I have main but java command not working

Question

I know Java need main function and I have one. This code compile with no error (This code is source code of the Design Pattern book) ,but when I execute

D:\ java SwingObserverExample 
Error: Could not find or load main class SwingObserverExample

here is source code:

package headfirst.designpatterns.observer.swing;

import java.awt.*;
import javax.swing.*;

public class SwingObserverExample{
    JFrame frame;

public static void main(String[] args) {
    SwingObserverExample example = new SwingObserverExample();
    example.go();
}

public void go() {
    frame = new JFrame();

    JButton button = new JButton("Should I do it?");

    // Without lambdas
    //button.addActionListener(new AngelListener());
    //button.addActionListener(new DevilListener());

    // With lambdas
    button.addActionListener(event -> 
        System.out.println("Don't do it, you might regret it!")
    );
    button.addActionListener(event ->
        System.out.println("Come on, do it!")
    );
    frame.getContentPane().add(BorderLayout.CENTER, button);

    // Set frame properties 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(BorderLayout.CENTER, button);
    frame.setSize(300,300);
    frame.setVisible(true);
}

/*
 * Remove these two inner classes to use lambda expressions instead.
 * 
class AngelListener implements ActionListener {
    public void actionPerformed(ActionEvent event) {
        System.out.println("Don't do it, you might regret it!");
    }
}

class DevilListener implements ActionListener {
    public void actionPerformed(ActionEvent event) {
        System.out.println("Come on, do it!");
    }
}
*/

why java can't find this program's main class ?

directory of my java file is D:\workplace\Head-First-Design-Patterns-master\Head-First-Design-Patterns-master\src\headfirst\designpatterns\observer\swing and I run java command from there.

Reimeus · Accepted Answer · 2015-03-23T19:42:31.073

2

Include the package name in the run command

java headfirst.designpatterns.observer.swing.SwingObserverExample

Also Java expects to find the class in a directory headfirst\designpatterns\observer\swing relative to the current location to match the package name so run the application from

D:\workplace\Head-First-Design-Patterns-master\Head-First-Design-Patterns-maste‌‌r\src

edited Mar 23 '15 at 19:42

answered Mar 23 '15 at 19:20

Reimeus

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You havent said what your directory structure is like but Java expects the class to be in a directory `headfirst\designpatterns\observer\swing` relative to the current location.... – Reimeus Mar 23 '15 at 19:30
my directory is `D:\workplace\Head-First-Design-Patterns-master\Head-First-Design-Patterns-master\src\headfirst\designpatterns\observer\swing` – Esterlinkof Mar 23 '15 at 19:31
In that case you can use a runtime location of `D:\workplace\Head-First-Design-Patterns-master\Head-First-Design-Patterns-maste‌r\src` – Reimeus Mar 23 '15 at 19:33
It's worked. please include this in your answer and I will mark your answer as solution. It's better to include link about java package tutorial. Thanks again – Esterlinkof Mar 23 '15 at 19:37
I have irrelevant question (about you), I beg leave to say in this comment ? – Esterlinkof Mar 23 '15 at 19:45

score 1 · Answer 2 · answered Mar 23 '15 at 19:20

1

This is because of the line package headfirst.designpatterns.observer.swing. The class is compiled in the package headfirst.designpatterns.observer.swing.

Either remove that or (better) run it using

java headfirst.designpatterns.observer.swing.SwingObserverExample

answered Mar 23 '15 at 19:20

John Bupit

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I removed firs line and it worked, why this is happened? – Esterlinkof Mar 23 '15 at 19:24
@saman, beacuse now it is in default package. And that's why you don't have to mention the package information. I think Reimeus solution also works fine in this case – Razib Mar 23 '15 at 19:24
@Razib where can I find some information about this or what keyword I must search? – Esterlinkof Mar 23 '15 at 19:27
@saman, Search for [java packages](https://docs.oracle.com/javase/tutorial/java/package/packages.html). – John Bupit Mar 23 '15 at 19:28

score 0 · Answer 3 · answered Mar 23 '15 at 19:23

0

While executing the code using java command use classpath - cp option and add the fully qualified class name where the main method exist -

java -cp headfirst.designpatterns.observer.swing.SwingObserverExample

answered Mar 23 '15 at 19:23

Razib

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I have main but java command not working

3 Answers3