静态方法返回一个类的实例是什么意思？

Question

Sometimes when I look at code other people have written I see something like the following:

<?php

namespace sys\database;

class Statistics {

    public function __construct() {
        // Database statistics are gathered here using
        // private methods of the class and then set to
        // class properties
    }

    public static function getInstance() {
        return new \sys\database\Statistics();
    }

    // ...

}

So the static function getInstance() simply returns an object of the class it belongs to. Then, somewhere else in the code I come across this:

$stats = \sys\database\Statistics::getInstance();

Which simply sets $stats to an instance of the Statistics object, ready for its class properties to be accessed to get various database statistics.

I was wondering why it was done this way as opposed to just using $stats = new \sys\database\Statistics();. At the end of the day, all the logic to gather statistics is in the constructor and the getInstance() method doesn't do anything other than returning a new object.

Is there something I'm missing here?

Answer 1

This is supposed to be an implementation of the Singleton pattern: http://www.oodesign.com/singleton-pattern.html

The pattern is used to never allow more than one instance of the class to be created.

However, there are a couple of flaws with the implementation you provided: the constructor should be private, and there should be a single private static instance of the class, returned every time the getInstance method is called.