Java Collectors.toList() 返回什么样的 List<E> ？
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What kind of List<E> does Collectors.toList() return?
提问by skiwi
I am reading State of the Lambda: Libraries Edition, and am being surprised by one statement:
我正在阅读Lambda 的状态：图书馆版，并对一项声明感到惊讶：
Under the section Streams, there is the following:
在Streams部分下，有以下内容：
List<Shape> blue = shapes.stream()
.filter(s -> s.getColor() == BLUE)
.collect(Collectors.toList());
The document does not state what shapesactually is, and I do not know if it even matters.
该文件没有说明shapes实际情况，我不知道它是否重要。
What confuses me is the following: What kind of concrete Listdoes this block of code return?
令我困惑的是：List这段代码返回什么样的具体内容？
- It assigns the variable to a
List<Shape>, which is completely fine. stream()norfilter()decide what kind of list to use.Collectors.toList()neither specifies the concrete type ofList.
- 它将变量分配给 a
List<Shape>，这完全没问题。 stream()也不filter()决定使用什么样的列表。Collectors.toList()两者都没有指定 的具体类型List。
So, what concretetype (subclass) of Listis being used here? Are there any guarantees?
那么，这里使用的是什么具体类型（子类）List？有任何保证吗？
采纳答案by Rohit Jain
So, what concrete type (subclass) of List is being used here? Are there any guarantees?
那么，这里使用的是 List 的什么具体类型（子类）？有任何保证吗？
If you look at the documentation of Collectors#toList(), it states that - "There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned". If you want a particular implementation to be returned, you can use Collectors#toCollection(Supplier)instead.
如果您查看 的文档Collectors#toList()，它会指出- “对返回的列表的类型、可变性、可序列化性或线程安全性没有任何保证”。如果您希望返回特定的实现，则可以Collectors#toCollection(Supplier)改用。
Supplier<List<Shape>> supplier = () -> new LinkedList<Shape>();
List<Shape> blue = shapes.stream()
.filter(s -> s.getColor() == BLUE)
.collect(Collectors.toCollection(supplier));
And from the lambda, you can return whatever implementation you want of List<Shape>.
从 lambda 中，你可以返回任何你想要的实现List<Shape>。
Update:
更新：
Or, you can even use method reference:
或者，您甚至可以使用方法引用：
List<Shape> blue = shapes.stream()
.filter(s -> s.getColor() == BLUE)
.collect(Collectors.toCollection(LinkedList::new));
回答by Peter Lawrey
It doesn't matter, but the concrete type is non-generic as indeed all types are non-generic at runtime.
没关系，但具体类型是非泛型的，因为实际上所有类型在运行时都是非泛型的。
So, what concrete type (subclass) of List is being used here? Are there any guarantees?
那么，这里使用的是 List 的什么具体类型（子类）？有任何保证吗？
I don't think so, but ArrayList or LinkedList seem likely.
我不这么认为，但 ArrayList 或 LinkedList 似乎很可能。
回答by whyem
Navigating through Netbeans (Ctrl + Click), I landed in this code. It seems to be using an ArrayList as Supplier.
在 Netbeans 中导航（Ctrl + 单击），我进入了这段代码。它似乎使用 ArrayList 作为供应商。
public static <T> Collector<T, ?, List<T>> toList() {
return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
(left, right) -> { left.addAll(right); return left; },
CH_ID);
}
