Multiple instances of a class

**Lora_91** · October 20th, 2013, 12:46 PM

Yeah sorry, it's a little bit difficult to explain so I understand if you're confused. I'll try to explain it better though.

So I have two JFrames, the first one is in the MainFrame.java and has the button "addmemberbutton", which is supposed to open up a second JFrame, wait until the user has entered all the information in the fields and when the user presses the "completebutton" the information will be stored in the new Members instance we just created.

So here I create the new instance of Membes:

        if(e.getSource() == AddMemberButton)
        {
            while(cancel == false)
            {
                        MA++;
                        int loginValue = 0;
                        Members muffin;
                        muffin = memberFrame.createAndShowMemberFrameGUI();

Which runs the createAndShowMemberFrameGUI:

	public static Members createAndShowMemberFrameGUI() 
	{
		//JFrame.setDefaultLookAndFeelDecorated(true);
		//JFrame frame = new JFrame(WindowName);
		memberFrame MF = new memberFrame();
		MF.frame.setContentPane(MF.createContentPane());
		MF.frame.setSize(WindowWidth,WindowHeight);
		MF.frame.setLocationRelativeTo(null);
		MF.frame.setResizable(false);
		MF.frame.setVisible(true);
 
		while(true)
		{
			if(done == true)
			{
				return newMember;
			}
 
			if(cancel == true)
			{
				return null;
			}
 
			/*
			try 
			{
				Thread.sleep(1000);
 
			} catch (InterruptedException e) 
			{
				e.printStackTrace();
			}
			*/
		}
 
 
 
	}

And that creates the second JFrame within:

	public JPanel createContentPane()
	{
//lots of code setting up the JFrame which isn't relevant to the issue so I won't bore you with it =)

The issue however is that createAndShowMemberFrameGUI() needs to return a value to muffin = memberFrame.createAndShowMemberFrameGUI(); when called upon, and those values are meant to be taken from the second JFrame which is just being created. So I need to hold off on the return of the value until the user has actually submitted these values in the second JFrame and pressed the complete button, but currently createAndShowMemberFrameGUI(); doesn't want to do that but rather return the values before the second JFrame is even opened.

I hope this explained the issue I'm having better. =/

**GregBrannon** · October 20th, 2013, 01:04 PM

Better explanation. I stand by my previous comments. Use a modal JDialog instead of a JFrame to collect and return data from the user before continuing.

**Lora_91** · October 22nd, 2013, 05:35 AM

Edit: I figured out a couple of the issues but ran into a new problem, it claims not to be able to reach the code where I return the values, even though it does allow me to enter the values and press the completebutton:

    public static Members createAndShowMemberFrameGUI()
    {
        //JFrame.setDefaultLookAndFeelDecorated(true);
        //JFrame frame = new JFrame(WindowName);
 
 
    	frame = new JFrame(WindowName);
 
        memberFrame MF = new memberFrame();
 
 
        dialog1 = new JDialog(frame);
        dialog2 = new JDialog(dialog1, "", Dialog.ModalityType.APPLICATION_MODAL);
 
        memberFrame.dialog2.setContentPane(MF.createContentPane());
        memberFrame.dialog2.setSize(WindowWidth,WindowHeight);
        memberFrame.dialog2.setLocationRelativeTo(null);
        memberFrame.dialog2.setResizable(false);
        memberFrame.dialog2.setVisible(true);
 
        while(true)
        {
            if(done == true)
            {
                return newMember;
                JOptionPane.showMessageDialog(null,"Returning"); //Error, unreachable code
            }
 
            if(cancel == true)
            {
                return null;
            }
 
 
            try
            {
                Thread.sleep(1000);
 
 
            } catch (InterruptedException e)
            {
                e.printStackTrace();
            }
 
        }
 
 
 
 
    }

    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource() == CompleteButton)
        {
            done = true;

Also, with this method the JDialog needs to be declared within the static method which means it has to be a static variable? I've heard from quite a few people that static variables should be avoided if possible, and I'm trying not to use them, but I consistently get errors along the lines of "cannot make a static reference to the non-static ---" or something like that. It all links back to the

public static void main(String [] args){

which starts up the program, and I'm pretty sure that one has to be static? =/

**aussiemcgr** · October 22nd, 2013, 06:58 AM

The main method is static. All methods and instance variables used in a static method must also be static. Now, there is a simple way to get around this: do your work in the class constructor instead of the main.
Have a look at the two code examples below. They do the same thing, but the first one requires the methods to be static, while the second one does not:
With Statics:

public class SomeClass {
	/**
	 * This variable must be static because it is directly referenced in the
	 * static getBool() method, and in the main
	 */
	public static boolean bool;
 
	/**
	 * This method must be static because it is directly referenced in the main
	 */
	public static boolean getBool() {
		return bool;
	}
 
	public static void main(String[] ags) {
		bool = true;
		System.out.println(getBool());
	}
}

Without Statics:

public class SomeClass {
	/**
	 * This variable does not need to be static because it is not directly 
	 * referenced in any static methods
	 */
	public boolean bool;
 
	/**
	 * Default Constructor for this class
	 */
	public SomeClass() {
		bool = true;
		System.out.println(getBool());
	}
 
	/**
	 * This method does not need to be static because it is not directly 
	 * referenced in the main
	 */
	public boolean getBool() {
		return bool;
	}
 
	public static void main(String[] ags) {
		/* We simply create a new instance of this class. Since the new instance
		 * exists only inside of the static method, the instance and all of its
		 * methods and variables do not need to be static. The constructor is
		 * referencing the bool variable and the getBool() method, not the main
		 * method.
		 */
		new SomeClass(); 
	}
}

Tell me if that makes sense. Once we get this resolved, we can see if your other problems still exist.

**Lora_91** · October 23rd, 2013, 04:54 AM

Thanks, that does make sense and I was able to get rid of all of the static variables and methods! =)

Although without the static my method doesn't seem to know how to find the other class' method, even though it's public:

(MainFrame.java)

        if(e.getSource() == AddMemberButton)
        {
            while(cancel == false)
            {
                        Members muffin;
                        muffin = createGUI(); //error: the method createGUI() is undefined for the type MainFrame

Trying to refer to the memberFrame class' method createGUI():

(memberFrame.java)

    public Members createGUI()
    {	
    	frame = new JFrame(WindowName);
 
        memberFrame MF = new memberFrame();
 
 
        dialog1 = new JDialog(frame);
        dialog2 = new JDialog(dialog1, "", Dialog.ModalityType.APPLICATION_MODAL);
 
        dialog2.setContentPane(MF.createContentPane());
        dialog2.setSize(WindowWidth,WindowHeight);
        dialog2.setLocationRelativeTo(null);
        dialog2.setResizable(false);
        dialog2.setVisible(true);
 
        while(true)
        {
            if(done == true)
            {
                return newMember;
                //JOptionPane.showMessageDialog(null,"Returning");
               // frame.dispose();
            }
 
            if(cancel == true)
            {
                return null;
            }   
        }
    }

It seems to only be looking within the MainFrame class, and memberFrame.createGUI can only be used if it's static, which I would prefer to avoid.

**aussiemcgr** · October 23rd, 2013, 07:15 AM

createGUI() does not need to be static. You need to invoke the createGUI() method on a MemberFrame object.

For example, I assume somewhere in your MainFrame class, you created a MemberFrame object (for this example, we'll name it: memberFrame). To invoke the createGUI() method, all you have to say is:

MemberFrame memberFrame = ...; // At some point in your MainFrame class, you have already done this
...
/* With the exception of class instance variables, 
 * all variables should explicitly be given a default 
 * value. For objects, this value is usually null 
 */
Member muffin = null; 
 
/* Invoke the method on your memberFrame
 * object you created earlier in the class.
 */
muffin = memberFrame.createGUI();
...

**Lora_91** · October 23rd, 2013, 08:26 AM

Hm, I had missed doing that, and it would appear I'm not entirely sure of how to do it. I added:

	private memberFrame memberFrame;
    memberFrame MemberFrame = memberFrame;

Into the variables of MainFrame.java, and it did cause muffin = memberFrame. to find the createGUI(); without any errors.

However, when running the program, and pressing the AddMemberButton I got a bunch of errors all starting with:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at LoginSystem.MainFrame.actionPerformed(MainFrame.ja va:253)
Which is the muffin = memberFrame.createGUI(); line

So I'm guessing I didn't set up the memberFrame object variable correctly?

**aussiemcgr** · October 23rd, 2013, 09:31 AM

I don't exactly understand what you did. Can you post your MainFrame class? If it is too large, just post the sections were you mention MemberFrame.

**Lora_91** · October 23rd, 2013, 09:35 AM

I added it at the top together with all of the other variables:

public class MainFrame implements ActionListener
{
 
	public memberFrame memberFrame;
    memberFrame MemberFrame = memberFrame;

And this is when I use it in the actionlistener:

        if(e.getSource() == AddMemberButton)
        {
            while(cancel == false)
            {
                Members muffin;
                muffin = memberFrame.createGUI(); //nullpointer exception

**aussiemcgr** · October 23rd, 2013, 10:58 AM

Why do you have the second MemberFrame variable? And do you ever actually initialize your memberFrame object (call it's constructor)?

**Lora_91** · October 23rd, 2013, 12:58 PM

I was confused with what to initialize it with and got an error that asked me to change it into that. =/

public class MainFrame implements ActionListener
{
 
    memberFrame MemberFrame = null;

I must be initializing it incorrectly here as well since I get a nullpointer exception, which wouldn't be strange considering it's equal to null. =/

**aussiemcgr** · October 23rd, 2013, 01:04 PM

Ok, so in the constructor of your MainFrame class, initialize it with the constructor of the memberFrame class. If you have not created a constructor in the memberFrame class, you can just use the default constructor:

MemberFrame = new memberFrame();

**Lora_91** · October 23rd, 2013, 02:10 PM

Ah thanks that worked! =)

Only issue that seems to be appearing now is that the memberFrame.java class is having issues closing down the second created frame, which is odd considering it worked while it was static. It claims nullpointerexception:

else if(e.getSource() == CancelButton)
        {
 
            int check = 0;
            String[] options = new String[]{"Yes", "No"};
            check = JOptionPane.showOptionDialog(null,
                    "Are you sure you wish to cancel?",
                    "Attention!",
                    JOptionPane.DEFAULT_OPTION,
                    JOptionPane.PLAIN_MESSAGE, null, options,
                    options[0]);
 
 
            if(check == 0)
            {
            	dialog2.dispose();
                //frame.dispose();
            }
 
        }

Not sure if it should be dialog2.dispose or frame.dispose, but both give the same error, and the JFrame remains open. =/

**aussiemcgr** · October 23rd, 2013, 02:26 PM

That's not odd. Static methods do not require an initialized instance of an object in order to work.
In case you do not know what "static" does, making a class variable static makes the variable "instance-less". If ANY instance of a class changes that static variable, ALL instances of that class reflect the change. Here is a simple example you can execute:

public class Prog {
    /**
     * Static variable
     */
    private static int var;
    /**
     * Constructor
     */
    public Prog() {
        var = 0;
    }
    /**
     * Getter
     */
    public int getVar() {
        return var;
    }
    /**
     * Setter
     */
    public void setVar(int v) {
        var = v;
    }
 
    /**
     * Main
     */
    public static void main(String[] args) {
        Prog prog1 = new Prog();
        Prog prog2 = new Prog();
        Prog prog3 = new Prog();
        System.out.println("Prog1: " + prog1.getVar());
        System.out.println("Prog2: " + prog2.getVar());
        System.out.println("Prog3: " + prog3.getVar());
        prog2.setVar(5);
        System.out.println("Prog1: " + prog1.getVar());
        System.out.println("Prog2: " + prog2.getVar());
        System.out.println("Prog3: " + prog3.getVar());
    }
}

The output of this program is:

Prog1: 0
Prog2: 0
Prog3: 0
Prog1: 5
Prog2: 5
Prog3: 5

Notice how we only called the getter on prog2, but all instances of Prog changed? This was because var is static. If var was not static, the output would be:

Prog1: 0
Prog2: 0
Prog3: 0
Prog1: 0
Prog2: 5
Prog3: 0

That's a simple little intro on what static means. I know it confused the hell out of me when I first started. Maybe that will make it a little easier to understand.

If you are getting a null pointer, it means that dialog2 must be null. Have you initialized it with a constructor?

**Lora_91** · October 23rd, 2013, 03:17 PM

Static is indeed confusing, which is one of the reasons I try to avoid it the best I can. =/

I thought I was initializing them with:

        frame = new JFrame(WindowName);
 
        memberFrame MF = new memberFrame();
 
 
        dialog1 = new JDialog(frame);
        dialog2 = new JDialog(dialog1, "", Dialog.ModalityType.APPLICATION_MODAL);

But perhaps not, because after doing it when they are created:

    JFrame frame = new JFrame();
    JDialog dialog1 = new JDialog();
    JDialog dialog2 = new JDialog();

I no longer receive any errors, but the frame isn't closed down either. It's like it's closing down the wrong instance of it?

**aussiemcgr** · October 23rd, 2013, 03:58 PM

If you are receiving null pointer exceptions after initializing them, you'll need to post the full stack-trace of the error message.
Perhaps check does not equal 0.

**Lora_91** · October 23rd, 2013, 04:05 PM

You misunderstand I didn't receive any null pointer exception with this, but I can't seem to close the frame either.

            	dialog2.dispose();
                frame.dispose();

One of those two lines is supposed to close down the frame, no? However nothing happens when I try to do it. =/

**aussiemcgr** · October 23rd, 2013, 04:14 PM

I thought you said you were getting a null pointer exception (back at post #38).
Put a print statement inside the if statement for those calls. Perhaps the if statement is never being entered in the first place.

**Lora_91** · October 23rd, 2013, 04:44 PM

I did say that at post 38 yes, but it was resolved in post 40, I apologize if I was unclear. =/

I added a JOptionPane.showMessageDialog(null,"Dispose");

And it's being called as it should, but the frame remains open. No errors are given, the message is shown, but nothing else happens. =/

**aussiemcgr** · October 23rd, 2013, 04:49 PM

If the null pointer was resolved by using the default constructors instead of the other constructors, this is probably not a good sign.
Are you able to post the entire code for that class?

**Lora_91** · October 23rd, 2013, 04:59 PM

Seems to work, this is the entire memberFrame.java class:

package LoginSystem;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.lang.reflect.*;
import java.util.List;
import java.util.ArrayList;
 
import javax.swing.*;
import javax.swing.event.DocumentListener;
 
public class memberFrame implements ActionListener
{
 
    JPanel titlePanel;
    JPanel buttonPanel;
    JPanel textPanel;
 
    JLabel StringWelcome;
 
    private JButton CompleteButton;
    private JButton CancelButton;
 
    JTextField firstName;
    JTextField lastName;
 
    JLabel titleLabel = new JLabel("Please fill in your name and desired password below:");
    JLabel firstNameLabel = new JLabel("First name:");
    JLabel lastNameLabel = new JLabel("Last name:");
    JLabel passwordLabel = new JLabel("Password:");
    JLabel repeatPasswordLabel = new JLabel("Repeat password:");
 
    JPasswordField password;
    JPasswordField repeatPassword;
 
    //**********************************************
    //*!CONSTANTS!!CONSTANTS!!CONSTANTS!!CONSTANTS!*
    //*!CONSTANTS!!CONSTANTS!!CONSTANTS!!CONSTANTS!*
    //**********************************************
 
    private int WindowWidth = 370;
    private int WindowHeight = 210;
 
    private String WindowName = "User configuration";
 
    private Members newMember;
    private boolean done = false;
    private boolean cancel = false;
 
 
    JFrame frame = new JFrame();
    JDialog dialog1 = new JDialog();
    JDialog dialog2 = new JDialog();
    public JPanel createContentPane()
    {
        JPanel totalGUI = new JPanel();
        totalGUI.setLayout(null);
 
        int titleSize = 320;
 
        titlePanel = new JPanel();
        titlePanel.setLayout(null);
        titlePanel.setLocation((WindowWidth/2)-titleSize/2,0);
        titlePanel.setSize(titleSize,30);
 
 
        textPanel = new JPanel();
        textPanel.setLayout(null);
        textPanel.setLocation(20,30);
        textPanel.setSize(WindowWidth-40,80);
 
        buttonPanel = new JPanel();
        buttonPanel.setLayout(null);
        buttonPanel.setLocation(20,WindowHeight-80);
        buttonPanel.setSize(WindowWidth-40,40);
 
        titleLabel.setLayout(null);
        titleLabel.setLocation(0,0);
        titleLabel.setSize(titleSize,20);
        titlePanel.add(titleLabel);
 
 
        firstNameLabel.setLayout(null);
        firstNameLabel.setLocation(0,0);
        firstNameLabel.setSize(150,20);
        textPanel.add(firstNameLabel);
 
        firstName = new JTextField();
        firstName.setLayout(null);
        firstName.setLocation(0,20);
        firstName.setSize(150,20);
        textPanel.add(firstName);
 
 
 
        lastNameLabel.setLayout(null);
        lastNameLabel.setLocation(180,0);
        lastNameLabel.setSize(150,20);
        textPanel.add(lastNameLabel);
 
 
        lastName = new JTextField();
        lastName.setLayout(null);
        lastName.setLocation(180,20);
        lastName.setSize(150,20);
        textPanel.add(lastName);
 
 
 
        passwordLabel.setLayout(null);
        passwordLabel.setLocation(0,40);
        passwordLabel.setSize(150,20);
        textPanel.add(passwordLabel);
 
 
        password = new JPasswordField();
        password.setLayout(null);
        password.setLocation(0,60);
        password.setSize(150,20);
        textPanel.add(password);
 
 
        repeatPasswordLabel.setLayout(null);
        repeatPasswordLabel.setLocation(180,40);
        repeatPasswordLabel.setSize(150,20);
        textPanel.add(repeatPasswordLabel);
 
        repeatPassword = new JPasswordField();
        repeatPassword.setLayout(null);
        repeatPassword.setLocation(180,60);
        repeatPassword.setSize(150,20);
        textPanel.add(repeatPassword);
 
        CompleteButton = new JButton("Complete");
        CompleteButton.setLayout(null);
        CompleteButton.setLocation(50,0);
        CompleteButton.setSize(100,40);
        buttonPanel.add(CompleteButton);
        CompleteButton.addActionListener(this);
 
        CancelButton = new JButton("Cancel");
        CancelButton.setLayout(null);
        CancelButton.setLocation(180,0);
        CancelButton.setSize(100,40);
        buttonPanel.add(CancelButton);
        CancelButton.addActionListener(this);
 
 
        totalGUI.add(titlePanel);
        totalGUI.add(textPanel);
        totalGUI.add(buttonPanel);
 
        totalGUI.setOpaque(true);
        return totalGUI;
    }
 
 
    public Members createGUI()
    {
        //JFrame.setDefaultLookAndFeelDecorated(true);
        //JFrame frame = new JFrame(WindowName);
 
 
        frame = new JFrame(WindowName);
 
        memberFrame MF = new memberFrame();
 
 
        dialog1 = new JDialog(frame);
        dialog2 = new JDialog(dialog1, "", Dialog.ModalityType.APPLICATION_MODAL);
 
        dialog2.setContentPane(MF.createContentPane());
        dialog2.setSize(WindowWidth,WindowHeight);
        dialog2.setLocationRelativeTo(null);
        dialog2.setResizable(false);
        dialog2.setVisible(true);
 
        while(true)
        {
            if(done == true)
            {
                return newMember;
                //JOptionPane.showMessageDialog(null,"Returning");
               // frame.dispose();
            }
 
            if(cancel == true)
            {
                return null;
            }  
        }
    }
 
 
    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource() == CompleteButton)
        {
            done = true;
            String Completefirstname = firstName.getText();
            String Completelastname = lastName.getText();
            String Completepassword = "Yey";
            newMember = new Members(Completefirstname,Completelastname,Completepassword);
            frame.dispose();
        }else if(e.getSource() == CancelButton)
        {
 
            int check = 0;
            String[] options = new String[]{"Yes", "No"};
            check = JOptionPane.showOptionDialog(null,
                    "Are you sure you wish to cancel?",
                    "Attention!",
                    JOptionPane.DEFAULT_OPTION,
                    JOptionPane.PLAIN_MESSAGE, null, options,
                    options[0]);
 
 
            if(check == 0)
            {
            	dialog2.dispose();
                frame.dispose();
                JOptionPane.showMessageDialog(null,"dispose");
            }
 
        }
    }
 
 
 
 
 
}

**aussiemcgr** · October 23rd, 2013, 07:37 PM

Why are you doing this in a while loop?

while(true)
        {
            if(this.done == true)
            {
                return this.newMember;
                //JOptionPane.showMessageDialog(null,"Returning");
               // frame.dispose();
            }
 
            if(this.cancel == true)
            {
                return null;
            }  
        }

And you get no exceptions or anything? What happens if you use the CompleteButton? Does frame get disposed?

**Lora_91** · October 24th, 2013, 12:12 AM

Currently, nothing happens when I press the complete button, I added a JOptionPane.showMessageDialog(null,"Complete"); just to check and it is being run as it should, but the frame isn't closed down same as with cancel. Since it isn't closed down the code never goes back to the loop and a value is never returned.

The while loop was an idea from my friend, the idea being that the loop would wait for the user until he/she was done putting in all of the information needed and then return the value. Although with JDialog it might not be needed.

**aussiemcgr** · October 24th, 2013, 07:23 AM

The while loop was an idea from my friend, the idea being that the loop would wait for the user until he/she was done putting in all of the information needed and then return the value. Although with JDialog it might not be needed.

That is not how the while loop would operate. In reality, it locks up the thread it is running on, since it runs forever and never moves on. An infinite loop is really only arguably needed if you are listening to a server socket. In almost every other situation, it can be devastating to your program.
I'm not sure if it will make a difference, but try removing the while loop and see if the frame closes. I don't think it will make a difference for you, but if that while loop is operating on the same thread as the frames, and the while loop is going forever, that may be why the frame is not closing: the thread is waiting for the while loop to finish before it can process the frame's disposal.

**Lora_91** · October 24th, 2013, 11:02 AM

Removed the while loop but it didn't make any difference, since the code never returns back to that part until the JDialog/frame is closed down, and it refuses to be shut down with the dispose call. =/

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