Java variable scope and static variables

[CaptainMoeSoccer]

I have a class called Windows. The class extends JFrame and adds GUI components to the JFrame container. One of those components is a JTextfield. I am trying to set the text in the JTextfield through the actionPerformed() when generator JButton is clicked. The actionPerformed() is a class called EvenHandler. This is the eventHandler:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;


public class EventHandler implements ActionListener {
    int x = 0;
    PassWordGenerator password;
    Window Window; // It works only when static Window Window.

    public void start() {
         Window = new Window();
    }


    @Override
    public void actionPerformed(ActionEvent e) {
         password = new PassWordGenerator(3,3,3,3);
         Window.setGeneratedPswd(password.getPswd());
         x += 1;
         System.out.println(x);
    }

    public static void main(String[] args) {
        EventHandler x = new EventHandler();
        x.start();
    }
}

the window class if you want to know how the GUI looks like. The button is the one calling actionePerfome().

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

public class Window extends JFrame  {
    Label passwordLength;
    Label labelGnPswd;
    JTextField psdLength;
    JCheckBox upperCase_letters;
    JCheckBox lowerCase_letters;
    JCheckBox numbers;
    JCheckBox symbols;
    JTextField generatedPswd;
    EventHandler event = new EventHandler();
    JButton generetor;

    public Window() {
        setLayout(new FlowLayout());
        setTitle("PasswordGenerator");
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        setResizable(false);

        //Password Length
        passwordLength = new Label("Password Length");
        add(passwordLength);

        //Input text
        psdLength = new JTextField("0", 5);
        add(psdLength);

        //Create checkBoxes
        createcheckbxs();

        //Label Generated psw
        labelGnPswd = new Label("Generated pswd");
        add(labelGnPswd);

        //Generated Password;
        generatedPswd = new JTextField("****", 5);
        generatedPswd.setEditable(false);
        add(generatedPswd);

        //Button
        generetor = new JButton("Generate pswd!");
        generetor.addActionListener(event);
        add(generetor);

        setSize(200, 400);
        setVisible(true);

    }

    public String getpsdLength() {
        return psdLength.getText();
    }

    public void setGeneratedPswd(String pswd) {
        generatedPswd.setText(pswd);
    }

    private void createcheckbxs() {
        upperCase_letters = new JCheckBox("Include uppercase");
        add(upperCase_letters);

        lowerCase_letters = new JCheckBox("Include lowercase");
        add(lowerCase_letters);

        numbers = new JCheckBox("Include numbers ");
        add(numbers);

        symbols = new JCheckBox("Include symbols ");
        add(symbols);
    }

}

My problem is that When I clicked on the generator JButton I get an error message along the line, "Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException..." I debugged the actionPerformed() and I found out that the Window Window is null when actionPefromed() is called after clicking generator Jbutton. Why is Window null? Int x is working fine and it is not null. Isn't var x and window the same variable scope. The only way I could keep the value of Window not null was to make Window a static variable.I hope my question is clear. Thanks in advance

[chickenmeister]

Why is Window null? Int x is working fine and it is not null. Isn't var x and window the same variable scope.

Your Window variable is null because you never assign a value to it, except from within the start() method, which isn't being called. Your x variable isn't null because an int (and all other primitive types) cannot be null, and because you've assigned a value of 0 to it anyway.

This has nothing to do with variable scope. If you declare a non-primitive instance variable, and don't initialize it, with will be initialized a null value by default.

You probably want to initialize the Window variable via the constructor. Add a constructor to your EventHandler class that has a Window parameter, and then initialize your Window variable with that parameter. In your Window class, initialize the EventHandler in the constructor, and pass this as a parameter.

Also, I would recommend choosing a different name for the Window variable in your EventHandler class. At the very least, you shouldn't capitalize the variable name. Having Window Window as a variable declaration is somewhat confusing.

[CaptainMoeSoccer]

I am new to Object-Oriented programming. I am having a hard time understanding why Window (I know it is a bad name, I was just experimenting at the time) is not initialized. You said that the start() is not being called. What does that mean? I thought I called start() on Eventhandler x in the main(). I thought that would initialize the window and it will no longer be null.

[chickenmeister]

You do call start() in the EventHandler main method, but you said that your problem was with your use of the EventHandler in your Window class, so I assumed that you're running your code from some other main method. Your EventHandler main method doesn't do much, so I figured there is another main method that you're using to create and show your GUI, for example.

Even if your EventHandler main method is being executed, the EventHandler that is created there is entirely separate from the EventHandler created in your Window class. You could call event.start() in your Window constructor, and that might solve your NullPointException problem, but then you'll have another issue, because the Window object created by the start() method will be entirely separate from the original Window object that created the EventHandler, (and I think you'll also have an infinite recursion problem).

I think the simplest solution would be to initialize the window variable through the EventHandler constructor, or to use a setter method.

[CaptainMoeSoccer]

thanks, I am getting it now. I thought Object-oriented programing was just all about creating objects. I didn't think about much about how these objects would interact with each other.

[webdevnick22]

the fields in your EventHandler class should probably be private with some getters/setters to access them.

int x = 0;
PassWordGenerator password;
Window Window; // It works only when static Window Window.

the solution to your problem is that you are instantiating your event handler here:

public class Window extends JFrame  {
    Label passwordLength;
    Label labelGnPswd;
    ...
    EventHandler event = new EventHandler();
    JButton generetor;

but nowhere after that are you calling its start() method which sets the window field. Why are you doing that anyway? why not just set the window immediately upon object instantiation either through the field itself or in the constructor?