delegate; the programmer can choose which UI delegate to use.

• Advantages: Many new features are available under Java programmer control in Swing, including:

  • programmable look and feel,
  • components with non-rectangular regions,
  • many new component types,
  • ability to change views dynamically (while the program runs),
  • ability to add interesting borders to components,
  • serializability (e.g., send a graphics object over network)
  • accessibility (e.g., make GUI applications handicap-accessible),
  • image backgrounds for buttons.

• Disadvantages:

  • Mixing lightweight and heavyweight components in the same GUI can lead to unexpected effects, e.g., one component covering up another.
  • Issues with multithreaded applications: for example, a race condition may arise if the model is changed while Java is redrawing the view.