java无法查找符号错误，空指针

3 月，2 周 Questions & Answers 154

现在没有编译错误，值不会显示，只有消息“number:” 我已经来过这里了，除了在它显示“number:NULL”之前，我想我已经到了那里。。。谢谢大家。我已经读了大约一个星期的文章，但今晚我和你一起学到了更多

登录类

public class Login_JPanel extends JPanel
{
    JLabel welcomeMessage,
    mainPicture;

    JButton playerregistrationJB = new JButton ( "Player Registration" );   

    JLabel userAccountNumberJLabel  = new JLabel("<HTML><CENTER><FONT SIZE=6 
    COLOR=YELLOW>Please, enter your account number below and then click on player
    registration to continue </COLOR></CENTER></HTML>");

    JTextField useraccountnumberJTF  = new JTextField();

    public Login_JPanel()
    {   
        //========================================================
        //SET UP USERNAME JLABEL AND TEXT FIELD
        //========================================================
        add(userAccountNumberJLabel);
        userAccountNumberJLabel.setBounds( 322, 335, 300, 155 );

        add(useraccountnumberJTF);
        useraccountnumberJTF.setBounds (330, 500,  90, 25 ); 

        playerregistrationJB.setBounds( 350, 600, 325, 75 );
        playerregistrationJB.setFont( new Font( "Broadway", Font.BOLD, 30 ) );
        playerregistrationJB.setForeground( Color.red );
        playerregistrationJB.setBackground( Color.YELLOW );
        add( playerregistrationJB);         

        add( welcomeMessage = new JLabel( "Welcome!!" ) );
        welcomeMessage.setBounds(0,0,50,23);

        add( mainPicture = new JLabel( new ImageIcon("henkidama.jpg") ) );
        mainPicture.setBounds(0,0,50,50);

        setLayout(null);
        setBounds(0,0,1000,800);    
    }   
}

这是playerregistration面板

public class PlayerRegistration_JPanel extends JPanel
{

    Login_JPanel loginJP = new Login_JPanel();

    JLabel welcomeMessage,
    mainPicture;

    JLabel useraccountnumberJL = new JLabel();

    JButton submitJB = new JButton ( "Submit" );    

    public PlayerRegistration_JPanel()
    {
        add(useraccountnumberJL);
        useraccountnumberJL.setText("number: " +  
                       loginJP.useraccountnumberJTF.getText());    
        useraccountnumberJL.setBounds( 100, 75, 625, 200 );
        useraccountnumberJL.setFont( new Font( "Broadway", Font.BOLD, 18 ) );

        submitJB.setBounds( 350, 600, 325, 75 );
        submitJB.setFont( new Font( "Broadway", Font.BOLD, 30 ) );
        submitJB.setForeground( Color.red );
        submitJB.setBackground( Color.YELLOW );
        add( submitJB); 

        add( welcomeMessage = new JLabel( "Welcome to Building Another Panel!!" ) );
        welcomeMessage.setBounds(0,0,50,23);

        add( mainPicture = new JLabel( new ImageIcon("henkidama.jpg") ) );
        mainPicture.setBounds(0,0,50,50);

        setLayout(null);
        setBounds(0,0,1000,800);        
    }   
}

有一个JLabel请求用户输入一个数字，到目前为止，用户应该单击String，然后数字出现在PlayerRegistration_JPanel中。此外，还有一个ProccessedJPanel，我通过ActionListener调用所有按钮，也是一个finalJpanel，我在一个帧中有我的主帧。我不知道问题出在哪里，因为我的JTextField是全局的（尽管我们在Java中没有全局变量这样的东西）

Tags:

共 (5) 个答案

# 1 楼答案

你的JTextField useraccountnumberJTF不是全局的。它是Login_JPanel类的成员变量。这意味着Login_JPanel类的每个实例都有一个这样的文本字段。你应该如何知道你指的是哪个领域

如果想要访问该字段，请在PlayerRegistration_JPanel的构造函数中传递Login_JPanel的实例，并要求该实例提供其字段

这是一个相当基本的OO概念。也许重读一些教程是好的，比如this one

# 2 楼答案

你的useraccountnumberJTF在哪里。你必须在另一个类中声明它。为了访问其他类属性，必须在类中创建其他类的对象，然后访问其他类属性。此外，这两个类必须在同一个包中

public class PlayerRegistration_JPanel extends JPanel
{
     Login_JPanel login = new Login_JPanel();
     public PlayerRegistration_JPanel() 
     {
          add(useraccountnumberJL);
          useraccountnumberJL.setText(login.useraccountnumberJTF.getText());
          useraccountnumberJL.setBounds( 100, 75, 625, 200 );
          useraccountnumberJL.setFont( new Font( "Broadway", Font.BOLD, 18 ) );
      }
}

# 3 楼答案

为什么你认为你可以在第二节课上直接使用useraccountnumberJTF

您正在混合两个类的变量。如果不引用第一个类，就不能在第二个类中直接使用在一个类中定义的变量。在第二个类中没有useraccountnumberJTF，所以它在该类中给出了错误。你能做的就是找到一种方法把这个变量值传递给第二类

# 4 楼答案

您必须声明useraccountnumberJTF：

import java.awt.Color;
import java.awt.Font;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Login_JPanel extends JPanel {
    JLabel welcomeMessage, mainPicture;

    JButton playerregistrationJB = new JButton("Player Registration");
    JLabel userAccountNumberJLabel = new JLabel("<HTML><CENTER><FONT SIZE=6 COLOR=YELLOW>Please, enter your account number below and then click on playerregistration to continue </COLOR></CENTER></HTML>");
    JTextField useraccountnumberJTF = new JTextField();

    public Login_JPanel() {
        // ========================================================
        // SET UP USERNAME JLABEL AND TEXT FIELD
        // ========================================================
        add(userAccountNumberJLabel);
        userAccountNumberJLabel.setBounds(322, 335, 300, 155);

        add(useraccountnumberJTF);
        useraccountnumberJTF.setBounds(330, 500, 90, 25);

        playerregistrationJB.setBounds(350, 600, 325, 75);
        playerregistrationJB.setFont(new Font("Broadway", Font.BOLD, 30));
        playerregistrationJB.setForeground(Color.red);
        playerregistrationJB.setBackground(Color.YELLOW);
        add(playerregistrationJB);

        add(welcomeMessage = new JLabel("Welcome!!"));
        welcomeMessage.setBounds(0, 0, 50, 23);

        add(mainPicture = new JLabel(new ImageIcon("henkidama.jpg")));
        mainPicture.setBounds(0, 0, 50, 50);

        setLayout(null);
        setBounds(0, 0, 1000, 800);
    }

    public String getUseraccountnumberJTFText() {
        return useraccountnumberJTF.getText();
    }
}

PlayerRegistration_JPanel：

import java.awt.Color;
import java.awt.Font;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class PlayerRegistration_JPanel extends JPanel
{
    JLabel welcomeMessage,
           mainPicture;
    JLabel useraccountnumberJL = new JLabel();
    JButton submitJB = new JButton ( "Submit" );    

    public PlayerRegistration_JPanel(Login_JPanel panel)
    {
        add(useraccountnumberJL);
        useraccountnumberJL.setText(panel.getUseraccountnumberJTFText());
        useraccountnumberJL.setBounds( 100, 75, 625, 200 );
        useraccountnumberJL.setFont( new Font( "Broadway", Font.BOLD, 18 ) );

        submitJB.setBounds( 350, 600, 325, 75 );
        submitJB.setFont( new Font( "Broadway", Font.BOLD, 30 ) );
        submitJB.setForeground( Color.red );
        submitJB.setBackground( Color.YELLOW );
        add( submitJB); 

        add( welcomeMessage = new JLabel( "Welcome to Building Another Panel!!" ) );
        welcomeMessage.setBounds(0,0,50,23);

        add( mainPicture = new JLabel( new ImageIcon("henkidama.jpg") ) );
        mainPicture.setBounds(0,0,50,50);

        setLayout(null);
        setBounds(0,0,1000,800);
    }   
}

请阅读以下内容：http://java.about.com/od/javasyntax/a/nameconventions.htm

编辑：

课程的声明应该是：

private Login_JPanel loginPanel;

private void theMethodWhoDeclareLoginJPanel() {
    loginPanel = new Login_JPanel();
}

private void theMethodWhoDeclarePlayerRegistrationJPanel() {
    new PlayerRegistration_JPanel(loginPanel);
}

# 5 楼答案

您没有在类PlayerRegistration_JPanel中声明useraccountnumberJTF（它只在Login_JPanel类中声明），但您在这一行中调用了它。这是你的错误

Python中文网

有 Java 编程相关的问题?

java无法查找符号错误，空指针

共 (5) 个答案

# 1 楼答案

# 2 楼答案

# 3 楼答案

# 4 楼答案

# 5 楼答案