java paint() 方法的使用问题

Question

我写的程序如下，没有报错，但是为什么不能 run as java 程序?
package main;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class PersonToComputerMode extends JFrame {

private JPanel BoardPane;//用来画棋盘格子的pane
private void pain(Graphics g) {
g.drawLine(30, 30, 60, 60);

}

public static void mian(String args[]){
PersonToComputerMode pm =new PersonToComputerMode();

}
public PersonToComputerMode(){
super("人机对战");

//init compments
this.BoardPane=new JPanel();
BoardPane.setLayout(new FlowLayout());

Graphics g = BoardPane.getGraphics();
repaint();

//add action listenner

//common methods

this.add(this.BoardPane);
BoardPane.setBounds(350, 350, 200, 200);
this.setBounds(300, 300, 300, 300);
this.setLayout(new FlowLayout());
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setVisible(true);
setResizable(false);
this.show();
}
}
现在可以运行了，mian 写错了，但是为什么不会产生直线？

Answer 1

一共改了六处错误见注释。
完整的程序如下：
package main;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class PersonToComputerMode extends JFrame {

private JPanel BoardPane;//用来画棋盘格子的pane
public void paint(Graphics g) { //这里pain函数改成paint函数
g.drawLine(30, 30, 60, 60);

}
public static void main(String args[]){ //这里mian函数改成main函数
PersonToComputerMode pm =new PersonToComputerMode();
}
public PersonToComputerMode(){
super("人机对战");

//init compments
this.BoardPane=new JPanel();
BoardPane.setLayout(new FlowLayout());

Graphics g = BoardPane.getGraphics();
repaint();

//add action listenner

//common methods

this.add(this.BoardPane);
//BoardPane.setBounds(350, 350, 200, 200);//这里注释掉一句
//this.setBounds(300, 300, 300, 300);//这里注释掉一句
this.setLayout(new FlowLayout());
this.setSize(300,300); //这里加一句
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setVisible(true);
setResizable(false);
//this.show(); //这里注释掉一句
}
}