如何在没有代码错误的情况下修复 .NullPointerException 错误？

Question

很抱歉 post 是我第一次 post 访问 Whosebug。我正在做我的家庭作业，它包括创建一个银行账户模拟、存款和退休，当我完成我的程序时，控制台向我显示：

Exception in thread "main" java.lang.NullPointerException

    at java.desktop/java.awt.Container.addImpl(Container.java:1117)

    at java.desktop/java.awt.Container.add(Container.java:436)

    at Banco.createGUI(Banco.java:29)

    at Banco.main(Banco.java:16)

Process finished with exit code 1

这是我的代码，我花了一些时间试图修复它，我找不到我的值为 null 的变量在哪里，这就是我寻求帮助的原因。

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Banco extends JFrame implements ActionListener {

    private JLabel label1, label2;
    private JTextField Square1, Square2, Square3;
    private JButton Button1, Button2;
    private Account account;

    public static void main(String[] args) {
        Banco Ventana = new Banco();
        Ventana.setSize(300,300);
        Ventana.createGUI();
        Ventana.setVisible(true);
    }

    private void createGUI() {
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        Container window = getContentPane();
        window.setLayout(new FlowLayout());

        label1 = new JLabel("Balance");
        window.add(label1);

        Square1 = new JTextField(5);
        window.add(Square2);

        label2 = new JLabel("State");
        window.add(label2);

        Square2 = new JTextField(9);
        window.add(Square2);

        Button1 = new JButton("Deposit");
        Button1.addActionListener(this);
        window.add(Button1);

        Button2 = new JButton("Retire");
        Button2.addActionListener(this);
        window.add(Button2);

        Square3 = new JTextField(5);
        window.add(Square3);

        account = new Account();
    }

    public void actionPerformed(ActionEvent event) {
        int balance;
        int Amount;

        balance = account.getBalance();
        Amount = Integer.parseInt(Square3.getText());

        if(event.getSource() == Button1){
            account.setBalance(balance);
            account.depositBalance(Amount);
            balance = account.getBalance();
        }

        if (event.getSource() == Button2){
            account.setBalance(balance);
            account.retireBalance(Amount);
            balance = account.getBalance();
        }

        if (balance < 0){
            Square2.setText("Overdrawn");
        }
        else {
            Square2.setText("OK");
        }
        Square1.setText(Integer.toString(balance));
    }
}

class Account{

    int Balance=0;

    public void setBalance(int Amount){
        Balance = Amount;
    }

    public int getBalance(){
        return Balance;
    }

    public void depositBalance(int Amount){
        Balance = Balance + Amount;
    }

    public void retireBalance(int Amount){
        Balance = Balance - Amount;
    }
}

Answer 1

这看起来很可疑：

Square1 = new JTextField(5);
window.add(Square2);

您初始化 Square1，但尝试将 Square2 添加到 window。

Answer 2

所以在堆栈跟踪中，它说

Exception in thread "main" java.lang.NullPointerException
    at java.desktop/java.awt.Container.addImpl(Container.java:1117)
    at java.desktop/java.awt.Container.add(Container.java:436)
    at Banco.createGUI(Banco.java:29)
    at Banco.main(Banco.java:16)
Process finished with exit code 1

您可以跳过前两个 at，因为它不在您的代码中。第三行说 (Banco.java:29) 这是您的代码中发生错误的地方。

检查该行，您在第 28 行中为 Square1 赋值，但您在第 34 行中用 Square2 调用 add()。这意味着，在第 29 行，Square2 的值为 null，这就是为什么您得到 NullPointerException

如何在没有代码错误的情况下修复 .NullPointerException 错误？

How can i fix the .NullPointerException error without code errors?

java

nullpointerexception