Java 不兼容的类型 List of List 和 ArrayList of ArrayList
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Incompatible types List of List and ArrayList of ArrayList
提问by Kraken
The below line gives me error :
以下行给了我错误：
Incompatible Types.
List<List<Integer>> output = new ArrayList<ArrayList<Integer>>();
What is the reason?
是什么原因？
EDIT
编辑
I understand if I change my second ArrayList to List, it does not give me error. I want to know the reason of error though. Thanks
我明白如果我将第二个 ArrayList 更改为 List，它不会给我错误。我想知道错误的原因。谢谢
采纳答案by Dawood ibn Kareem
If you had a List<List<Integer>>then you'd be able to add a LinkedList<Integer>to it. But you can't do this for an ArrayList<ArrayList<Integer>>, so the latter can't possibly be a type of List<List<Integer>>.
如果你有一个，List<List<Integer>>那么你就可以LinkedList<Integer>向它添加一个。但是您不能为 an 执行此操作ArrayList<ArrayList<Integer>>，因此后者不可能是List<List<Integer>>.
回答by Ankit Lamba
From Generics, Inheritance, and Subtypes
从泛型、继承和子类型
This is a common misunderstanding when it comes to programming with generics, but it is an important concept to learn.
Box<Integer>is not a subtype of Box even though Integer is a subtype of Number.
在使用泛型编程时，这是一个常见的误解，但这是一个需要学习的重要概念。
Box<Integer>不是 Box 的子类型，即使 Integer 是 Number 的子类型。
回答by CoderCroc
It is clearly stated in Java Doc
Java Doc 中有明确说明
In general, if Foo is a subtype (subclass or subinterface) of Bar, and G is some generic type declaration, it is not the case that
G<Foo>is a subtype ofG<Bar>. This is probably the hardest thing you need to learn about generics, because it goes against our deeply held intuitions.
一般来说，如果foo是酒吧的子类型（子类或子接口），并且G是一些通用的类型声明，它不是的情况下
G<Foo>是的子类型G<Bar>。这可能是你需要学习的关于泛型的最难的事情，因为它违背了我们根深蒂固的直觉。
Same thing happens here it's Bar = List<Integer>and Foo = ArrayList<Integer>as ArrayList<ArrayList<Integer>>is not sub type of List<List<Integer>>
同样的事情发生在这里它是Bar = List<Integer>，Foo = ArrayList<Integer>因为ArrayList<ArrayList<Integer>>它不是子类型List<List<Integer>>
回答by lpiepiora
The reason is that generics are not covariant.
原因是泛型不是协变的。
Consider simpler case:
考虑更简单的情况：
List<Integer> integers = new ArrayList<Integer>();
List<Number> numbers = integers; // cannot do this
numbers.add(new Float(1337.44));
Now List holds a Float, which is certainly bad.
现在 List 持有一个 Float，这当然很糟糕。
Same for your case.
你的情况也一样。
List<ArrayList<Integer>> al = new ArrayList<ArrayList<Integer>>();
List<List<Integer>> ll = al; // cannot do this
ll.add(new LinkedList<Integer>())
Now you have a list llwhich holds LinkedList, but the alis declared as a List of ArrayLists.
现在您有一个ll包含的列表LinkedList，但al被声明为ArrayLists的列表。
回答by spiralmoon
The correct writing should be:
List<List<Integer>> ret = new ArrayList<List<Integer>>();Since in this way, you can add not only ArrayListbut also LinkedListto ret
正确的书写应该是：
List<List<Integer>> ret = new ArrayList<List<Integer>>();既然这样，你不仅增加ArrayList，而且LinkedList以ret
回答by Pavel Shkleinik
Less text more fixes:
更少的文字更多的修复：
List<List<Integer>> lists = new ArrayList<>();
or
或者
List<List<Integer>> lists = new ArrayList<List<Integer>>();
