新手循环判断问题，出现莫名其妙的报错，求大神，有代码

循环代码如图，已有List数据，从一个JTextField中获得文本数据，进入到循环，但获得的数据与List中的数据相同时，正常运行，但是获得的数据和List中的数据没有相同时，会在第一个if那报错，为什么啊?是判断条件的问题吗？public void updateUser(User user) {
for (int i = 0, len = userList.size(); i < len; i++) {
if (user.getNum().equals(userList.get(i).getNum())) {
String inputName = JOptionPane.showInputDialog(null,
"请输入新的用户名：\n");
if (inputName == null){
return;
}
if (inputName.equals("")) {
JOptionPane.showMessageDialog(null, "请输入修改后信息",
"提示", JOptionPane.ERROR_MESSAGE);
return;
}else{
user.setName(inputName);
userList.get(i).setName(user.getName());
break;
}
}
if (i == len - 1 &&!user.getNum().equals(userList.get(i).getNum())) {

            JOptionPane.showConfirmDialog(null, "未知用户",
                    "提示", JOptionPane.CLOSED_OPTION);
        }
    }
}

吧len=userList.size()放for外面去

我觉得你要受限判断一下userList是否为null

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at com.niit.frame.UserDAOImpl.updateUser(UserDAOImpl.java:46)
at com.niit.frame.StudentFrame.actionPerformed(StudentFrame.java:163)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6535)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6300)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4891)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

user.getNum()跟userList.get(0).getNum()打出来看看

public void updateUser(User user) {
for (int i = 0, len = userList.size(); i < len; i++) {

if(user.getNum()!=null){

            if (user.getNum().equals(userList.get(i).getNum())) {
                String inputName = JOptionPane.showInputDialog(null,
                        "请输入新的用户名：\n");
                if (inputName == null){
                    return;

        }
    }