Java 带有sql语句的Actionlistener中的NullPointerException

我在从sql语句中获取密码时遇到了问题，当我尝试在一个简单的类中使用密码时，它对我来说非常有效，但当我尝试在GUI actionlistener中使用密码时，我获取了NullPointerException。这是我的一部分代码，我希望你能找到解决这个问题的办法

代码：

我的sql语句的代码：

    public String login(String username) throws ClassNotFoundException, SQLException, IndexOutOfBoundsException, NullPointerException
{
    DBConnection db = new DBConnection("sdj");
    ArrayList<Object[]> result;
    result = db.query("SELECT password FROM `users` WHERE username = ?", username);
    Object[] temp;
    String incorrect = "incorrect";
    try
    {
    temp = result.get(0);
    }
    catch (IndexOutOfBoundsException exception)
    {
        return incorrect;
    }
    String usr = (String) temp[0];
    return usr;
}

---

您不必在代码中的任何地方初始化变量

private DBController db

。然后在

actionPerformed（）

方法中，尝试调用此空变量的login方法：

temp = db.login(loginText.getText());

您可能会将其与您在登录方法中声明的db混淆在一起，这是不同的。

1）为了更快地获得更好的帮助，请发布一个。2） 源代码中只有一行空白就足够了。

{

之后或

}

之前的空行通常也是多余的。

java.lang.NullPointerException
at GUI.Main$LoginListener.actionPerformed(Main.java:90)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$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$200(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)

temp = db.login(loginText.getText());