經(jīng)常會在筆試或面試中碰到的問題，記錄一下官方文檔的解釋

Load

Invoked whenever a class or category is added to the Objective-C runtime; implement this method to perform class-specific behavior upon loading.

+ (void)load;

當一個類或一個分類添加到Objective-C 運行時環(huán)境中時，即該類被加載到程序中時，該方法會被調(diào)用。

Initialize

Initializes the class before it receives its first message.

+ (void)initialize;

當它接收到第一條消息之前調(diào)用，即調(diào)用它的第一個類方法或?qū)嵗椒ㄖ罢{(diào)用。