What causes “Can't find Symbol” and how to fix

问题:

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

回答1:

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 :)

回答2:

password is local to the Password2 constructor.

It should either be passed around, or an instance variable.

回答3:

Your class doesn't have a definition for password. Hence, the error when passing it to the equals method.

回答4:

It can't find the variable password, which, as you've coded it, only exists in the Password2 constructor. You'll need to either make password a class member variable or pass it to the constructor of your Handler classes, so they can have a reference to it.

回答5:

password 

is a local variable declared in the constructor of Password2. It is not in scope in your EnterButtonHandler.actionPerformed method. Make it an instance variable to resolve.