Java 小程序没有按应有的方式重印

Question

这是一个我正在忙着制作的小程序，但它没有按应有的方式更新。任何人都可以告诉我哪里出了问题吗？这是我正在学习的类(class)的一部分，因此它必须或多或少保持原样。 请记住，我是 java 的初学者，谢谢大家......

import java.awt.*;
import java.applet.*;
import java.awt.event.*;


public class PasswordApplet extends Applet implements ActionListener
{
   //Declaring variables
   String id, password;
   boolean success;
   String idArray[] = {"Reign", "Test", "Others"};
   String passwordArray[] = {"130489", "1234", "1111"};


   //Create components for applet
   Label headerLabel = new Label("Please type your ID and Password");


   Label idLabel = new Label("ID:");
      TextField idField = new TextField(8);

   Label passwordLabel = new Label("Password:");
      TextField passwordField = new TextField(8);


   Button loginButton = new Button("Login");

   public void init()
   {
      //Set color, layout, and add components
      setBackground(Color.orange);

      setLayout(new FlowLayout(FlowLayout.LEFT,50,30));

      add(headerLabel);

      add(idLabel);
         add(idField);
         idField.requestFocus();

      add(passwordLabel);
         add(passwordField);
         passwordField.setEchoChar('*');

      add(loginButton);
         loginButton.addActionListener(this);

    }

   public void actionPerformed(ActionEvent e)
   {
       success = false;

      //Sequential search
      for (int i = 0; i<idArray.length; i++)
      {
   if ((idArray[i].compareTo(id)==0)&&(passwordArray[i].compareTo(password)==0))
            success=true;
      }

      if(success=true)
      {
        headerLabel.setText("Login Successful");
        headerLabel.repaint();


      }
      else
      {
        headerLabel.setText("Invalid. Try Again");
        headerLabel.repaint();

      }
        repaint();

    }
}

Answer 1

在 ActionListener 中比较其值之前，您需要分配 id 字段

id = idField.getText();

否则将抛出NPE，阻止任何 UI 更新