无法从静态上下文引用非静态变量

Question

我正在尝试编写一段代码，当我选中两个复选框中的一个复选框时，它将更改我选择按钮时显示的消息。

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

public class  FirstWindow extends JFrame{

        String message;

        public static void main(String[] args){

            //the string to show the message for button 1
            message = "goodjob \n";

            //all of the main window
            JFrame frame = new JFrame("heading");
            frame.setVisible(true);
            frame.setSize(600,400);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setLocationRelativeTo(null);

            //makes seperate panels
            JPanel panel1 = new JPanel();
            JPanel panel2 = new JPanel();
            JPanel panel3 = new JPanel(new GridBagLayout());

            //set of buttons
            JButton button1 = new JButton("Button 1");
            JButton button2 = new JButton("Button 2");

            //makes an ACTION LISTENER, which tells the button what to do when
            //it is pressed.
            button1.addActionListener(new ActionListener(){

                    //the command to button 
                    @Override
                    public void actionPerformed(ActionEvent e) {
                    JOptionPane.showMessageDialog(null, message);

                    }
        });

            //for panel adds buttons
            panel1.add(button1);
            panel1.add(button2);

            //set of checkboxes 
            JCheckBox checkBox = new JCheckBox("Option1");
            JCheckBox checkBox2 = new JCheckBox("Option2");

            //adds text if the box is checked
                if (checkBox.isSelected())
                {
                    message += "you picked option 1";
                }
            if (checkBox2.isSelected())
                {
                    message += "you picked option 2";
                }   

            //for panel2 adds checkboxes
            panel2.add(checkBox);
            panel2.add(checkBox2);

            //makes a new label, text area and field
            JLabel label = new JLabel("This is a label");
            JTextArea textArea = new JTextArea("this is a text area");
            JTextField textField = new JTextField("text field");

            //makes an invisible grid
            GridBagConstraints grid = new GridBagConstraints();
            grid.insets = new Insets(15, 15, 15, 15);

            //sets the the coordinates 0,0. adds the label, and sets position
            grid.gridx = 0;
            grid.gridy = 0;
            panel3.add(label, grid);

            //sets the the coordinates 0,1. adds the textArea, and sets position
            grid.gridx = 0;
            grid.gridy = 1;
            panel3.add(textArea, grid);

            //sets the the coordinates 0,2. adds the text field, and sets position
            grid.gridx = 0;
            grid.gridy = 2;
            panel3.add(textField, grid);

            //adds each PANEL to the FRAME and positions it
            frame.add(panel1, BorderLayout.SOUTH);
            frame.add(panel3, BorderLayout.CENTER);
            frame.add(panel2, BorderLayout.NORTH);

        }
}

我收到的错误消息是：

"FirstWindow.java:12：错误：不能从静态上下文消息中引用非静态变量消息="good job\n"；"

对于第12、37、53、57行。我已尝试在main中声明字符串变量，但我只会收到错误：

“FirstWindow。java:38：错误：从内部类引用的局部变量必须是final或有效的final JOptionPane。showMessageDialog（null，message）；”

如何解决这个问题？

Answer 1

我建议您创建一个构造函数，并避免在这种情况下使用静态变量：

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

public class  FirstWindow extends JFrame{

    private String message;

    FirstWindow (){

        super("heading");

        //the string to show the message for button 1
        message = "goodjob \n";

        //all of the main window
        // remove this: JFrame frame = new JFrame("heading");
        // remove this: frame.setVisible(true);

        // change frame.setSize(600,400); by:
        setSize(600,400);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);

        // the rest of your code with the JFrame instantiation
        // ...

        //adds each PANEL to the FRAME and positions it
        // here also change frame.add(... by just add(... as follows
        add(panel1, BorderLayout.SOUTH);
        add(panel3, BorderLayout.CENTER);
        add(panel2, BorderLayout.NORTH);
    }

    public static void main(String[] args){

        new FirstWindow().setVisible(true);
    }

}

Answer 2

这里的问题是，您在静态方法public static void main（）之外声明了非静态变量字符串消息。所以有三种可能的解决方案

>

在main方法内声明字符串消息

public static void main(String []args){
String message = "goodjob \n";

根据上面的答案，您可以将String消息更改为静态

static String message;
public static void main(String []args){
message = "goodjob \n";

创建FirstWindow类的对象，然后访问字符串消息

FirstWindow sampleFirst = new FirstWindow();
sampleFirst.message = "goodjob \n";

Answer 3

将消息字段设置为静态：

static String message;

您的主类是静态的（必须如此）。这意味着您没有类的实例。但字段消息目前不是静态的，这意味着它只作为实例的一部分存在。由于您没有实例，因此无法访问它。

作为附带说明：我强烈建议您找人对这段代码进行代码审查。有很多东西需要学习。考虑将其提交给Code Review。