Java：在一个类中使用Actionlistener来引用另一个类中的变量

Question

I have a JTextField and a button in a class called Main. I have an ActionListener in another class called Action. I want the ActionListener to be able to refrence the JTextField. I keep getting a null pointer exception. I want to keep the JTextField and ActionListener seperate. I am going to be having many ActionListeners and it would be easier for me to organize it this way.

public class Main  {

    public JTextField text;

    public JTextField getText(){
        return this.text;
    }

public  static void main(String[] args) {

        Main main=new Main();
        main.blah();
    }

public  void blah(){

    JFrame myWindow=new JFrame("ff");
    myWindow.setSize(500,500);
    myWindow.setVisible(true);
    myWindow.setDefaultCloseOperation(3);
    text=new JTextField(10);
    JLabel lengthL = new JLabel("Enter a number",SwingConstants.CENTER  );
    JButton button=new JButton("Click button");
    myWindow.getContentPane();
    myWindow.setLayout(new GridLayout(4,4));
    myWindow.add(lengthL);
    myWindow.add(text);
    myWindow.add(button);

    Action hand=new Action();
    button.addActionListener(hand);
}
}



public  class Action  implements ActionListener{

    @Override
    public void actionPerformed(ActionEvent e) {
        Main main=new Main();
        double length=Double.parseDouble(main.text.getText());
        System.out.println(length);

    }
}

Answer 1

Why not pass the JTextField when you create the ActionListener, like so:

public  class Action  implements ActionListener{
    private JTextField text;

    public Action(JTextField text) {
        this.text = text;
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        double length=Double.parseDouble(text.getText());
        System.out.println(length);
    }
}

//in Main:
public void blah(){
    JFrame myWindow=new JFrame("ff");
    myWindow.setSize(500,500);
    myWindow.setVisible(true);
    myWindow.setDefaultCloseOperation(3);
    text=new JTextField(10);
    JLabel lengthL = new JLabel("Enter a number",SwingConstants.CENTER  );
    JButton button=new JButton("Click button");
    myWindow.getContentPane();
    myWindow.setLayout(new GridLayout(4,4));
    myWindow.add(lengthL);
    myWindow.add(text);
    myWindow.add(button);

    Action hand=new Action(text);  //change this line
    button.addActionListener(hand);
}

Answer 2

I keep getting a null pointer exception

That's because you're creating a new Main instance when you handle the event:

public void actionPerformed(ActionEvent e) {
    Main main=new Main();
    ...

You should better have the action a reference of the container class ( Main ) and a method to access that textfield value:

   Main aMain; 
   public void actionPerformed(ActionEvent e) {
    this.aMain.getText();
    ....

Or even better:

    public void actionPerformed(ActionEvent e) {
       double lenght = this.main.getValue();// returns double 
       ...