Java error with WindowAdapter and WindowDestroyer

Question

I'm just starting out in java and trying to use some example code I found online to get started, but for some reason, I am unable to compile this code. I on Ubuntu 16.04 and I have the "default-jdk" installed.

Here's the code:

import java.awt.*;
import java.awt.event.WindowListener;
import javax.swing.*;
import java.io.*; 


public class Test extends JFrame{
public static void main (String argv []) 
{
    new Test("Window Application");
}

public Test(String title) 
{
    super(title);
    setSize(200, 100);
    addWindowListener((WindowListener) new WindowDestroyer());                 
    setVisible(true);
}

private class WindowDestroyer extends WindowAdapter 
{      
    public void windowClosing(WindowEvent e) 
    {    
        System.exit(0);  
    }                                                             
}

}

When I try doing javac Test.java I get 2 cannot find symbol errors.

private class WindowDestroyer extends WindowAdapter

public void windowClosing(WindowEvent e)

*I get 2 cannot find symbol errors.* - so did you import the class? If you don't import the class, then you can't access the method either. Read the API for the WindowAdapter class to see what you need to import. — camickr, Dec 10 '18 at 03:00

score 0 · Answer 1 · answered Dec 10 '18 at 04:28

From the Java 8 docs for WindowAdapter, it is defined as java.awt.event.WindowAdapter.

You need to import the class first:

import java.awt.event.WindowAdapter;

in addition to your other imports.

import java.awt.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowListener;

As a side note, you might be tempted to just do

import java.awt.event.*;

to avoid import errors like that in the future.

I suggest reading the discussions on Why is using a wild card with a Java import statement bad? to have an idea on the pro's and con's of doing so.

Oleg Cherednik · Answer 2 · 2018-12-11T10:13:01.693

0

I can see, that you create simple Swing application window and close it close window. You do it in incorrect way. It is much better to use setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE) (only if you not plan to do smth. special before). And use SwingUtilities.invokeLater() to execute asynchronously on the AWT event dispatching thread:

public class Test extends JFrame {

    public static void main(String... ars) {
        SwingUtilities.invokeLater(() -> new Test().setVisible(true));
    }

    public Test() {
        super("Window Application");
        setSize(200, 100);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
     }
}

edited Dec 11 '18 at 10:13

answered Dec 10 '18 at 05:00

Oleg Cherednik

17,377
4
21
35

You don’t need a qualified access to `WindowConstants` when you are in a subclass of `JFrame`. Just `setDefaultCloseOperation(EXIT_ON_CLOSE);` will do. Further, you don’t need to call `.setVisible(true)` twice… – Holger Dec 10 '18 at 08:47
It depends on. Personally I do not like static constant imports. – Oleg Cherednik Dec 10 '18 at 08:53
In this case, it’s not about static import, as `JFrame` implements the `WindowConstants` interface, hence, inherits the constants. They are in scope, whether you import them or not. – Holger Dec 10 '18 at 09:09
In this case it is OK. Thanks. – Oleg Cherednik Dec 10 '18 at 09:10

Java error with WindowAdapter and WindowDestroyer

2 Answers2