[英]Why any type can EXTENDS interface?
Today I got lost in Java polymorphism. 今天我迷失了Java多态性。 I have done something that I have thought that is impossible.
我做过一些我认为不可能完成的事情。 Namely - I have EXTENDED interface.
即 - 我有EXTENDED界面。
I have created empty interface: 我创建了空接口:
public interface Figure {}
Nothing new right now. 现在没什么新鲜的。 Then I have created parametrized class Board with paramether that EXTENDS my interface!
然后我创建了带参数的参数化类板,扩展了我的界面!
public class Board< T extends Figure > {
T[][] board;
}
I thought it is not be possible! 我以为这是不可能的! Nevertheless I was going further with my not understanding.
尽管如此,我还是因为不理解而走得更远。
I have created class implementing Figure interface: 我已经创建了实现Figure接口的类:
public class Cross implements Figure{}
And to my suprise I was able to do that: 令我惊讶的是,我能够做到这一点:
Board b = new Board<Cross>();
Please help me and explain me this strange situation. 请帮帮我解释一下这种奇怪的情况。
I know a little about polymorphism and I understand that Cross is Figure. 我对多态性了解不多,我知道Cross就是图。 I am confused howcome any type is able to EXTEND interface, and howcome classes that implements interaface (not extending interface) are correct as paramether which extends interface.
我很困惑,任何类型都能够EXTEND接口,并且实现交互式(不扩展接口)的howcome类是正确的,作为扩展接口的paramether。
Please help me and explain this polymorphicall mess. 请帮助我解释这个多态的混乱。 Thanks.
谢谢。
When you say public class Board< T extends Figure >
, it means that the Board can accept either Figure
or sub-interfaces of Figure
or any classes which implement these interfaces. 当你说
public class Board< T extends Figure >
,这意味着董事会可以接受Figure
或子接口Figure
或实现这些接口的类。 So, when you say: 所以,当你说:
public class Cross implements Figure{}
and 和
Board b = new Board<Cross>();
This means that Cross has implemented Figure
, and thus Board
can accept Cross
, since Cross
IS-A Figure
这意味着Cross已经实现了
Figure
,因此Board
可以接受Cross
,因为Cross
IS-A Figure
As far as extending an interface is concerned, only interfaces can extend interfaces. 就扩展接口而言,只有接口可以扩展接口。 So, the following is legal:
所以,以下是合法的:
public interface TwoDimensionalFigure extends Figure{}.
So any class which implements TwoDimensionalFigure
can also be accepted by Board
. 因此,任何实现
TwoDimensionalFigure
类也可以被Board
接受。
By doing 通过做
public class Board< T extends Figure > {
T[][] board;
}
You are creating a bounded generic type, T
which should have its super class of type Figure
您正在创建一个有界泛型类型,
T
应该具有类型为 Figure
超类
As explained in this thread, there is no difference between definig a generic type which extends an interface or a class . 正如本主题中所解释的那样,定义扩展 接口或类的泛型类型之间没有区别。
What compiler makes sure is that, when you create Board
, T
will be supplied with any concrete class that either extends Figure
or implements Figure
编译器确保的是,当您创建
Board
, T
将提供任何具体的类,可以extends Figure
或implements Figure
Actully you didn't extend the interface. 实际上你没有扩展界面。 You just create a generic class with the type Cross.
您只需创建一个Cross类型的泛型类。 The definition of your class merely specifies that the the parameterized type must be derived from Figure, ie implementing Figure or a derivative from it.
您的类的定义仅指定参数化类型必须从图中派生,即实现图或其衍生。
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