如何实现ActionListener

Question

I am trying to simply change the value of a JLabel when a button is clicked.

This is the GUI code:

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

public class login extends JFrame implements ActionListener
{
       public login (){

         //Create (Frame, Label, Text input for doctors name & Password field):

       JFrame frame = new JFrame("Doctor Login");
       JLabel label = new JLabel("Login Below:");
       JTextField name = new JTextField(10);
       JPasswordField pass = new JPasswordField(10);
       JButton button = new JButton("Login");

        //Exit program on close:
       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

       //Label
       label.setPreferredSize(new Dimension(175, 100));
       label.setLocation(100,100);

       //Add Elements to frame:
       frame.setLayout(new FlowLayout());
       frame.getContentPane().add(label);
       frame.getContentPane().add(name);
       frame.getContentPane().add(pass);
       frame.getContentPane().add(button);

       //Create border for text field:
       Border nameBorder = BorderFactory.createLineBorder(Color.BLACK, 1);

       name.setBorder(nameBorder);
       pass.setBorder(nameBorder);

       //Set Size of frame:
       frame.setSize(500, 250);

       //Set Location of the frame on the screen:
       frame.setLocation(200,200);

       //Display
       frame.setVisible(true);


       //Compiler Gives an error here - Illegal Start of Expression
       public void actionEvent(ActionEvent event){
           label.setText("Logged in");
       }
    }

}

main class code:

import java.util.*;
import java.util.Date;

public class main
{

public static void main(String[] args) {

   login login = new login();

}


}

The actionEvent method in the login class returns the error Illegal Start of Expression.

Answer 1

If you just want to add the ActionListener to the button , then you should delete

implements ActionListener

and do:

button.addActionListener(new ActionListener()
{
    @Override
    public void actionPerformed(ActionEvent e)
    {
        label.setText(...);
    }
});

Notes:

• Note that you are getting that error because you are adding that method inside the constructor.
• You should follow Java naming conventions, use names like SomeClass for classes and someVariable for variables or methods.

Answer 2

You've misplaced and misnamed the actionPerformed method. Right now you have it nested inside of the constructor, and Java does not allow methods to be nested inside of other methods or constructors (same for constructors not being allowed to be nested in methods or constructors). Move it out of the constructor, call it actionPerformed(ActionEvent e) , and see what happens.