Question

我是一名初学程序员，我正在从一本书中学习。而且我在java中使用getSource()方法时遇到了麻烦。请看下面的代码，它有很多评论可以帮助您了解我的想法，并且写得很好谢谢，这是我第一次使用计算器。

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

public class EventDemo extends JFrame implements ActionListener {

    public EventDemo() //constructor
    {

        super("Sign in");    //title for JFrame

        setSize(300, 250);  //size for JFrame

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    // this make sure the JFrame close when click on exit

        JLabel username = new JLabel("Enter Username");  //promt user for username

        username.setFont(new Font("Arial", Font.BOLD, 16));  //set the font for the username label

        JTextField usernameTextField = new JTextField(20);  // where user type in username

        JLabel password = new JLabel("Enter password"); // prompt user for password

        password.setFont(new Font("Arial", Font.BOLD, 16)); // set font for password label

        JTextField passwordTextField = new JTextField(20);  // where user enter password

        JButton login = new JButton("Login");    // login button created

        JButton back = new JButton("Back");   // back button

        setLayout(new FlowLayout());     // our layout      

        setVisible(true);   // making sure we see our frame when we run our program :) i know it's better to this in main   

        login.addActionListener(this);   // telling our login button to listen for an event
        back.addActionListener(this);  // telling our back button to listen for an event

        add(username); // adding components to frame, you guys know this :)
        add(usernameTextField);
        add(password);
        add(passwordTextField);
        add(login);
        add(back);

    }

    @Override
    public void actionPerformed(ActionEvent x) // THIS IS WHERE I'M GOING CRAZY, INSIDE THE METHOD. i'm trying to see which button is causing the event. 
    {

        // this is the method the book (Java Programming Seventh Edition Joyce Farrell) taught me, and i also see this online as well, but is not working 
        Object source = x.getSource(); //no error shows up here

        if (source == login) // or if(x.getSource == login) // Eclipse is saying "login cannot be resolved to a variable"
        {
            // do this..
        } else {
            // to this.... it's only two buttons :)
        }

        // this method below i got online, but it works. But it said i have to set the buttons name after i created them see the first 4 line of code below
        JButton login = new JButton();
        JButton back = new JButton();
        login.setText("Login");
        back.setText("Back");
        // the above 4 line of code would go inside the constructor EventDemo, and not inside the actionPerformed method

        String source = x.getActionCommand();

        if (source.equals("login")) {
            // do this..
        } else {
            // do this...
        }

    }

}

Answer 1

Eclipse抱怨的原因＆＃34;登录无法解决＆＃34;是因为它需要一个尚未声明的登录变量。您的登录变量声明在EventDemo构造函数中，并且在actionPerformed（）方法中不可用。如果在构造函数之前移动声明并在构造函数内部初始化，则不应再抱怨。

public class EventDemo extends JFrame implements ActionListener
{
   JButton login;



public EventDemo()   //constructor
{
//....
// do stuff

login = new JButton("Login");
// ...

}

Answer 2

您需要在构造函数之前在类JButton login;中声明EventDemo（如全局）。然后你可以在构造函数中初始化它。这样就可以在代码中的任何位置访问login。

此外，您恰好已声明source两次（Object source = x.getSource();和String source = x.getActionCommand();），因此请注意并将String source = x.getActionCommand();更改为source = x.getActionCommand();。

   public class EventDemo extends JFrame implements ActionListener {

        JButton login;

        public EventDemo() //constructor
        {
            //....
            // do stuff

            login = new JButton("Login");
            // ...

        }

        public void actionPerformed(ActionEvent x) // THIS IS WHERE I'M GOING CRAZY, INSIDE THE METHOD. i'm trying to see which button is causing the event. 
        {
            //your code
            source = x.getActionCommand();//take out the String

            if (source.equals("login")) {
                // do this..
            } else {
                // do this...
            }

        }
    }

如何在Java中使用getSource方法？

2 个答案: