Java Swing Toolkit Graphics

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Java Swing Toolkit Graphics

• The key to effectively using graphics in Java is
  understanding
• the basic components of the graphics library
• the patterns that are used to combine components

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Some patterns used

• OBSERVER
• STRATEGY
• COMPOSITE
• DECORATOR

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Components

• All displayable objects in the graphics hierarchy
  extend this base class. You will probably never
  create an instance of this class directly, but
  instead use the pre-defined GUI elements that
  Java provides.

4
Containers

• Containers are the basic enclosing element of all
  graphical applications. You cant have a
  graphical app without an outer enclosing
  container to put the rest of your graphical
  components in.
• The top level container for an application is
  usually a Frame (Or JFrame)

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Frames

• A frame is a top-level window with a title and a
  border.
• Frames also support menu bars.
• Lets look at some code to create a basic frame.
• FrameTest.java

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Hello World, Swing Style

• public class FrameTest
• JFrame frame new JFrame()
• public void init()
• frame.getContentPane().add(new JLabel(Hello
  World))
• frame.pack()
• frame.show()
• public static void main (String args)
• FrameTest app new FrameTest()
• app.init()

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Composite Pattern
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Composite Pattern

• The whole graphical framework is a hierarchy of
  components. This is an almost textbook example
  of the composite pattern. A frame contains many
  children, which all comply to the same interface
  (Component), and a call to, for example,
  repaint() on the frame will also call repaint()
  on all its children.

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Input Elements

• The whole point of a GUI is to accept user input
  and do something with it. To this end, there are
  a large number of input components in Swing.
• Lets first examine the JButton.
• FrameButton.java

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Button Example

• JButton button new JButton(Push Me)
• Frame.getContentPane.add(button)
• button.addActionListener(new ActionListener()
• public void actionPerformed(ActionEvent e)
• label.setText(Clicked)
• )

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OBSERVER PATTERN

• Idea decouple event from event handling

Abstract Observable
Abstract Observable
Abstract Observable
Abstract Observer
attach(Observer) detach(Observer) notifyObservers(
)
update()
0..
Concrete Observable
Concrete Observer
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JFC use of OBSERVER

• The observer pattern is used for event
  notification.
• Observables (classes like JButton) generate
  events.
• An observable can have many observers.

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OBSERVER PATTERN

• Terminology differs slightly in JFC classes

Abstract Observable
Abstract Observable
JButton
ActionListener
addActionListener(ActionListener)
actionPerformed(ActionEvent)
0..
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Multiple Observers

• We can add more than one observer to our button.
  All observers are notified when a button event
  occurs. Lets add an observer that will also
  change the color of the label.
• button.addActionListener(new ActionListener()
• public void actionPerformed(ActionEvent e)
  setLabelColor()
• )

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The Text Field

• Lets look at another input widget in Javas
  toolkit, the Text Field
• Text fields are one of the most common data input
  elements for an application. They can also be
  one of the most prone to problems, because they
  allow entry of free-format information, pushing
  validation to the client.
• For example, if your text field is collecting a
  number from the user, nothing is stopping them
  from entering q, which may cause problems for
  the client.
• See TextWidget.java

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The Combo Box

• Often, the input that you want from the users is
  one of a finite set of choices. In order to keep
  the user from entering an invalid value via a
  text field widget, you can give them a combo box
  (dropdown list) that allows them to pick one of a
  restricted set of values. This allows you to
  guard against invalid input from the user.
• See Combo.java

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The List

• Weve already seen many implementations of the
  List ADT. Now lets take a look at one way in
  which we can represent a list graphically using
  Swing. The class is called (Surprise, surprise)
  JList.
• See ListWidget.java

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DECORATOR

• A decorator adds functionality while maintaining
  an interface.
• One example
• InputStreamReader wraps InputStream
• BufferedReader wraps InputStreamReader
• Another example
• JScrollPane wraps Jlist

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But, some list elements dont show up!

• Swing makes use of the decorator pattern to
  provide scroll functionality to various
  components (those that implement the scrollable
  interface) via the ScrollPane.
• See ScrollListWidget.java

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Why is the list so scrunched up?

• In our example program so far, we can see that
  the layout of the various components is not
  exactly ideal. The list doesnt show up the
  right size, the label is not centered, maybe we
  would like to move the list to the center, or any
  number of other things. What controls how the
  components appear in the frame? The answer is
  the Layout Manager.

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STRATEGY

• A layout manager has responsibility for laying
  out components within a container.
• Unlike in NGP, JFC containers do not have fixed
  layout managers.
• Layout managers are treated as strategies.
• Strategies can be swapped.

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Layout Managers

• Swing has several different layout managers.
  Some of the most common are FlowLayout,
  BorderLayout, GridLayout, and GridBag Layout.
  Each has advantages and disadvantages.
• The JFrame component defaults to BorderLayout.

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BorderLayout

• The BorderLayout manager creates an object that
  resembles a picture with a four sided frame, or
  border.

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FlowLayout

• FlowLayout is arguably the simplest layout
  manager. It just stacks up components in a row,
  right to left. If it runs out of space, it wraps
  to a new line. This is the default layout
  manager for JPanel. You can see the behavior in
  how the button, text area, and combo box are laid
  out.

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GridLayout

• GridLayout arranges its components in an equally
  spaced grid of cells. Will take as much space as
  is available to it.

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GridBag Layout

• GridBag is both the most flexible of the layout
  managers, and the most difficult to use. It
  allows you customize the size and growth of all
  the components separately, but setting it up is a
  pain.