ICS 22 Klefstad Week 4 Java Graphical User Interfaces

Graphical User Interfaces

java.awt provides common GUI widgets
Swing is similar but all class names start with a 'J'
User actions generate events handled by event listeners
Widgets are placed automatically by LayoutManagers

Class Component

Component is the superclass of all GUI widgets
key methods are:
- setBackground, setForeground, setFont
- repaint, paint, update - for graphics
- setVisible - hides or shows this component

Class Container

is a Component that may hold other Components
has an associated LayoutManager for placing objects
key methods are:
- setLayout - sets the layout manager
- add, remove - add or remove components
- getComponents - gets an array of components in this container

Class Panel

is a Container
used for grouping components
each panel has its own LayoutManager
default LayoutManager is FlowLayout (left to right)

Class Applet

An Applet is a Panel
A complete program that runs within a Web page
key methods are:
- init, start, stop, destroy, resize, showStatus

Class Window

A Window is a Container
LayoutManager is BorderLayout (on the Center, North, South, etc)
key methods are:
- init, start, stop, destroy, resize

Class Frame

A Frame is a Window
Allows stand-alone GUI program
Frame is initially invisible, so you must setVisible(true)
key methods are:
- init, start, stop, destroy, resize

Events

GUIs typically wait for some physical event to trigger an action
Events are triggered by mouse motion, button press, key press, etc.

A Component object can respond to an event by overriding the event handler

import java.applet.Applet;
public class SimpleEvent
  extends Applet
  implements MouseListener
{
  public void init()
  {
    addMouseListener(this);
  }
  public void mousePressed(MouseEvent e)
  {
    getGraphics().drawString("hi", e.getX(), e.getY());
  }
  ... // other MouseListener functions
}

Event Delivery

all events are derived from java.awt.Event
AWT events are of subtype java.awt.AWTEvent
they are defined in package java.awt.event
EG
```
ActionEvent, MouseEvent, KeyEvent
```
events are sent to all listeners associated with the target object

ActionEvents

usual event for buttons, lists, etc

import java.applet.Applet;
import java.awt.Button;
import java.awt.event.ActionListener;
public class SimpleButtonEvent
  extends Applet
  implements ActionListener
{
  public void init()
  {
    Button b = new Button("Press me");
    b.addActionListener(this);
    add(b);
  }
  public void actionPerformed(ActionEvent e)
  {
    play(getCodeBase(), "audio/beep.au");
  }
}

Important Listener interfaces and Their Event Methods

ActionListener
- actionPerformed
MouseListener
- mouseClicked, mousePressed, mouseReleased, mouseEntered, mouseExited
MouseMotionListener
- mouseDragged, mouseMoved
KeyListener
- keyPressed, keyReleased, keyTyped
ItemListener
- itemStateChanged
TextListener
- textValueChanged
Adaptor classes provide stubs for all functions
- useful when overridding only one or two of the methods

Using inner classes for events

inner classes work well for handling events

import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
public class SimpleButtonEvent
  extends Applet
{
  Button b;
  class ActionHandler
    extends ActionAdaptor
  {
    String message;
    ActionHandler(String msg)
    {
      message = msg;
    }
    public void actionPerformed(ActionEvent e)
    {
      getGraphics().drawString(message, e.getX(), e.getY());
    }
  }
  public void init()
  {
    b = new Button("Press me");
    b.addActionListener(new ActionHandler("hi"));
    add(b);
  }
}

Label

A displayed String object

It has no events

import java.applet.*;
import java.awt.*;
public class LabelTest
  extends Applet
{
  public void init()
  {
    add(new Label("A label"));
    add(new Label("Another label", Label.RIGHT)); // right justify
  }
}

Buttons

A pushbutton widget with label
May be pushed with a mouse click
ActionListener is notified via call to ActionPerformed

Check Boxes

A Checkbox is a label with a small pushbutton
The state of a Checkbox is true or false (the default)
Clicking on a Checkbox toggles the state and notifies ItemListener

Checkbox Groups

Controls behavior of a group of Checkbox objects
Only one of the Checkbox objects may be true
Sometimes called "radio" buttons

List

A scrollable list of Strings
Allows user to select one or more items
Each selection notifies ActionListener or ItemListener

Scrollbar

A "slider" widget containing an integer value
Range of the integer value is set at construction
Both horizontal and vertical sliders are available.
Uses AdjustmentEvent and AdjustmentListener

TextComponent

superclass of TextArea and TextField
key methods are:
- getText, setText
- addTextListener

TextField

displays a line of (optionally editable) text
can be scrolled left and right
The width and initial string are set at construction

TextArea

displays several lines of (optionally editable) text
can be scrolled left/right and up/down
key methods are:
```
append, insert, replaceRange
```

GUI Object Layout

A LayoutManager places Components within a Container
Regions are divided into panels with different layout manager
For example, a telephone has two panels
- the keypad is a grid of numbers
- the display has current number and meters for signal and battery
Regions can also be defined to isolate events (such as mouse clicks)

Class FlowLayout

Arranges Components left to right
When no more fit on current row, wrap to next line
Each line of Component objects is centered in the Container
FlowLayout is the default behavior of Panel and Applet

Class GridLayout

Arranges components into a matrix
Specify rows and columns at construction

Classes GridBagLayout & GridBagConstraints

A grid of cells like the GridLayout
Components may span several grid cells either direction
GridBagConstraints control size and position
add() makes a copy of GridBagConstraints

Class BorderLayout

Components may be placed at "North", "East", "South", "West", and "Center"
Center component is resized to fill extra space left by others

Class CardLayout

Displays only one component at a time
Each component ("card") expands to fill the entire container
Cards are referenced by name (stored CardLayout)
Card is "brought to front" by show(cardName)

Example import java.awt.*; import java.awt.event.*; // The main class, builds user interface to a List public class ListTest extends Frame { // Allows Exit via clicking on Windows X-box // in upper right corner class WindowDestroyer extends WindowAdapter { public void windowClosing(WindowEvent e) { System.exit(0); } } ListTest() { addWindowListener(new WindowDestroyer()); setSize(800,700); setLayout(new GridLayout(2,1)); add(new ListManager(new LinkedList())); setVisible(true); } public static void main(String[] argv) { ListTest t = new ListTest(); } }

Example (Continued) class ListManager extends Panel { private List myList; TextArea output; class Inserter extends Panel implements ActionListener { TextField t; Button b; Inserter() { add(b = new Button("Insert")); add(t = new TextField("Hello")); // so my actionPerformed will be called b.addActionListener(this); } public void actionPerformed(ActionEvent e) { String s = t.getText().trim(); if ( s.length() > 0 && !myList.isFull() ) myList.insert(s); redrawList(); } } class Remover extends Panel implements ActionListener { TextField t; Button b; Remover() { add(b = new Button("Remove")); add(t = new TextField(20)); // so my actionPerformed will be called b.addActionListener(this); } public void actionPerformed(ActionEvent e) { if ( !myList.isEmpty() ) { t.setText((String)myList.pop()); redrawList(); } } } void redrawList() { output.setText(myList.toString()); } ListManager(List s) { setLayout(new GridLayout(3,1)); myList = s; add(new Inserter()); add(new Remover()); output = new TextArea(); add(output); } }

Example (Continued) abstract class List { abstract void insert(Object o); abstract Object remove(); abstract boolean isEmpty(); abstract boolean isFull(); } class LinkedList extends List { private static class ListNode { Object info; Node next; ListNode(Object newInfo, Node newNext) { info = newInfo; next = newNext; } } private ListNode head; LinkedList() { head = null; } void insert(Object o) { head = new Node(o, head); } Object remove() { Object temp = head.info; head = head.next; return temp; } boolean isEmpty() { return head == null; } boolean isFull() { return false; } public String toString() { String result = ""; for (Node p = head; p != null; p = p.next) result += " " + p.info; return result; } }