Changing a JTextField causing exception in cleanup method?

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;

public class MainForm extends 开发者_C百科JFrame{

    private JPanel p;
    private JButton clear;
    private JLabel nameLabel;
    private JTextField nameText;
    private JLabel genderLabel;
    private ButtonGroup genderButtonGroup;
    private JTextField courseText;

    public MainForm() {
        super("Some application");
        p = new JPanel();
        this.setLayout(new GridBagLayout());
        GridBagConstraints c = new GridBagConstraints();

        JLabel nameLabel = new JLabel("Student Name");
        c.gridx=0;
        c.gridy=0;
        c.gridwidth=1;
        c.gridheight=1;
        c.weightx=0.0;
        c.weighty=0.0;
        c.fill = GridBagConstraints.VERTICAL;
        c.insets= new Insets(4,4,4,4);
        this.getContentPane().add(nameLabel,c);

        JTextField nameText = new JTextField(20);
        c.gridx=1;
        c.gridy=0;
        c.gridwidth=1;
        c.gridheight=1;
        c.weightx=0.0;
        c.weighty=0.0;
        c.fill = GridBagConstraints.VERTICAL;
        c.insets= new Insets(4,4,4,4);
        this.getContentPane().add(nameText,c);
        nameText.setText("fsdf"); //works fine

        JButton clearButton = new JButton("Clear");
        clearButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { clearMainForm(); } });
        c.gridx=0;
        c.gridy=8;
        c.gridwidth=1;
        c.gridheight=1;
        c.weightx=0.0;
        c.weighty=0.0;
        c.fill = GridBagConstraints.VERTICAL;
        c.insets= new Insets(4,4,4,4);
        this.getContentPane().add(clearButton,c);

    }



    public void clearMainForm() {
        System.out.println("clearing");
        nameText.setText(""); // causes exception
    }



}

changing nameText just after it is created works fine, but trying it in clearMainFOrm, after the clear button is pressed causes an exception.

1) It helps to actually say what the exception is.

2) This line:

JTextField nameText = new JTextField(20);

sets a local variable, not the class variable. Change it to:

nameText = new JTextField(20);

and it'll work.

3) You're not setting any of your class variables. You're very shortly going to have more problems.