Object-Oriented Programming with Java Lecture Notes

22 April 2008 • Swing Components

Outline

• Why Swing?

• Some Swing components.

  • Text-oriented components.

  • The MVC pattern.

  • Choice Components

  • Actions

Wither the AWT?

• The AWT failed by being too ambitions.

  • Device-independent graphics and an abstract windowing toolket.

• By jettsoning the windowing part, the AWT became more focused and successful.

  • And mis-named.

• But what about windowing?

Swing

• Swing was developed to take over the abstract windowing part of the AWT.

  • Swing is built using AWT's graphics capabilities.

• Swing was a joint development between Netscape and Sun.

• There are other Swing-like packages too, notably Eclipse’s Standard Widget Toolkit (SWT).

Swing Parts

• Roughly, Swing has three parts:

  1. the components (widgets),

  2. layout managers, and

  3. interaction protocols.

• These parts are many (protocols) or complicated (layout manages) or both (components), and they all interact.

Text I-O

• Three basic text components show text, and allow varying amounts of text to be input.

  • Text handling can get complicated in other, more advanced components.

  • Upto rendering HTML source.

  

Text Labels

• JLabels are the simplest Swing components.

  • A short piece of read-only text, or an image, or both.

    Install the Java plug-in to see this applet. See the code.

Reactive Text Labels

• The JLabel class is a JComponent ancestor, and JComponents can have listeners attached.

  • A JLabel with a listener attached can react to events.

    Install the Java plug-in to see this applet. See the code.

• Listening to the keyboard would allow text I-O.

Text Fields

• A JTextField is a short, one-line piece of editable text.

  Install the Java plug-in to see this applet. See the code.

Text Areas

• A JTextArea is a larger, multi-line piece of editable text.

  Install the Java plug-in to see this applet. See the code.

Scroll Bars

• A JTextArea scrolls horizontally to reveal long lines.

  • But not vertically, and only as determined by line length.

• A JScrollPane is a container that is able to scroll the component it contains.

  Install the Java plug-in to see this applet. See the code.

Scrolling

A JScrollPane opens a view-port (JViewport) onto its contained component. Scrolling involves moving the view-port over the contained component.

• A JScrollPane can manage any component that implements the Scrollable interface.

  • The Scrollable interface answers queries about view-port characteristics.

Component Parts

• Any particular Swing component can be divided into three parts:

  1. Content: the component's state.

  2. Appearance: how component's displayed.

  3. Behavior: how the component reacts to events.

Interactions

• Separation into appearence, behavior, and content is clean and understandable.

  You may not agree with it, but that's a different problem. The separation also allows straightforward interactions between parts. Although the pressure to optimize is intense.

Model View Controller

• The appearence-behavior-content separation maps reaidly into object-oriented design.

  • Each interaction between parts is an interface.

  • Each class implements one of the interfaces:

    The model implements the changes interface.
    The view implements the state interface.
    The controller implements the events interface.

• The result is known as the model-view-controller pattern ) (MVC).

Text-Field MVC Example

• Consider a JTextField instance:

  The view shows the text. Install the Java plug-in to see this applet. The model has the text displayed and its attributes. The controller deals with keystrokes and mouse events.

MVC Interactions

• Take a text-field instance again:

  

MVC Advantages

• A clean and definite separation between parts.

  • Initial design responsibilites can be straightforward.

• A flexible implementation structure.

  • The state, change, and event interfaces make part substitution easy.

• It's a well understood (or at least heard-of) pattern.

  • From Smalltalk, not the Gang of Four.

MVC Disadvantages

• The interface design can be tricky.

  • What's the controller do with a double click?

    • It depends on what's been double clicked, which the controller doesn't know about.

• There's intense pressure to optomize (mush together) the MVC parts.

  • Eliminate state queries to the model.

Choice

• Choice swing components allow selection among a small number of items.

  • Selection can be one-of or many.

  

Check Boxes

• A JCheckBox provides multiple yes-no selections.

  Install the Java plug-in to see this applet. See the code.

Radio Buttons

• A JRadioButton provides one-of-several selections.

  Install the Java plug-in to see this applet. See the code.

• A ButtonGroup enforces the one-of-many rule.

  • Exactly one radio button in a group is selected.

  • Separate button groups are independent.

Combo Box

• A JComboBox also provides one-of-several selections.

  • It's a less intrusive version of radio buttons.

  • It's also better than radio buttons for hangling many possible choices.

    • Although not that much better.

Install the Java plug-in to see this applet. See the code.

Sliders

• A JSlider provides one-of-several selections from a large range of selections.

  Install the Java plug-in to see this applet. See the code.

Using Swing Components

• We've seen what (some) Swing components can do.

  • But what can you do with Swing components?

  • And how?

• How are Swing Components used in an application?

  • What Swing components can do for your application depends on the component and your imagination.

Swing-Application Interaction

• Swing Components interact (via intermediaries) with the environment through events.

  

• How do Swing components interact with the rest of the application?

  • One obvious answer is through query methods.

Component Query Methods

• A Swing component supports one or more query methods (getters) to determine the component state.

  JSlider.getModel(): return a slider's minimum, maximum, and current value.

  ButtonGroup.getSelected(): return a radio-button group's selected button.

  JTextField.getText(): return a text field's displayed text.

• And so on.

Using Query Methods

• Query methods are simple and do the job, but have one big problem:

  When should an application call a component's query method?

• Calling too often is wasteful, calling too infrequently is unresponsive.

  • Also, periodic querying complicates the application's structure.

• The Swing components have to take the lead.

Component Notification

• Swing components should notify the application when something of note happens.

  • The application can then query for the details.

• Java's event model is ready-made to handle (asynchronous) notifications.

  • The application registers a listener with the component.

  • The component calls-back to the application through the listener when something interesting happens.

Actions

• The ActionListener interface defines notification-receiving behavior.

  • A specific notifications have a class implementing ActionListener.

  • The TextAction, for example

• An interested application provides an action-listener instance to a Swing component.

  • The component calls-back through the instance.

Action-Listener Behavior

• The action-listener interface defines the

  void actionPerformed(ActionEvent e)
  

  call-back method.

• The ActionEvent argument provides information about the event:

  String  getActionCommand()
     int  getModifiers()
    long  getWhen()
  String  paramString()
  

State-Picking Case Study

• Combo boxes don't work well when there's a lot of choices.

  • The user's dealing with too many moving parts.

    Install the Java plug-in to see this applet. See the code.

• Can we do better? Is there a better way of picking a state?

Slider Picking

• How about a fifty-position slider and associated text label?

  • Each slider position is a state name.

  • The text label shows the slider's current chosen state.

  Install the Java plug-in to see this applet. See the code.

Slider-Label Coupling

• The slider's change listener is responsible for coupling the slider value and the label's text.

  slider.addChangeListener(
    new ChangeListener() {
      public void
      stateChanged(ChangeEvent e) {
        JSlider slider = 
          (JSlider) e.getSource();
        stateLabel.setText(
          stateNames[slider.getValue()]);
        }
      });
  

Recursion!

• The coupled slider-label is a new component (or could be).

  • How does an application deal with it?

  • The same way: query methods and listeners.

• Also, notice how easily the MVC pattern can get mushed into a single class.