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Thread: JComboBox null pointer Exception error

  1. #1
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    Default JComboBox null pointer Exception error

    I can not figure out this JComboBox null pointer exception error. It happens when I attempt to use getSelectedItem() inside an ActionListener. The error doesn't actually initiate until I select an item with the combo box on the GUI when I run it. I don't understand why its null and after trying to figure it out for the past couple of hours I am stumped. where the combo box is created and initialized is highlighted in red. So is where the error is coming from.



    import java.awt.event.*;
    public class GUIController implements ActionListener
    {
       //members
       private GUIView view;
     
     
       public GUIController()
       {
          view = new GUIView(this);
     
       }
       public static void main(String [] args) 
       {  
          //GUIView one = new GUIView();
          GUIController control = new GUIController();
       }
     
     
       public void actionPerformed (ActionEvent evt)
       {            
          //Clear all text input
          if (evt.getActionCommand().equals("Clear Input"))
          {
     
             view.getNameTxt().setText("");
             view.getSocialSecurityTxt().setText("");
             view.getMaritalStatusTxt().setText("");
             view.getSalaryTxt().setText("");
             view.getBossNameTxt().setText("");
             view.getDepartmentTxt().setText("");
             view.getTitleTxt().setText("");
             view.getBonusTxt().setText("");
     
          }
     
         [COLOR="Red"] //null exception error occurs here
          String input = (String)view.getEmployeeTypeBox().getSelectedItem();[/COLOR]
          if (input.equals("Worker"))
            view.getBonusTxt().setEditable(false);
     
       }
    }


    And the GUIView Class.......................

    import java.awt.*;
    import java.awt.event.*;
     
    import javax.swing.*;
    import javax.swing.event.AncestorListener;
    public class GUIView extends JFrame
    {
       //members
       private ActionListener controller;
     
       //size of Frame
       private static final int LENGTH = 700;
       private static final int WIDTH = 400;
       //size of input field bar
       private static final int BAR_SIZE = 10;
       //the final JPanel to be put onto the JFrame
      // private JPanel mainPanel;
     
       private JList employeeList;
     
       private String input;
     
     
       //=========For construction of input panel (West region of GUI)===
     
       //panels for construction of west GUI portion
       private JPanel gridInputPanel;
       private JPanel inputPanel;
       //Text fields for inputPanel
       private JTextField txtName;
       private JTextField txtSocialSecurity;
       private JTextField txtMaritalStatus;
       private JTextField txtSalary;
       private JTextField txtBossName;
       private JTextField txtDepartment;
       private JTextField txtTitle;
       private JTextField txtBonus;
       //labels for inputPanel
       private JLabel lblName;
       private JLabel lblSocialSecurity;
       private JLabel lblMaritalStatus;
       private JLabel lblSalary;
       private JLabel lblBossName;
       private JLabel lblDepartment;
       private JLabel lblTitle;
       private JLabel lblBonus;
     
       //JButtons
       private JButton addEmployee;
       private JButton clearInputButton;
     
     [COLOR="Red"]  //combo box is declared here
       private JComboBox employeeTypeBox;
       private  String[] employeeTypes = {"  ","Employee", "Manager", "Worker"};
       [/COLOR]
       //=========For construction of RIGTH region of GUI===
       private JPanel gridEmploySelection;
       //buttons
       private JButton sortListButton;
       private JButton clearListButton;
     
     
       //may want to add another argument for different actions
       GUIView(ActionListener ctr)
       {
          super("Employee GUI");
     
          controller = ctr;
          setSize(LENGTH, WIDTH);
          setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          setLayout(new GridLayout(1,4));
          //build the panels
          buildInputPanel();
          //for crdating sort button and list
          configureListComponents();
          buildSortPanel();
          //Add input Panel to frame
          add(gridInputPanel, BorderLayout.WEST);
          //Add listPanel to fram
          employeeList.setVisible(true);
          add(employeeList, BorderLayout.CENTER);
          add(gridEmploySelection, BorderLayout.EAST);
          setVisible(true);
       }
     
     
     
       private void configureListComponents()
       {
          employeeList = new JList();
          employeeList.setVisibleRowCount(3);
     
       }
     
     
     
       private void  buildSortPanel()
       {
           gridEmploySelection = new JPanel();
           gridEmploySelection.setLayout(new GridLayout(10,1));
          //buttons
           sortListButton = new JButton("Sort List");
           clearListButton = new JButton("Clear List");
           gridEmploySelection.add(sortListButton);
           gridEmploySelection.add(clearListButton);
           //add rodio buttons
       }
     
       /*================
          buildInputPanel():
          is responsible for construcing
          the West portion of the GUI.
          Any other Jpanels used to help build
          inputPanel will also be constructed 
          inside buildInputPanel() and then
          added to inputPanel.
        ==================
        */
       private void buildInputPanel()
       { 
          //initializes the JPanels
          gridInputPanel = new JPanel();
          gridInputPanel.setLayout(new GridLayout(18,2));
     
          [COLOR="Red"]//Combo box is initailized here
          JComboBox employeeTypeBox = new JComboBox(employeeTypes);
          //Add action to combo box
          employeeTypeBox.addActionListener(controller);[/COLOR]
     
          //initialize input text fields
          txtName = new JTextField(BAR_SIZE);
          txtSocialSecurity = new JTextField(BAR_SIZE);
          txtMaritalStatus = new JTextField(BAR_SIZE);
          txtSalary = new JTextField(BAR_SIZE);
          txtBossName = new JTextField(BAR_SIZE);
          txtDepartment = new JTextField(BAR_SIZE);
          txtTitle = new JTextField(BAR_SIZE);
          txtBonus = new JTextField(BAR_SIZE);
          //initialize txt label fields
          lblName = new JLabel("Name");
          lblSocialSecurity = new JLabel("Social Security");
          lblMaritalStatus = new JLabel("Marital Status");
          lblSalary = new JLabel("Salary");
          lblBossName = new JLabel("Boss Name");
          lblDepartment = new JLabel("Department");
          lblTitle = new JLabel("Title");
          lblBonus = new JLabel("Bonus");
     
     
          //======Add components to gridInputPanel==========
          addEmployee = new JButton("Add Employee");
          clearInputButton = new JButton("Clear Input");
     
          //adding txt input
          gridInputPanel.add(txtName);
          gridInputPanel.add(lblName);
          gridInputPanel.add(txtSocialSecurity);
          gridInputPanel.add(lblSocialSecurity);
          gridInputPanel.add(txtMaritalStatus);
          gridInputPanel.add(lblMaritalStatus);
          gridInputPanel.add(txtSalary);
          gridInputPanel.add(lblSalary);
          gridInputPanel.add(txtBossName);
          gridInputPanel.add(lblBossName);
          gridInputPanel.add(txtDepartment);
          gridInputPanel.add(lblDepartment);
          gridInputPanel.add(txtTitle);
          gridInputPanel.add(lblTitle);
          gridInputPanel.add(txtBonus);
          gridInputPanel.add(lblBonus);
     
     
          //Adding buttons and other stuff
          gridInputPanel.add(addEmployee);
          gridInputPanel.add(employeeTypeBox);
          gridInputPanel.add(clearInputButton);
     
     
          //adding actions to buttons
          clearInputButton.addActionListener(controller);
     
       }
     
       /*PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP
     
                 ACCESSOR METHODS
     
        PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP*/
        public JTextField getNameTxt()
        {
           return txtName;
        }
     
        public JTextField getSocialSecurityTxt()
        {
           return txtSocialSecurity;
        }
     
        public JTextField getMaritalStatusTxt()
        {
           return txtMaritalStatus;
        }
     
        public JTextField getSalaryTxt()
        {
           return txtSalary;
        }
     
        public JTextField getBossNameTxt()
        {
           return txtBossName;
        }
     
        public JTextField getDepartmentTxt()
        {
           return txtDepartment;
        }
     
        public JTextField getTitleTxt()
        {
           return txtTitle;
        }
     
        public JTextField getBonusTxt()
        {
           return txtBonus;
        }
     
       [COLOR="Red"]//Combo box from GUI is accessed from Controller class here
        public JComboBox getEmployeeTypeBox()
        {
     
           return employeeTypeBox;
        }[/COLOR]
    }





    Thank you


  2. #2
    Administrator copeg's Avatar
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    Default Re: JComboBox null pointer Exception error

    With the following code:

    JComboBox employeeTypeBox = new JComboBox(employeeTypes);
    You are instantiating a local variable rather than your class variable, and so your class variable remains null. If you do the following you will instantiate the class variable instead.
    employeeTypeBox = new JComboBox(employeeTypes);

  3. The Following User Says Thank You to copeg For This Useful Post:

    F_Arches (November 29th, 2009)

  4. #3
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    Default Re: JComboBox null pointer Exception error

    Oh my gosh! Thanks! I didn't even realize that mistake even while looking for the problem. It works now. I can now finally continue with my program.


    Thank you so much

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