未知 java class 错误

Question

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JButton;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JTextField;
public class autos extends JPanel implements ActionListener {
    private static final long serialVersionUID = 1L;
    String get;
    int logic = 0, b;
    JFrame frame = new JFrame("Tic Tac Toe");
    JTextField text = new JTextField(1);
    public autos() {
        JButton button = new JButton("LOAD");
        add(button);
        add(text);
        setLayout(null);
        button.setBounds(70, 70, 80, 40);
        text.setBounds(140, 140, 20, 20);
        text.addActionListener(this);
        if (duvoi.equals("x")) logic = 1;
        else if (get.equals("0")) logic = 2;
        else logic = 0;
    }
    public void paint(Graphics g) {
        super.paint(g);
        if (logic == 1) 
        g.drawString("You entered x", 200, 100);
        else if (logic == 2) 
        g.drawString("You entered", 200, 100);
        else b = 23;
        repaint();
    }
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.getContentPane().add(new autos());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 400);
        frame.setVisible(true);
    }
    @Override
    public void actionPerformed(ActionEvent e) {
        get = text.getText();
    }
}

当我尝试运行代码时，出现以下错误：

Exception in thread "main" java.lang.NullPointerException at autos.(autos.java:27) at autos.main(autos.java:48)

我正在使用 eclipse.Why 会出现这些错误吗？我该怎么做才能消除这些错误？

Answer 1

声明后永远不会初始化 get。

String get;

然后你打电话给

else if(get.equals("0"))

此时，get 为 null。因此，您说的 else if (null.equals("0")) 导致了 NullPointerException。一种可能的解决方案是 Yoda conditions。喜欢，

else if("0".equals(get)) // <-- won't throw NPE.

未知 java class 错误

Unknown java class errors

java

compiler-errors

jpanel

jframe