PHP致命错误：第11行的/var/www/cli/application.php中找不到类'GreetCommand'

I am using symfony console component

When I run this command by console

php application.php 

Then this command give me this error

PHP Fatal error: Class 'GreetCommand' not found in /var/www/cli/application.php on line 11

I configured all setting according to this link

http://symfony.com/doc/current/components/console/introduction.html

My application.php file code

<?php
// application.php

require __DIR__.'/vendor/autoload.php';

//use GreetCommand;
use Symfony\Component\Console\Application;

$application = new Application();
$application->add(new GreetCommand());
$application->run();

And My Greet Command exist in same location of the application.php file Code is:

<?php
//namespace Command;

use Symfony\Component\Console\Command\Command;
use Symfony\Component\Console\Input\InputArgument;
use Symfony\Component\Console\Input\InputInterface;
use Symfony\Component\Console\Input\InputOption;
use Symfony\Component\Console\Output\OutputInterface;

class GreetCommand extends Command
{
    protected function configure()
    {
        $this
            ->setName('demo:greet')
            ->setDescription('Greet someone')
            ->addArgument(
                'name',
                InputArgument::OPTIONAL,
                'Who do you want to greet?'
            )
            ->addOption(
               'yell',
               null,
               InputOption::VALUE_NONE,
               'If set, the task will yell in uppercase letters'
            )
        ;
    }

    protected function execute(InputInterface $input, OutputInterface $output)
    {
        $name = $input->getArgument('name');
        if ($name) {
            $text = 'Hello '.$name;
        } else {
            $text = 'Hello';
        }

        if ($input->getOption('yell')) {
            $text = strtoupper($text);
        }

        $output->writeln($text);
    }
}

So please help me what is the problem in my application.php file when executing this command by using symfony console component