Retrieve value from MySql database to Java

Question

I use the following code to retrieve the data from MySql database. GContnStr is a connection method I call to connect to the database.But,when I use this code, I got the following exceptions. How can I solve this problem?

package designstudent;

import java.sql.*;
import java.awt.event.*;
import java.beans.EventHandler;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.table.DefaultTableModel;
import java.sql.*;
import java.util.ArrayList;
import java.util.jar.Attributes.Name;
import javax.swing.JOptionPane;
import javax.swing.ListSelectionModel;
import java.util.ArrayList.*;
import java.util.Vector;
import javax.swing.*;
public class combobutton extends javax.swing.JFrame {

     Statement TmpStmnt=null;
     ResultSet TmpDetlRs=null;
     GContnStr GCS=new GContnStr();
     String   GStrSql=null;


      private  String PFldname="StudClass";
      private  String PTName="studentmaster";



       enum options{
           DBTABLE,
           DBQryDef
                  }

     String PoptnStr;

     options val = options.valueOf(PoptnStr);

    public combobutton() throws SQLException {
        combo(PoptnStr, TmpStmnt, TmpDetlRs, GStrSql, PFldname, PTName);
        initComponents();
    }
public void combo(String PoptnStr,Statement TmpStmnt,ResultSet TmpDetlRs,String StrSql,String PFldName,String PTName) throws SQLException
    {
      cbx1.removeAllItems();
      cbx2.removeAllItems();
      cbx3.removeAllItems();
      String DBTABLE = null;
      try{
        switch(val)
              {
          case DBTABLE:
          case DBQryDef:
               if(PoptnStr==DBTABLE)
               {
                   GCS.GContnStr();
                   GStrSql="select '"+ PFldName+"' from '"+PTName+"'";
                   TmpDetlRs=TmpStmnt.executeQuery(GStrSql);
                    while(TmpDetlRs.next())
                    {
                        String TmpOb1=TmpDetlRs.getString("StudClass");
                        System.out.println(TmpOb1);
                    }

               }
      }
        }
      catch(Exception e){
          System.out.println(e);

      }
    }

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    new combobutton().setVisible(true);

                } catch (SQLException ex) {
                    Logger.getLogger(combobutton.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JComboBox cbx1;
    private javax.swing.JComboBox cbx2;
    private javax.swing.JComboBox cbx3;
    // End of variables declaration                   

}

ERROR

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Name is null
        at java.lang.Enum.valueOf(Enum.java:236)
        at designstudent.combobutton$options.valueOf(combobutton.java:30)
        at designstudent.combobutton.<init>(combobutton.java:37)
        at designstudent.combobutton$1.run(combobutton.java:122)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:660)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
BUILD SUCCESSFUL (total time: 1 second)

Answer 1

This problem has nothing to do with the database. Your problem is right here:

 String PoptnStr; 
 options val = options.valueOf(PoptnStr); 

You create your String which is initialized to null , then immediately use it, so you've written options.valueOf(null);

Also, by convention, class names should start with capital letters (ie options should be Options ) and instance variable names should be camel case (ie PoptnStr should be poptnStr ).

Answer 2

The initialization of your field

options val = options.valueOf(PoptnStr);

causes the NPE. At the point this code is executed there is no value assigned to PoptnStr . Move the initialization to the constructor and make sure PoptnStr is already set will fix this

Answer 3

can you try after initialize the PoptnStr either as below

String PoptnStr = options.DBQryDef.name();

String PoptnStr = options.DBTABLE.name();

And didnt understand why you are switching on val for DBTABLE and DBQryDef and also comparing with DBTABLE null string:

 //assigning the val variable 
    options val = options.valueOf(PoptnStr);

 //..... your other code

    //And comparing it as below:
   String DBTABLE = null;
      try{
        switch(val)
              {
          case DBTABLE:
          case DBQryDef:
               if(PoptnStr==DBTABLE)
               {

Can you elaborate it a bit more?