Download javax.swing.JPanel

javax.swing.JPanel
javax.swing.JPanel, a subclass of JComponent, builds general-purpose GUI containers
that you can use to organize other components or display computer graphics.
To use a JPanel object to organize other components:
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2.
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Build the JPanel object.
Embellish it as desired.
Give it an appropriate layout manager.
Add components to it.
Add the panel to the containment hierarchy.
JComponent
JPanel
Example
Here is a window that is an interface to a desktop calculator.
The buttons are organized into three separate JPanel
objects, containing:
clear buttons
number buttons
entry buttons
The clear and entry buttons are in JPanel objects that use 2 × 1 grid layout managers. Here’s
the code that builds the clear buttons:
JButton btnCE = new JButton( "CE" );
JButton btnC = new JButton( "CLEAR" );
The following code builds the left panel and adds these buttons to it.
JPanel leftPanel = new JPanel( );
leftPanel.setLayout( new GridLayout( 2, 1, 2, 2 ) );
leftPanel.add( btnC );
leftPanel.add( btnCE );
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The following code builds the center panel:
JPanel centerPanel = new JPanel( );
centerPanel.setLayout( new GridLayout( 4, 3, 2, 2 ) );
The number buttons are kept in an array. Here’s the code to build them:
JButton [] btnList = new JButton[10];
for ( int k = 0; k < btnList.length; k++ )
btnList[k] = new JButton( Integer.toString( k ) );
The decimal point button must be built separately. The bottom left corner of the number pad is
an unused button containing a blank caption added to the panel as filler.
JButton btnDot = new JButton( "." );
JButton btnBlank = new JButton( " " );
Components are added to a grid layout in row-major order – i.e. starting with coordinate (0, 0)
and proceeding by rows. Since this doesn’t match the order of the buttons in the array, it is
easiest to add each button to the panel separately.
centerPanel.add(
centerPanel.add(
centerPanel.add(
centerPanel.add(
centerPanel.add(
centerPanel.add(
etc.
btnList[7]
btnList[8]
btnList[9]
btnList[4]
btnList[5]
btnList[6]
);
);
);
);
);
);
The entry buttons are similarly built and added to a third panel. Finally, the three panels are
added to the west, center and east areas of the frame’s border layout.
win.add( leftPanel, BorderLayout.WEST );
win.add( centerPanel, BorderLayout.CENTER );
win.add( rightPanel, BorderLayout.EAST );
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A JPanel can also be used to render computer graphics. The following material assumes that
you are familiar with the material in the collection Graphics, Color, Font. In particular you must
understand the topic Graphics Context.
A JPanel object is visually rendered by its own internal paint method named
paintComponent, whose declaration appears as:
void paintComponent( Graphics g )
Whenever the JVM determines that the JPanel object needs to be rendered – such as when it is
resized or maximized – it automatically calls paintComponent and passes the graphics
context to parameter g. Thus, to customize the rendering of a JPanel object:
Create your own subclass of JPanel.
Morph its inherited paintComponent method.
Within paintComponent call the appropriate Graphics methods in parameter g.
Example
This application creates the window
shown to the right.
The subclass of JPanel is shown on the
next page.
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import javax.swing.*;
public class MyApp
{
public static void main( String [] args )
{
JFrame win = new JFrame( "Panel Graphics" );
win.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
win.add( new MyPanel( ) );
win.setSize( 350, 150 );
win.setVisible( true );
}
}
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import javax.swing.*;
import java.awt.*;
public class MyPanel extends JPanel
{
public void paintComponent( Graphics g )
{
super.paintComponent( g ); // call superclass
g.setColor( Color.RED );
g.setFont( new Font( "Trebuchet MS", Font.PLAIN, 24 ) );
g.drawString( "Welcome to Graphics", 20, 50 );
g.setColor( Color.BLUE );
g.drawRect( 20, 52, 250, 6 );
}
}
Exercises
1.
Using the code from the first example, write a complete
Java program (application or applet) that uses JPanel
objects to build and display the window shown to the right.
The calculator’s display (showing 0) is a JTextField
that has been added to the north area of the window.
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2.
Modify your solution to exercise #1 so that
the window includes a display of the
calculator’s internal accumulator (see the
picture to the right). This is yet another
JPanel object added to the south area of
the window. It contains a JLabel object
(containing the text Accumulator) and a
JTextField object (showing 0).
3.
Write a complete Java program that builds and displays the GUI shown below. Two JPanel
objects are used: one for the full name and one for the mailing address. Put titled borders around
them.
4.
Write a complete Java application that uses a JPanel object
painting the picture shown to the right. The picture must resize in
proportion to the window, as shown below.
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5.
Write a complete Java application that uses a JPanel object painting a complicated picture of
your own design. Here are two samples to inspire your creativity.
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