Java不支援多继承 这是个BUG?

先从Java 8之前说起。要区分“宣告多继承”与“实现多继承”。

Java是不允许“实现多继承”，简称不允许“多继承”。但是Java支援“宣告多继承”——Java的界面的多继承——一个类可以实现多个界面（“继承”了多个界面上的方法宣告），而一个界面可以继承多个界面（同样是“继承”了多个界面上的方法宣告）。

界面只允许有方法宣告而不允许有实现，因而不会出现像C++那样的实现多继承的决议问题；抽象类可以有方法实现，但要遵循Java类的单继承限制，也避免了实现多继承的问题。这是早期Java为了与C++区分开的一个决定。

至于早期Java为何要做这样的设计取舍，不必猜测，看看高司令老人家自己是怎么说的：

James Gosling on Java, May 1999

Bill Venners:In Java you included multiple inheritance of interface, but left out multiple inheritance of implementation. Were you trying to say anything to designers by making the interface a separate construct? Were you trying to say anything by leaving out multiple inheritance of implementation? How did that come about?

James Gosling: It listened to people from the C++ and Objective-C camps, and I tried to sort out who had the most happy experiences. The Objective-C notion of a pure interface with no implementation seemed to have worked out really well for people. It avoids a lot of the sticky issues such as disambiguation that people get into in C++. It\'s still kind of messy. It\'s an area that I don\'t feel totally happy with.

Bill Venners: In what way?

James Gosling: Another way for doing these things is a technique called delegation. Some ideas in the delegation camp felt good, but I never came up with anything that really worked. I ended up with the interface construct, which felt simple enough to be comprehensible, sophisticated enough to be useful. It also avoided any of the tar pits that the other folks got into.

纯粹是高司令老人家的taste。

然后，从Java 8开始，界面允许为方法提供“预设实现”了——预设方法（default method）。因而实质上Java 8的界面多继承其实也会涉及到实现多继承，并且语言层面有专门规定去解决实现多继承时选择哪个版本的问题——哪个都不选择，而是在发现会继承多个预设方法实现并且没有override时报错，逼使使用者显式override可能冲突的方法。

这使得Java 8开始界面可以当作traits来使用，达到实现多继承的目的。

Java 8语言规范的一些相关新规定：

8.4.8. Inheritance, Overriding, and Hiding

9.4. Method Declarations

13.5.6. Interface Method Declarations

作者：RednaxelaFX

连结：https://www.zhihu.com/question/24317891/answer/65097560