什么原因导致“无法找到符号”和如何解决它？ [英] What causes "Can't find Symbol" and how to fix it?

问题描述

我一直在努力想出这一点，我在不同的程序运行它，所以它绝对在代码中。也许很容易。错误提示

Password2.java:90：错误：找不到符号
if（pw.equals（password））
^
symbol：variable password
location：class Password2.EnterButtonHandler
1错误

这里是代码：

  // Password1.java 
 
 import javax.swing。 
 import java.awt。*; 
 import java.awt.event.ActionEvent; 
 import java.awt.event.ActionListener; 
 
 public class Password2 extends JFrame //继承JFrame类
 {
 //静态最终变量来保存框架尺寸（以像素为单位）
 private static final int WIDTH = 400; 
 private static final int HEIGHT = 120; 
 
 //声明标签，字段，按钮等。
 private JLabel enterLabel，validLabel，resultLabel; 
 private JTextField pwTextField; 
 private JButton enterB，clearB; 
 
 private EnterButtonHandler ebHandler; 
 private ClearButtonHandler cbHandler; 
 
 public Password2（）//构造函数定义框架
 {
 setTitle（Password Checker）; //设置框架的标题
 setSize（WIDTH，HEIGHT）; //设置框架大小
 
 //准备容器
容器窗格= getContentPane（）; 
 GridLayout aGrid = new GridLayout（3，2，5，5）; // create a 3 row 2 column layout 
 pane.setLayout（aGrid）; //设置框架的布局
 
 String password =hello; 
 
 //实例化JLabels 
 enterLabel = new JLabel（Enter Password：）; 
 validLabel = new JLabel（Validation：）; 
 resultLabel = new JLabel（）; 
 
 //实例化文本字段
 pwTextField = new JPasswordField（30）; 
 
 //实例化按钮
 enterB = new JButton（Enter）; 
 clearB = new JButton（Clear）; 
 
 //初始化按钮处理程序
 ebHandler = new EnterButtonHandler（）; 
 enterB.addActionListener（ebHandler）; 
 
 //初始化按钮处理程序
 cbHandler = new ClearButtonHandler（）; 
 clearB.addActionListener（cbHandler）; 
 
 
 pane.add（enterLabel）; 
 pane.add（pwTextField）; 
 pane.add（validLabel）; 
 pane.add（resultLabel）; 
 pane.add（enterB）; 
 pane.add（clearB）; 
 
 //调用中心框架方法
 centerFrame（WIDTH，HEIGHT）; 
 
} //结构构造函数
 
 //在屏幕上显示中心GUI 
 public void centerFrame（int frameWidth，int frameHeight）
 {
 //创建工具箱对象
 Toolkit aToolkit = Toolkit.getDefaultToolkit（）; 
 
 //创建带有用户屏幕信息的维对象
尺寸屏幕= aToolkit.getScreenSize（）; 
 
 //分配框架左上角的x，y位置
 int xUpperLeft =（screen.width  -  frameWidth）/ 2; 
 int yUpperLeft =（screen.height  -  frameHeight）/ 2; 
 
 //在用户屏幕上定位框架的方法
 setBounds（xUpperLeft，yUpperLeft，frameWidth，frameHeight）; 
} 
 
 private class EnterButtonHandler implements ActionListener 
 {
 public void actionPerformed（ActionEvent e）
 {
 String pw = pwTextField.getText ）; 
 
 if（pw.equals（password））
 {
 resultLabel.setText（Password Accepted）; 
 pwTextField.setText（）; 
} 
 else 
 {
 resultLabel.setText（Password Rejected）; 
 pwTextField.setText（）; 
} 
} 
} 
 private class ClearButtonHandler implements ActionListener 
 {
 public void actionPerformed（ActionEvent e）
 {
 resultLabel .setText（）; 
 pwTextField.setText（）; 
} 
 
} 
 public static void main（String [] args）
 {
 JFrame aPassword2 = new Password2（）; //创建JFrame对象
 aPassword2.setDefaultCloseOperation（JFrame.EXIT_ON_CLOSE）; 
 aPassword2.setVisible（true）; 
} 
} //类结束
  

解决方案

阅读错误消息，喜欢错误消息。

这需要一些练习，但是一段时间后，它更容易看清楚： ）

错误：找不到符号 [...]

符号：变量密码

位置： [in] Password2。 EnterButtonHandler

没有名为的密码 （ EnterButtonHandler ）。

p>

---

提示：在不同的变量中有一个 em> scope / context ...也许它不应该是一个本地变量？有关更多信息，请参见 Java教程：变量）。

I've been trying to figure this out, I've run it in different programs so it's definitely in the code. Probably something easy too. The error says

Password2.java:90: error: cannot find symbol if(pw.equals(password)) ^ symbol: variable password location: class Password2.EnterButtonHandler 1 error

Here is the code:

// Password1.java

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

public class Password2 extends JFrame // inherits from the JFrame class 
{
    // static final variables to hold frame dimensions (in pixels) 
    private static final int WIDTH = 400;
    private static final int HEIGHT = 120;

    //declare labels, fields, buttons, etc.
    private JLabel enterLabel, validLabel, resultLabel;
    private JTextField pwTextField;
    private JButton enterB, clearB;

    private EnterButtonHandler ebHandler;
    private ClearButtonHandler cbHandler;

    public Password2() // constructor defines frame 
    { 
            setTitle( "Password Checker" ); // set the title of the frame
        setSize( WIDTH, HEIGHT ); // set the frame size

        // prepare the container 
        Container pane = getContentPane();
        GridLayout aGrid = new GridLayout( 3, 2, 5, 5 ); // create a 3 row 2 column layout
        pane.setLayout( aGrid ); // set the layout for the frame

        String password = "hello";

        //instantiate JLabels
        enterLabel = new JLabel("Enter Password: ");
        validLabel = new JLabel("Validation: ");
        resultLabel = new JLabel("");

        //instantiate text fields
        pwTextField = new JPasswordField( 30 );

        //instantiate buttons
        enterB = new JButton("Enter");
        clearB = new JButton("Clear");

        //initialize button handler
        ebHandler = new EnterButtonHandler();
        enterB.addActionListener(ebHandler);

        //initialize button handler
        cbHandler = new ClearButtonHandler();
        clearB.addActionListener(cbHandler);


        pane.add(enterLabel);
        pane.add(pwTextField);
        pane.add(validLabel);
        pane.add(resultLabel);
        pane.add(enterB);
        pane.add(clearB);

        //calls center frame method
        centerFrame( WIDTH, HEIGHT );

    }// end constructor

    //methood to center GUI on screen
    public void centerFrame( int frameWidth, int frameHeight)
    {
        //create toolkit object
        Toolkit aToolkit = Toolkit.getDefaultToolkit();

        //create a dimension object with user screen information
        Dimension screen = aToolkit.getScreenSize();

        //assign x, y position of upper left corner of frame
        int xUpperLeft = ( screen.width - frameWidth ) / 2;
        int yUpperLeft = ( screen.height - frameHeight ) / 2;

        //method to position frame on user's screen
        setBounds( xUpperLeft, yUpperLeft, frameWidth, frameHeight );
    }

    private class EnterButtonHandler implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            String pw = pwTextField.getText();

            if(pw.equals(password))
            {
                resultLabel.setText("Password Accepted");
                pwTextField.setText("");
            }
            else
            {
                resultLabel.setText("Password Rejected");
                pwTextField.setText("");
            }
        }
    }
    private class ClearButtonHandler implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            resultLabel.setText("");
            pwTextField.setText("");
        }

    }
    public static void main(String [] args) 
    {
        JFrame aPassword2 = new Password2(); // create the JFrame object
        aPassword2.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        aPassword2.setVisible(true);
    }
    } // end of class

解决方案

Read the error message, love the error message.

It takes some practice, but after awhile it's easy to see it more clearly: just read across the bold text below as a sentence :)

error: cannot find symbol [...]

symbol: variable password

location: [in] class Password2.EnterButtonHandler

There is nothing named password in that scope/context (EnterButtonHandler).

Happy coding.

---

Hint: there is a local variable with the same name in a different scope/context... perhaps it shouldn't be a local variable? See The Java Tutorial: Variables for more :)

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