Panels

A panel is a window component that provides a rectangular subarea in a window. The Java class javax.swing.JPanel allows you to add widgets to a panel with different layouts, in the same manner as you do with window frames. In BreezySwing, the class GBPanel, a subclass of javax.swing.JPanel, provides a more limited functionality. However, panels are useful for decorating windows, displaying graphs and charts, developing interactive sketchpads, and other graphics-based applications.allow programmers perform graphics operations, such as drawing geometric shapes, text, and so forth. A panel can also detect mouse events, such as button presses and releases, mouse cursor movement, and mouse dragging (pressing mouse button and moving the mouse cursor simultaneously).

A BreezySwing panel is added to a window like any other component. In this section, we present two simple examples that illustrate the display of colored panels and the response to mouse presses in them.

The first example program displays four colored panels at startup, and allows the user to change the colors of the panels by clicking a button. The next two screen shots show the initial window and the result of one button click.

The program uses an instance of the java.util.Random class to change each panel's background to a new random color when the button is clicked (each color is represented as an RGB value, where the red, green, and blue components are integers in the range 0..255). A Java list is used to organize the panels for simple processing. Here is the code for the complete program (PanelDemo1.java):

import javax.swing.*;

import BreezySwing.*;

import java.awt.Color;

import java.util.*;

public class PanelDemo1 extends GBFrame{

GBPanel northWest = addPanel(new GBPanel(), 1,1,1,1);

GBPanel southWest = addPanel(new GBPanel(), 1,2,1,1);

GBPanel northEast = addPanel(new GBPanel(), 2,1,1,1);

GBPanel southEast = addPanel(new GBPanel(), 2,2,1,1);

JButton changeColorBtn = addButton("Change color", 3,1,2,1);

private java.util.List<GBPanel> panels = new ArrayList<GBPanel>();

private Random numbers = new Random();

public PanelDemo1(){

northWest.setBackground(Color.red);

southWest.setBackground(Color.green);

northEast.setBackground(Color.blue);

southEast.setBackground(Color.yellow);

panels.add(northWest);

panels.add(southWest);

panels.add(northEast);

panels.add(southEast);

}

public void buttonClicked(JButton buttonObj){

// Reset all backgrounds to random colors

for (GBPanel panel : panels)

panel.setBackground(new Color(numbers.nextInt(255),

numbers.nextInt(255),

numbers.nextInt(255)));

}

public static void main (String[] args){

JFrame frm = new PanelDemo1();

frm.setSize (200, 200);

frm.setTitle("Random Colors");

frm.setVisible (true);

}

}

The second example program allows the user to change the color of a panel by clicking on it directly with the mouse. To support this response, the programmer must define a new subclass of GBPanel, in much the same manner as we have defined main window classes to be subclasses of GBFrame. The program now consists of two files, PanelDemo2.java (containing the main window class) and ColorPanel.java (containing the new subclass, named ColorPanel).

The code for the main window class creates four instances of ColorPanel, passes them their initial colors as arguments, and adds them to the window. The window's button goes away, as does its buttonClicked method and its constructor. Here is the code (PanelDemo2.java):

import javax.swing.*;

import BreezySwing.*;

import java.awt.Color;

public class PanelDemo2 extends GBFrame{

GBPanel northWest = addPanel(new ColorPanel(Color.red), 1,1,1,1);

GBPanel southWest = addPanel(new ColorPanel(Color.green), 1,2,1,1);

GBPanel northEast = addPanel(new ColorPanel(Color.blue), 2,1,1,1);

GBPanel southEast = addPanel(new ColorPanel(Color.yellow), 2,2,1,1);

public static void main (String[] args){

JFrame frm = new PanelDemo2();

frm.setSize (200, 200);

frm.setTitle("Random Colors");

frm.setVisible (true);

}

}

Note how much simpler this code is than the earlier code for PanelDemo1.java. That's because the new panel class now has the code for setting up the panel and responding to events.

The new panel class, defined in ColorPanel.java, sets the panel's initial background color in the constructor, and includes a method named mousePressed to handle a mouse pressed event. When the user presses the mouse in the rectangular area of a panel, this method, which does nothing in the GBPanel class, is triggered (much like buttonClicked for all buttons and menuItemSelected for all menus). The mousePressed method is overridden in the new panel class, to reset the panel's background to a new random color. Here is the code:

import javax.swing.*;

import BreezySwing.*;

import java.awt.Color;

import java.util.Random;

public class ColorPanel extends GBPanel{

private Random numbers = new Random();

public ColorPanel(Color color){

setBackground(color);

}

public void mousePressed(int x, int y){

setBackground(new Color(numbers.nextInt(255),

numbers.nextInt(255),

numbers.nextInt(255)));

}

}

Note that the mousePressed method takes two integer coordinates as arguments. These will be the coordinates of the mouse press in the panel's rectangular area when the method is triggered. You will see how to use this information to draw graphical shapes in our next examples.

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