Java：仅从集合中选择所提供类型的元素 [英] Java: select from collection only elements of provided type

问题描述

我有一个类型B和C的元素的集合，所有扩展A.我需要过滤集合，以获取元素只有B类型。

I have a collection of elements of type B and C, which all extends A. I need to filter the collection to get elements of only B type.

有任何方法除外：

for (A a : initCollection) {
  if (a instanceof B) {
    newCollection.add(a)?
  }
}

感谢

推荐答案

Guava 在其他答案中被提及，但不是特定的解决方案，这甚至比人们更容易实现：

Guava was alluded to in other answers, but not the specific solution, which is even simpler than people realize:

Iterable<B> onlyBs = Iterables.filter(initCollection, B.class);

这很简单，干净，做正确的事情，只创建一个实例，不会产生任何警告。

It's simple and clean, does the right thing, only creates a single instance and copies nothing, and doesn't cause any warnings.

（ Collections2.filter（） ，所以如果你真的想要一个 Collection ，你必须提供 Predicates.instanceOf（B.class）和所得到的集合仍然可能是 Collection 类型。）

(The Collections2.filter() method does not have this particular overload, though, so if you really want a Collection, you'll have to provide Predicates.instanceOf(B.class) and the resulting collection will still sadly be of type Collection<A>.)

这篇关于Java：仅从集合中选择所提供类型的元素的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！