array of JPanel in java....

[tweety_bird_bunny]

hi...
i am trying to write a program in java....the class uses ActionListener interface....
i have 2 create multiple jpanels so i created an array of jpanels.....
the array works fine in constructor....but as soon as as i try to use the panels of array in public void actionPerformed(ActionEvent ae)interface, the program compiles sucesfully but as soon as i click on the button when the program is runnning a huge list of errors comes in console....
plz help me to sort out the problem...
i'm pasting the code...

//start of code

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;

public class MyFrame1 implements ActionListener
{
JFrame frame;
JPanel main,panel[];
JButton next,previous,endexam;
JLabel lab1,lab2,lab3,lab4,lab5;
public MyFrame1()
{
int i;
frame = new JFrame("Welcome to My Frame");
main = new JPanel();
JPanel panel[]=new JPanel[2];
for(i=0;i<2;i++)panel[i]=new JPanel();


next = new JButton("Next");
previous = new JButton("Previous");

lab1 = new JLabel("Panel ONE");
lab2 = new JLabel("Panel TWO");


main.add(panel[0]);
main.add(panel[1]);
frame.getContentPane().add(main);
frame.setVisible(true);
frame.setSize(600,600);
panel[1].setVisible(false);
panel[0].add(lab1);
panel[0].add(next);
panel[0].add(previous);
next.addActionListener(this);
previous.addActionListener(this);
}

public void actionPerformed(ActionEvent e)
{
JPanel panel[]=new JPanel[2];
String s = e.getActionCommand();

if ("Next".equals(s))
{
String str ="panel";
panel[0].setVisible(false);
panel[1].add(lab2);
panel[1].add(next);
panel[1].add(previous);
panel[1].setVisible(true);
}
if ("Previous".equals(s))
{
panel[0].setVisible(true);
panel[0].add(lab1);
panel[0].add(next);
panel[0].add(previous);
panel[1].setVisible(false);
}
}
public static void main(String[] args)
{
MyFrame1 my = new MyFrame1();

}
}

// end of code.....

also plz refer me some more interactive compiler for java(j2sdk)....i am bored with this whole console interface....
some gui cmpiler (eg devc++ for c....similarly refer me for java) which supports all the basic commands...

[JGuru]

@Tweety, This is not the correct way to right a Java program!!!
If you want to align items say JButtons, or any other Component, you must use
LayoutManagers!!
See this Layout Manager Tutorial Click here
Download Java Tutorial from here

Use a Good Java IDE like JCreator: http://www.jcreator.com/

For aligning buttons use FlowLayout

Here is a Sample code using FlowLayout:

 

/*
 * FlowLayoutDemo.java requires no other files.
 */

import java.awt.Container;
import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JFrame;

import java.awt.Dimension;
import java.awt.ComponentOrientation;

public class FlowLayoutDemo {
    public static boolean RIGHT_TO_LEFT = false;

    public static void addComponents(Container contentPane) {
        if (RIGHT_TO_LEFT) {
            contentPane.setComponentOrientation(
                ComponentOrientation.RIGHT_TO_LEFT);
        }
        contentPane.setLayout(new FlowLayout());

        contentPane.add(new JButton("Button 1"));
        contentPane.add(new JButton("Button 2"));
        contentPane.add(new JButton("Button 3"));
        contentPane.add(new JButton("Long-Named Button 4"));
        contentPane.add(new JButton("5"));
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        //JFrames decorated by the Java look and feel
        //can't get smaller than their minimum size.
        //We specify a skinnier minimum size than the
        //content pane will cause the frame to request,
        //so that you can see what happens when you
        //drag the window so that it's narrower than a
        //single row.
        JFrame frame = new JFrame("FlowLayoutDemo") {
            public Dimension getMinimumSize() {
                Dimension prefSize = getPreferredSize();
                return new Dimension(100, prefSize.height);
            }
        };
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Set up the content pane.
        addComponents(frame.getContentPane());

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

[tweety_bird_bunny]

actually what i ment was that i want to create an array of jpanels and want the actionPerformed interface to change the panels whenever i click a button or something....
but when i use the notation panel[i].setVisible(true); in the interface then a null pointer exception occures while runtime...

i dont knw how to utilize the array of panels in the interface method though it works perfectly in the above class ....

[JGuru]

@tweety, Use CardLayout, CardLayout class lets you implement an area that contains different components at different times. A CardLayout is often controlled by a combo box, with the state of the combo box determining which panel (group of components) the CardLayout displays. An alternative to using CardLayout is using a tabbed pane, which provides similar functionality but with a pre-defined GUI. For further details, see How to Use CardLayout.

[code]
[b]
@tweety, Use CardLayout, CardLayout class lets you implement an area that contains different components at different times. A CardLayout is often controlled by a combo box, with the state of the combo box determining which panel (group of components) the CardLayout displays. An alternative to using CardLayout is using a tabbed pane, which provides similar functionality but with a pre-defined GUI.
See this example. Maybe this is what you want to do!!

/*
 * @(#)CardTest.java	@author JGuru
 */

import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;

class CardPanel extends Panel {
    ActionListener listener;

    Panel create(LayoutManager layout) {
	Button b = null;
	Panel p = new Panel();

	p.setLayout(layout);

	b = new Button("one");
	b.addActionListener(listener);
	p.add("North", b);

	b = new Button("two");
	b.addActionListener(listener);
	p.add("West", b);

	b = new Button("three");
	b.addActionListener(listener);
	p.add("South", b);

	b = new Button("four");
	b.addActionListener(listener);
	p.add("East", b);

	b = new Button("five");
	b.addActionListener(listener);
	p.add("Center", b);

	b = new Button("six");
	b.addActionListener(listener);
	p.add("Center", b);

	return p;
    }

    CardPanel(ActionListener actionListener) {
	listener = actionListener;
	setLayout(new CardLayout());
	add("one", create(new FlowLayout()));
	add("two", create(new BorderLayout()));
	add("three", create(new GridLayout(2, 2)));
	add("four", create(new BorderLayout(10, 10)));
	add("five", create(new FlowLayout(FlowLayout.LEFT, 10, 10)));
	add("six", create(new GridLayout(2, 2, 10, 10)));
    }

    public Dimension getPreferredSize() {
	return new Dimension(200, 100);
    }
}

public class CardTest extends Applet
		      implements ActionListener,
				 ItemListener {
    CardPanel cards;

    public CardTest() {
	setLayout(new BorderLayout());
	add("Center", cards = new CardPanel(this));
	Panel p = new Panel();
	p.setLayout(new FlowLayout());
	add("South", p);

	Button b = new Button("first");
	b.addActionListener(this);
	p.add(b);

	b = new Button("next");
	b.addActionListener(this);
	p.add(b);

	b = new Button("previous");
	b.addActionListener(this);
	p.add(b);

	b = new Button("last");
	b.addActionListener(this);
	p.add(b);

	Choice c = new Choice();
	c.addItem("one");
	c.addItem("two");
	c.addItem("three");
	c.addItem("four");
	c.addItem("five");
	c.addItem("six");
	c.addItemListener(this);
	p.add(c);
    }

    public void itemStateChanged(ItemEvent e) {
	((CardLayout)cards.getLayout()).show(cards,
	                                     (String)(e.getItem()));
    }

    public void actionPerformed(ActionEvent e) {
	String arg = e.getActionCommand();

	if ("first".equals(arg)) {
	    ((CardLayout)cards.getLayout()).first(cards);
	} else if ("next".equals(arg)) {
	    ((CardLayout)cards.getLayout()).next(cards);
	} else if ("previous".equals(arg)) {
	    ((CardLayout)cards.getLayout()).previous(cards);
	} else if ("last".equals(arg)) {
	    ((CardLayout)cards.getLayout()).last(cards);
	} else {
	    ((CardLayout)cards.getLayout()).show(cards,(String)arg);
	}
    }

    public static void main(String args[]) {
	Frame f = new Frame("CardTest");
	CardTest cardTest = new CardTest();
	cardTest.init();
	cardTest.start();

	f.add("Center", cardTest);
	f.setSize(300, 300);
	f.show();
    }
    
    public String getAppletInfo() {
        return "Demonstrates the different types of layout managers.";
    }
}