Exception in thread "AWT-EventQueue-0" java.lang.NoSuchMethodError:

Question

I'm trying to debug an issue with some code given to me by my professor. Ultimately I need to take the input from the text field in the LoginPanel and reference this later on as the username.

Here is my code:

import java.awt.*;
import java.awt.event.*;

public class MyCardFrame extends Frame{
    LoginPanel lp;
    ChatPanel cp;

    public MyCardFrame(){
        setLayout(new CardLayout());
        setTitle("Chat");
        setSize(500,500);
        lp = new LoginPanel(this);
        cp = new ChatPanel();
        add(lp, "login");
        add(cp, "chat");

        addWindowListener(new WindowAdapter(){
            public void windowClosing(WindowEvent we){
                System.exit(0);
            }
        });

        setVisible(true);
    }

    public static void main(String[] args){

        MyCardFrame mcf = new MyCardFrame();
    }
}

class LoginPanel extends Panel implements ActionListener{
    TextField tf;
    MyCardFrame mcf;

    public LoginPanel(MyCardFrame mcf){
        this.mcf = mcf;
        tf = new TextField(20);
        tf.addActionListener(this);
        add(tf);        
    }       
    public void actionPerformed(ActionEvent ae){
        //using setName instead of setUserName     *********
        mcf.cp.setUserName(tf.getText()); //call setUserName of chatpanel which is a member of mycardframe
        CardLayout cl = (CardLayout)(mcf.getLayout());
        cl.next(mcf);
        tf.setText("");

    }


}

class ChatPanel extends Panel{
    Label label1,label2;
    String userName;
    public ChatPanel(){
        label1 = new Label("Chat Panel: ");
        label2 = new Label("Name will go here");
        add(label1);
        add(label2);
    }
    public void setUserName(String userName){
        this.userName = userName;
        label2.setText(getUserName());
    }
    public String getUserName(){
        return userName;
    }
}

After this runs I get the following error:

Exception in thread "AWT-EventQueue-0" java.lang.NoSuchMethodError: 
    finalproject.ChatPanel.setUserName(Ljava/lang/String;)V

This is coming from the line:

mcf.cp.setUserName(tf.getText());

From what I can tell I have correctly called setUserName, and while the IDE provides no errors, on run it fails.

Also to clarify: Yes I know nobody uses AWT, but its required for this class and we are not permitted to use Swing.

Answer 1

NoSuchMethodError意味着非常具体的东西。它说当代码被编译时，类和方法被成功定位，但是当代码被执行时，即使加载了类，它也不包含所请求的方法。

这几乎总是意味着编译期间使用的jar文件的版本与运行时提供的版本不同。运行时版本缺少该方法，或签名不同，无法找到匹配方法。

来自NoSuchMethodError的Javadoc：

  

如果应用程序试图调用类的指定方法（静态或实例），并且该类不再具有该方法的定义，则抛出该异能。

     

通常，编译器会捕获此错误;如果类的定义发生不兼容的更改，则此错误只能在运行时发生。

解决方案是仔细检查在编译和运行时都有相同的jar文件。