Question

我使用以下代码从MySql数据库中检索数据。 GContnStr是我调用连接数据库的连接方法。但是，当我使用此代码时，我得到以下异常。我该如何解决这个问题？

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

此问题与数据库无关。你的问题就在这里：

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

您创建的String已初始化为null，然后立即使用，因此您已撰写options.valueOf(null);

此外，按照惯例，类名应以大写字母开头（即options应为Options），实例变量名称应为驼峰（即PoptnStr应为{{1} }}）。

Answer 2

您的字段的初始化

options val = options.valueOf(PoptnStr);

导致NPE。在执行此代码时，没有为PoptnStr分配值。将初始化移动到构造函数并确保已设置PoptnStr将修复此

Answer 3

你可以尝试初始化PoptnStr，如下所示

String PoptnStr = options.DBQryDef.name();

String PoptnStr = options.DBTABLE.name();

并且不明白为什么要为DBTABLE和DBQryDef启用val并且还要与DBTABLE null字符串进行比较：

 //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)
               {

你能详细说明一下吗？

从MySql数据库检索值到Java

3 个答案: