Guava ImmutableList copyOf vs Builder [英] Guava ImmutableList copyOf vs Builder

问题描述

我想知道哪个更有效率，为什么？

I was wondering which is more efficient and why?

1）

List<Blah> foo;
...
return ImmutableList.copyOf(foo);

或

2）

List<Blah> foo;
...
return new ImmutableList.Builder<Blah>().addAll(foo).build();

推荐答案

我看不出你应该使用的任何理由这里的构建器：

I don't see any reason why you should use builder here:

• ImmutableList.copyOf 比制作<$更具可读性c $ c> Builder 在这种情况下，


• Builder 不会推断泛型类型，你有用作单行时用
  指定类型，


• （来自文档） ImmutableList.copyOf 试图避免在安全的情况下实际复制数据）时，.Collection％29>做了很好的魔术，


• （来自源代码） Builder #addAll 在之前创建的<上调用 addAll code> ArrayList 而 copyOf 避免为零元素和单元素集合创建任何列表（分别返回空的不可变列表和单例不可变列表），


• （来自源） copyOf（Collection）实例不会创建临时 ArrayList （ copyOf（Iterable）和 copyOf（Iterator）这样做），


• （来自源代码）此外， Builder＃build 在以前内部填充的 ArrayList 上调用 copyOf ，这会带给您什么问题 - 为什么要使用 Builder 这里，当你有 copyOf ？

• ImmutableList.copyOf is much more readable than making a Builder in this case,
• Builder doesn't infer generic type and you have to specify type by yourself when used as one-liner,
• (from docs) ImmutableList.copyOf does good magic when invoked with another immutable collection (attempts to avoid actually copying the data when it is safe to do so),
• (from source) Builder#addAll invokes addAll on previously created ArrayList while copyOf avoids creating any list for zero- and one-element collections (returns empty immutable list and singleton immutable list respectively),
• (from source) copyOf(Collection) instance doesn't create temporary ArrayList (copyOf(Iterable) and copyOf(Iterator) does so),
• (from source) moreover, Builder#build invokes copyOf on previously internally populated ArrayList, what brings you to your question - why use Builder here, when you have copyOf?

PS我个人使用 ImmutableList.builder（）静态工厂而不是新的ImmutableList.Builder< Blah>（）构造函数 - 当分配给 Builder< Blah> 变量，第一个推断泛型类型，而后者不推断。

P.S. Personally I use ImmutableList.builder() static factory instead of new ImmutableList.Builder<Blah>() constructor - when assigned to a Builder<Blah> variable the first infers generic type while the latter doesn't.

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