Question

我正在分配我的组合框以处理Value或Car Name，所以我创建了这段代码，NullPointerException总是错误，我的代码是错误的吗？

 public CarRent() 
{
    initComponents();
    setLocationRelativeTo(null);

    try
    {
     AddItem();
    }
    catch(Exception e)
    {
       e.printStackTrace();
    }




}


public void Conn() throws Exception
{

  Class.forName("com.mysql.jdbc.Driver");
  conn = DriverManager.getConnection(conn1,user,pass);   

}

public void AddItem() throws Exception
{
            String s = "Select Car_Name from Car_Tbl";
            rs = st.executeQuery(s);

            while(rs.next())
            {
                System.out.println(cboCars);
                cboCars.addItem(rs.getString("Car_Name"));
            }



}

运行：显示java.lang.NullPointerException 在trypo.CarRent.AddItem（CarRent.java:77）在trypo.CarRent。（CarRent.java:50）在trypo.CarRent $ 4.run（CarRent.java:413）在java.awt.event.InvocationEvent.dispatch（InvocationEvent.java:312） at java.awt.EventQueue.dispatchEventImpl（EventQueue.java:745） at java.awt.EventQueue.access $ 300（EventQueue.java:103） at java.awt.EventQueue $ 3.run（EventQueue.java:706） at java.awt.EventQueue $ 3.run（EventQueue.java:704） at java.security.AccessController.doPrivileged（Native Method） at java.security.ProtectionDomain $ 1.doIntersectionPrivilege（ProtectionDomain.java:76） at java.awt.EventQueue.dispatchEvent（EventQueue.java:715） at java.awt.EventDispatchThread.pumpOneEventForFilters（EventDispatchThread.java:242） at java.awt.EventDispatchThread.pumpEventsForFilter（EventDispatchThread.java:161） at java.awt.EventDispatchThread.pumpEventsForHierarchy（EventDispatchThread.java:150） at java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:146） at java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:138） at java.awt.EventDispatchThread.run（EventDispatchThread.java:91）建立成功（总时间：3秒）

Answer 1

尝试

 Resultset rs = st.executeQuery(s)
 While(rs.next())
{
 cboCars.addItem(rs.getString("Car_Name");
}

您的rs变量需要初始化为ResultSet。

我也不知道你的代码在尝试之上。应该是声明

Statement st = null;
String s = "Select Car_Name from Car_Tbl";

填充数组cboCars后。

JComboBox carList = new JComboBox(cboCars);

.....

Statement stmt = null;
String q = "select Car_Name from Car_Tbl"
String[] cars ;
int i = 0;
try { 
    stmt = con.createStatement();
    ResultSet rs = stmt.executeQuery(query);
    while(rs.next()) {
    cars[i] = rs.getString("Car_Name");
    i++;
   } catch (SQLException e) {
    }finally{
     if(stmt !=null) { stmt.close();}
   }
}
// Now You have an array populated with cars . Create your combo box with the data

JComboBox carList = new JComboBox(cars);

将值从数据库传递到ComboBox

1 个答案: