building Java
GUI with the AWT
AWT (Abstract Window Toolkit)
Present in all Java implementations Described in most Java textbooks Adequate for many applications Uses the controls defined by your OS
therefore it's "least common denominator"
Difficult to build an attractive GUI import java.awt.*; import java.awt.event.*;
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Swing
Same concepts as AWT Doesn't work in ancient Java implementations (Java 1.1 and earlier) Many more controls, and they are more flexible
Some controls, but not all, are a lot more complicated
Gives a choice of "look and feel" packages Much easier to build an attractive GUI import javax.swing.*;
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Swing vs. AWT
Swing is bigger, slower, and more complicated
But not as slow as it used to be
Swing is more flexible and better looking Swing and AWT are incompatible--you can use either, but you can't mix them
Actually, you can, but it's tricky and not worth doing
Learning the AWT is a good start on learning Swing Many of the most common controls are just renamed
AWT: Button b = new Button ("OK"); Swing: JButton b = new JButton("OK");
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To build a GUI...
Make somewhere to display things--usually a Frame or Dialog (for an application), or an Applet Create some Components, such as buttons, text areas, panels, etc. Add your Components to your display area Arrange, or lay out, your Components Attach Listeners to your Components
Interacting with a Component causes an Event to occur A Listener gets a message when an interesting event occurs, and executes some code to deal with it
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Containers and Components
The job of a Container is to hold and display Components Some common subclasses of Component are Button, Checkbox, Label, Scrollbar, TextField, and TextArea A Container is also a Component
Some Container subclasses are Panel (and Applet), Window, and Frame
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This allows Containers to be nested
An Applet is Panel is a Container
java.lang.Object | +java.awt.Component | +java.awt.Container | +java.awt.Panel | +java.applet.Applet ...so you can display things in an Applet
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Example: A "Life" applet
Container (Applet) Containers (Panels) Component (Canvas) Components (Buttons) Components (TextFields) Components (Labels)
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Applets
An application has a public static void main(String args[ ]) method, but an Applet usually does not An Applet's main method is in the Browser To write an Applet, you extend Applet and override some of its methods The most important methods are init( ), start( ), and paint(Graphics g)
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To create an applet
public class MyApplet extends Applet { ... }
this is the only way to make an Applet
You can add components to the applet The best place to add components is in init( ) You can paint directly on the applet, but... ...it's better to paint on a contained component Do all painting from paint(Graphics g)
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Some types of components
Label Choice TextField Button List Button Checkbox
Scrollbar TextArea
Checkbox
CheckboxGroup
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Creating components
Label lab = new Label ("Hi, Dave!"); Button but = new Button ("Click me!"); Checkbox toggle = new Checkbox ("toggle"); TextField txt = new TextField ("Initial text.", 20); Scrollbar scrolly = new Scrollbar (Scrollbar.HORIZONTAL, initialValue, bubbleSize, minValue, maxValue);
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Adding components to the Applet
class MyApplet extends Applet { public void init () { add (lab); // same as this.add(lab) add (but); add (toggle); add (txt); add (scrolly); ...
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Creating a Frame
When you create an Applet, you get a Panel "for free" When you write a GUI for an application, you need to create and use a Frame:
Frame frame = new Frame(); frame.setTitle("My Frame"); frame.setSize(300, 200); // width, height ... add components ... frame.setVisible(true);
Or:
class MyClass extends Frame { ... setTitle("My Frame"); // in some instance method
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Arranging components
Every Container has a layout manager The default layout for a Panel is FlowLayout An Applet is a Panel Therefore, the default layout for a Applet is FlowLayout You could set it explicitly with setLayout (new FlowLayout( )); You could change it to some other layout manager
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FlowLayout
Use add(component); to add to a component when using a FlowLayout Components are added left-to-right If no room, a new row is started Exact layout depends on size of Applet Components are made as small as possible FlowLayout is convenient but often ugly
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Complete example: FlowLayout
import java.awt.*; import java.applet.*; public class FlowLayoutExample extends Applet { public void init () { setLayout (new FlowLayout ()); // default add (new Button ("One")); add (new Button ("Two")); add (new Button ("Three")); add (new Button ("Four")); add (new Button ("Five")); add Button (new ("Six")); } }
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BorderLayout
At most five components can be added If you want more components, add a Panel, then add components to it. setLayout (new BorderLayout());
add (new Button("NORTH"), BorderLayout.NORTH);
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BorderLayout with five Buttons
public void init() { setLayout (new BorderLayout ()); add (new Button ("NORTH"), BorderLayout.NORTH); add (new Button ("SOUTH"), BorderLayout.SOUTH); add (new Button ("EAST"), BorderLayout.EAST); add (new Button ("WEST"), BorderLayout.WEST); add (new Button ("CENTER"), BorderLayout.CENTER); }
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Complete example: BorderLayout
import java.awt.*; import java.applet.*; public class BorderLayoutExample extends Applet { public void init () { setLayout (new BorderLayout()); add(new Button("One"), BorderLayout.NORTH); add(new Button("Two"), BorderLayout.WEST); add(new Button("Three"), BorderLayout.CENTER); add(new Button("Four"), BorderLayout.EAST); add(new Button("Five"), BorderLayout.SOUTH); add(new Button("Six"), BorderLayout.SOUTH); } }
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Using a Panel
Panel p = new Panel(); add (p, BorderLayout.SOUTH); p.add (new Button ("Button 1")); p.add (new Button ("Button 2"));
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GridLayout
The GridLayout manager divides the container up into a given number of rows and columns: new GridLayout(rows, columns)
All sections of the grid are equally sized and as large as possible
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Complete example: GridLayout
import java.awt.*; import java.applet.*; public class GridLayoutExample extends Applet { public void init () { setLayout(new GridLayout(2, 3)); add(new Button("One")); add(new Button("Two")); add(new Button("Three")); add(new Button("Four")); add(new Button("Five")); } }
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Making components active
Most components already appear to do something-buttons click, text appears To associate an action with a component, attach a listener to it Components send events, listeners listen for events Different components may send different events, and require different listeners
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Listeners
Listeners are interfaces, not classes
class MyButtonListener implements ActionListener {
An interface is a group of methods that must be supplied When you say implements, you are promising to supply those methods
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Writing a Listener
For a Button, you need an ActionListener
b1.addActionListener (new MyButtonListener ( ));
An ActionListener must have an actionPerformed(ActionEvent) method
public void actionPerformed(ActionEvent e) { ... }
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MyButtonListener
public void init () { ... b1.addActionListener (new MyButtonListener ()); } class MyButtonListener implements ActionListener { public void actionPerformed (ActionEvent e) { showStatus ("Ouch!"); } }
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Listeners for TextFields
An ActionListener l...