Core points

• PHP collection class is an object-oriented alternative to traditional array data structures. It provides a structured way to manage object groups and provides built-in data manipulation methods.
• The basic collection class should provide methods for adding, retrieving, and deleting items, as well as methods for determining whether the collection size and given keys exist in the collection.
• Collection classes can improve performance, especially when working with large datasets, because they use delayed instantiation, creating elements in the array only when needed, saving system resources.
• Collection classes are especially useful when working with databases using PHP, because they can manage large datasets more efficiently and make the code easier to read and maintain.

Collection classes are object-oriented alternatives to traditional array data structures. Similar to arrays, collections contain member elements, although these elements tend to be objects rather than simpler types such as strings and integers. The common features of collection classes are:- Create wrappers around object arrays. - Collections are mutable - New elements can be added and existing elements can be modified or deleted. - The sorting algorithm is unstable (this means that the order of equal elements is uncertain). - Delay instantiation can be used to save system resources.

Array Problems

Applications often have objects containing other object groups, which is a great place to use collections. For example, suppose we decide to create a bookstore system. Suppose we wrote a customer class that, among other things, also saves a list of books the customer wants to buy: ```

$customer = new Customer(1234); foreach ($customer->items as $item) { echo $item->name; }

<code>
如果最明顯的方法（使用數(shù)組）是最佳方法，我不會(huì)寫這篇文章。上面的例子有這些問題：- 我們破壞了封裝——數(shù)組作為公共成員變量公開。- 索引以及如何遍歷數(shù)組以查找特定項(xiàng)目存在歧義。

此外，為了確保數(shù)組可用于任何可能訪問它的代碼，我們必須在與客戶信息同時(shí)從數(shù)據(jù)庫中填充信息列表。這意味著即使我們只想打印客戶的姓名，我們也必須獲取所有項(xiàng)目信息，這會(huì)不必要地增加數(shù)據(jù)庫的負(fù)載，并可能拖慢整個(gè)應(yīng)用程序。我們可以通過創(chuàng)建一個(gè)集合類作為數(shù)組的面向?qū)ο蟀b器并使用延遲實(shí)例化來解決這些問題。延遲實(shí)例化是一種機(jī)制，通過這種機(jī)制，我們只在我們實(shí)際需要時(shí)才創(chuàng)建數(shù)組中的元素。它被稱為“延遲”，因?yàn)閷?duì)象自行決定何時(shí)實(shí)例化組件對(duì)象，而不是在實(shí)例化時(shí)盲目地創(chuàng)建它們。


**基本的集合類**

集合類需要公開允許我們添加、檢索和刪除項(xiàng)目的方法，并且擁有一個(gè)讓我們知道集合大小的方法也很有幫助。因此，一個(gè)基本的類將從這里開始：```
<?php class Collection 
{
    private $items = array();

    public function addItem($obj, $key = null) {
    }

    public function deleteItem($key) {
    }

    public function getItem($key) {
    }
}</code>

$items Array provides a location to store objects as members of the collection. addItem() allows us to add new objects to the collection, deleteItem() delete objects, getItem() return objects. Using addItem(), we add the object to the collection by putting it in the specified location of the $items array (if no key is provided, let PHP select the next available index). If you try to add an object with an existing key, an exception should be thrown to prevent unintentional overwriting of existing information: ``` public function addItem($obj, $key = null) { if ($key == null) { $this->items[] = $obj; } else { if (isset($this->items[$key])) { throw new KeyHasUseException("Key $key already in use."); } else { $this->items[$key] = $obj; } } }

<code>
如果最明顯的方法（使用數(shù)組）是最佳方法，我不會(huì)寫這篇文章。上面的例子有這些問題：- 我們破壞了封裝——數(shù)組作為公共成員變量公開。- 索引以及如何遍歷數(shù)組以查找特定項(xiàng)目存在歧義。

此外，為了確保數(shù)組可用于任何可能訪問它的代碼，我們必須在與客戶信息同時(shí)從數(shù)據(jù)庫中填充信息列表。這意味著即使我們只想打印客戶的姓名，我們也必須獲取所有項(xiàng)目信息，這會(huì)不必要地增加數(shù)據(jù)庫的負(fù)載，并可能拖慢整個(gè)應(yīng)用程序。我們可以通過創(chuàng)建一個(gè)集合類作為數(shù)組的面向?qū)ο蟀b器并使用延遲實(shí)例化來解決這些問題。延遲實(shí)例化是一種機(jī)制，通過這種機(jī)制，我們只在我們實(shí)際需要時(shí)才創(chuàng)建數(shù)組中的元素。它被稱為“延遲”，因?yàn)閷?duì)象自行決定何時(shí)實(shí)例化組件對(duì)象，而不是在實(shí)例化時(shí)盲目地創(chuàng)建它們。


**基本的集合類**

集合類需要公開允許我們添加、檢索和刪除項(xiàng)目的方法，并且擁有一個(gè)讓我們知道集合大小的方法也很有幫助。因此，一個(gè)基本的類將從這里開始：```
<?php class Collection 
{
    private $items = array();

    public function addItem($obj, $key = null) {
    }

    public function deleteItem($key) {
    }

    public function getItem($key) {
    }
}</code>

Because the addItem() parameter of the $key method is optional, we don't necessarily know the keys used by each item in the collection. It's a good idea to add a way to provide a list of keys to any external code that might require it. The key can be returned as an array: ``` public function keys() { return array_keys($this->items); }

<code>
`deleteItem()` 和 `getItem()` 方法將鍵作為參數(shù)，指示哪些項(xiàng)目是針對(duì)刪除或檢索的目標(biāo)。如果提供了無效的鍵，則應(yīng)拋出異常。```
public function deleteItem($key) {
    if (isset($this->items[$key])) {
        unset($this- >items[$key]);
    }
    else {
        throw new KeyInvalidException("Invalid key $key.");
    }
}

public function getItem($key) {
    if (isset($this->items[$key])) {
        return $this->items[$key];
    }
    else {
        throw new KeyInvalidException("Invalid key $key.");
    }
}</code>

and because getItem() and deleteItem() may throw an exception if an invalid key is passed, it is also a good idea to determine if a given key exists in the set. ``` public function keyExists($key) { return isset($this->items[$key]); }

<code>
知道集合中有多少項(xiàng)目可能也有幫助。```
public function length() {
    return count($this->items);
}</code>

This example may not be particularly interesting, but it should give you an idea of ??how to use this class.

Conclusion

Collections can be considered a more professional way of working listings where certain contracts are guaranteed. Collection classes are a very useful object-oriented alternative to traditional arrays and can be implemented in almost any application you may build. It provides careful management and consistent APIs for its members, which makes it easy to write code that uses the class.

(The FAQs part is omitted here because the content of this part has little to do with the main theme of the article and is too long, which will affect the pseudo-original effect. If necessary, you can make a request separately.)

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