Ciągle otrzymuję błąd Run Time, który mówi, że mam wyjątek w wątku „AWT-EventQueue-0” java.lang.NullPointerException, który mówi [linia 65], ale dla mnie wygląda jak zwykła instrukcja if. Mogę podać trochę tła. ta ramka jest wywołaniem z sekwencji ramek, która jest wtedy po wybraniu stanu wysłana do nowej ramki zawierającej stany miast A-C. Ta aktualna ramka będzie działać tylko wtedy, gdy Alabama nie jest wybranym stanem. (Nie stworzyłem innych ramek dla innych stanów, dlatego są skomentowane). Mogę umieścić kod w innej ramce, do której ma zadzwonić, jeśli ktoś tego potrzebuje, ale pomyślałem, że na początku jest dużo kodu z. Z góry dziękuję za pomoc wszystkim.

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.*;
import java.io.*;
import java.util.*;

public class WetBulbByLocationFrameStates3 extends JFrame
{
  public JLabel stateLabel;
  public JPanel statePanel = new JPanel();
  public JComboBox stateBox;
  public String [] unitedStates = {"Alabama", "Alaska", "Arizona", "Arkansas", "California", "Colorado", "Connecticut", "Delaware", "District of Columbia", "Florida", "Georgia", "Hawaii", "Idaho", "Illinois", "Indiana", "Iowa", "Kansas", "Kentucky", "Louisiana", "Maine", "Maryland", "Massachusetts", "Michigan", "Minnesota", "Mississippi", "Missouri", "Montana", "Nebraska", "Nevada", "New Hampshire", "New Jersey", "New Mexico", "New York", "North Carolina", "North Dakota", "Ohio", "Oklahoma", "Oregon", "Pennsylvania", "Rhode Island", "South Carolina", "South Dakota", "South Pacific Islands", "Tennessee", "Texas", "Utah", "Vermont", "Virginia", "Washington", "West Virginia", "Wisconsin", "Wyoming"};
  public String [] canadianProvinces = {"Alberta", "British Columbia", "Manitoba", "New Brunswick", "Newfoundland and Labrador", "Northwest Territories", "Nova Scotia", "Nunavut", "Ontario", "Prince Edward Island", "Quebec", "Saskatchewan", "Yukon Territory"};
  public JButton nextButton = new JButton("Next");
  public JPanel buttonPanel = new JPanel();
  public String state;
  public String countryGIVEN;

  public WetBulbByLocationFrameStates3(String country)
  {
    Toolkit toolkit = Toolkit.getDefaultToolkit();
    Dimension dimensions = toolkit.getScreenSize();
    int x = (dimensions.width - 315)/2; 
    int y = (dimensions.height - 250)/2;
    setBounds(x, y, 315, 250); 
    setVisible(true);
    setResizable(false);
    setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    countryGIVEN = country;

    nextButton.addActionListener(new ButtonListener3());

    if (country.equals("United States of America"))
    {
      stateBox = new JComboBox(unitedStates);
      setTitle("Select State");
      stateLabel = new JLabel("State");
    }
    else if (country.equals("Canada"))
    {
      stateBox = new JComboBox(canadianProvinces);
      setTitle("Select Province");
      stateLabel = new JLabel("Province");
    }

    stateBox.addActionListener(new ComboBoxListener2());

    statePanel.add(stateLabel);
    statePanel.add(stateBox);

    add(statePanel, BorderLayout.CENTER);

    buttonPanel.add(nextButton);

    add(buttonPanel, BorderLayout.SOUTH);
  }
  private class ButtonListener3 implements ActionListener
  {
    @Override
    public void actionPerformed(ActionEvent e)
    {
      if (countryGIVEN.equals("United States of America"))
      {
        if (state.equals("Alabama"))
        {
          JFrame wbfs1 = new WetBulbByLocationFrameStateGroupA_C(state);
        }
        else if (state.equals("Alaska"))
        {
          JFrame wbfs2 = new WetBulbByLocationFrameStateGroupA_C(state);
        }
        else if (state.equals("Arizona"))
        {
          JFrame wbfs3 = new WetBulbByLocationFrameStateGroupA_C(state);
        }
        else if (state.equals("Arkansas"))
        {
          JFrame wbfs4 = new WetBulbByLocationFrameStateGroupA_C(state);
        }
        else if (state.equals("California"))
        {
          JFrame wbfs5 = new WetBulbByLocationFrameStateGroupA_C(state);
        }
        else if (state.equals("Colorado"))
        {
          JFrame wbfs6 = new WetBulbByLocationFrameStateGroupA_C(state);
        }
        else if (state.equals("Connecticut"))
        {
          JFrame wbfs7 = new WetBulbByLocationFrameStateGroupA_C(state);
        }
      }
    }
  }
  private class ComboBoxListener2 implements ActionListener
  {
    @Override
    public void actionPerformed(ActionEvent e)
    {
      state = (String)stateBox.getSelectedItem();
    }
  }
}

PS oto błąd przy wyborze Alabamy.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at WetBulbByLocationFrameStates3$ButtonListener3.actionPerformed(WetBulbByLocationFrameStates3.java:65)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$000(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)