Java OOP Programming language (Part 7) - Swing

Java Programming – Swing
Oum Saokosal
Master’s Degree in information systems,Jeonju
University,South Korea
012 252 752 / 070 252 752
oumsaokosal@gmail.com

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Swing Components
• Swing is a collection of libraries that contains
primitive widgets or controls used for designing
GraphicalUser Interfaces(GUIs).
• Commonly used classes in javax.swing package:
• JButton, JTextBox, JTextArea, JPanel,
JFrame, JMenu, JSlider, JLabel, JIcon, …
• There are many, many such classesto do anything
imaginablewithGUIs
• Here we onlystudy the basic architecture and do
simpleexamples

Swing components, cont.
• Each componentis a Java classwith a fairlyextensive
inheritencyhierarchy:
Object
Component
Container
JComponent
JPanel
Window
Frame
JFrame

Using Swing Components
•Very simple, just create object from
appropriate class – examples:
• JButton but = new JButton();
• JTextField text = new JTextField();
• JTextArea text = new JTextArea();
• JLabel lab = new JLabel();
•Many more classes. Don’t need to know every
one to get started.

Adding components
• Once a component is created, it can be added
to a container by calling the container’s add
method:
Container cp = getContentPane();
cp.add(new JButton(“cancel”));
cp.add(new JButton(“go”));
How these are laid out is determined by the layout
manager.
This is required

Laying out components
•Not so difficult but takes a little practice
•Do not use absolute positioning – not
very portable, does not resize well, etc.

Laying out components
• Use layout managers – basically tells form how
to align components when they’re added.
• Each Container has a layout manager
associated with it.
• A JPanel is a Container– to have different
layout managers associated with different
parts of a form, tile with JPanels and set the
desired layout manager for each JPanel, then
add components directly to panels.

Layout Managers
•Java comes with 7 or 8. Most common
and easiest to use are
• FlowLayout
• BorderLayout
• GridLayout
•Using just these three it is possible to
attain fairly precise layout for most
simple applications.

Setting layout managers
• Very easy to associate a layout manager with a
component. Simply call the setLayout method
on theContainer:
JPanel p1 = new JPanel();
p1.setLayout(new FlowLayout(FlowLayout.LEFT));
JPanel p2 = new JPanel();
p2.setLayout(new BorderLayout());
As Componentsare added to the container,the layout
manager determinestheir size and positioning.

What are events?
• All componentscan listen for one or more events.
• Typical examples are:
• Mouse movements
• Mouse clicks
• Hitting any key
• Hitting return key
• etc.
• Telling the GUI what to do when a particular event
occurs is the role of the event handler.

ActionEvent
• In Java, most components have a special
event called an ActionEvent.
• This is loosely speaking the most common
or canonical event for that component.
• A good example is a click for a button.
• To have any component listen for
ActionEvents, you must register the
component with anActionListener. e.g.
button.addActionListener(new MyAL());

Delegation, cont.
• This is referred to as the Delegation Model.
• When you register an ActionListener with a
component, you must pass it the class
which will handle the event – that is, do the
work when the event is triggered.
• For anActionEvent, this class must
implement the ActionListener interface.
• This is simple a way of guaranteeing that
the actionPerformed method is defined.

actionPerformed
• The actionPerformed method has the following
signature:
void actionPerformed(ActionEvent)
• The object of type ActionEvent passed to the
event handler is used to query information about
the event.
• Some common methods are:
• getSource()
• object reference to component generating event
• getActionCommand()
• some text associated with event (text on button, etc).

actionPerformed, cont.
•These methods are particularly useful
when using one eventhandler for
multiple components.

Java OOP Programming language (Part 7) - Swing

Simplest GUI
import javax.swing.JFrame;
class SimpleGUI extends JFrame{
SimpleGUI(){
setSize(400,400); //set frames size in pixels
setDefaultCloseOperation(EXIT_ON_CLOSE);
}
public static void main(String[] args){
SimpleGUI gui = new SimpleGUI();
System.out.println("main thread continues");
}
}

Another Simple GUI
import javax.swing.*;
SimpleGUI(){
setSize(400,400); //set frames size in pixels
JButton but1 = new JButton(“Click me”);
Container cp = getContentPane();//must do this
cp.add(but1);
}
}
}

Add Layout Manager
import javax.swing.*; import java.awt.*;
SimpleGUI(){
setSize(400, 400); //set frames size in pixels
JButton but1 = new JButton("Click me");
Container cp = getContentPane();//must do this
cp.setLayout(new FlowLayout(FlowLayout.CENTER));
cp.add(but1);
}
}
}

Add call to event handler
import javax.swing.*; import java.awt.*;
SimpleGUI(){
setSize(400,400);
JButton but1 = new JButton("Click me");
Container cp = getContentPane();
cp.setLayout(new FlowLayout(FlowLayout.CENTER);
but1.addActionListener(new MyActionListener());
cp.add(but1);
}
}
}

Event Handler Code
class MyActionListener implements ActionListener
{
public void actionPerformed(ActionEvent ae)
{
JOptionPane.showMessageDialog(
"I got clicked",
null
);
}
}

Add second button/event
SimpleGUI(){
JButton but1 = new Jbutton("Click me");
JButton but2 = new JButton(“exit”);
MyActionListener al = new MyActionListener();
but1.addActionListener(al);
but2.addActionListener(al);
cp.add(but1);
cp.add(but2);
}
}

How to distinguish events –Less
good way
class MyActionListener implents ActionListener{
public void actionPerformed(ActionEvent ae){
if (ae.getActionCommand().equals("Exit"){
System.exit(1);
}
else if (ae.getActionCommand().equals("Click me"){
JOptionPane.showMessageDialog(null, "I’m clicked");
}
}
}

Good way
class MyActionListener implents ActionListener{
public void actionPerformed(ActionEvent ae){
if (ae.getSource() == but2){
System.exit(1);
}
else if (ae.getSource() == but1){
JOptionPane.showMessageDialog(
null, "I’m clicked");
}
}
Question: How are but1, but2 brought into scope to do this?
Question:Why is this better?

Java OOP Programming language (Part 7) - Swing

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