在 Java 中出错，不知道我做错了什么

Question

I'm new to Java and I was trying to do the following on eclipse:

import javax.swing.*;

public class Hello_World {
  public class HelloWorld extends JFrame
  {  
     public static void main(String[] args) {
     JFrame frame = new HelloWorld();
     frame.setSize( 300, 200 );
     frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
     frame.setTitle( "Hello world" );
     frame.setVisible( true );
    }
  }
}

I have no idea what I doing wrong here. The compiler gives me following error:

Main method not found in class Hello_World, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application 

Can someone tell me what I'm doing wrong?

Answer 1

The compiler is complaining because you have defined your main method inside a nested class, instead of directly in the class that you are compiling.

Just move the main method into the HelloWorld class.

import javax.swing.*;
public class Hello_World {
    public static class HelloWorld extends JFrame
    {  

    }
    public static void main(String[] args) {
        JFrame frame = new HelloWorld();
        frame.setSize( 300, 200 );
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setTitle( "Hello world" );
        frame.setVisible( true );
    }
}

Answer 2

Here is a better solution:

package hello_world;

import javax.swing.*;

public class Hello_World extends JFrame {

    public static void main(String[] args) {
        JFrame frame = new Hello_World();
        frame.setSize( 300, 200 );
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setTitle( "Hello world" );
        frame.setVisible( true );
    }
}