Collections.emptyMap（）vs new HashMap（） [英] Collections.emptyMap() vs new HashMap()

问题描述

在某些情况下，我可以使用 Collections.emptyMap（）？文档说我可以使用这种方法，如果我想我的集合是不可变的。

为什么我想要一个不可变的空集合？ ，项目＃ 43 - 返回空数组或集合，非空演示返回一个空集合，甚至可以演示使用这些 emptyList /Class.class方法的$ / code>，，emptySet（）和集合还具有不可变的附加好处。 Item＃15 最小化可变性。

http://www.coderanch.com/t/536728/java/java/Collections-emptySet-Collections-emptyList-Collections\">Collections-emptySet-Collections-emptyList-Collections

它是一种编程习语。这是为不需要空变量的人。所以在set被初始化之前，他们可以使用空集。

注意：下面的代码只是一个示例（根据用例更改） p>

  private Set myset = Collections.emptySet（）; 
 
 void initSet（）{
 myset = new HashSet（）; 
} 
 void deleteSet（）{
 myset = Collections.emptySet（）; 
} 
  

这些方法有几个优点：

1. 它们更简洁，因为您不需要显式地输出集合的通用类型 - 它通常只是从方法调用的上下文。




2. 它们更有效率，因为他们不打扰创建新对象;他们只是重用一个现有的空和不可变的对象。

这个效果通常很小，但偶尔

What are some of the situations where I can use Collections.emptyMap() ? The Documentation says I can use this method if I want my collection to be immutable.

Why would I want an immutable empty collection? What is the point?

解决方案

From Effective Java, Item #43 - "Return empty arrays or collections, not null" demonstrates returning an empty collection and perhaps even demonstrates using these emptyList(), emptySet(), and emptyMap() methods on the Collections class to get an empty collection that also has the additional benefit of being immutable. From Item #15 "Minimize Mutability".

From Collections-emptySet-Collections-emptyList-Collections

Its a type of programming idiom. This is for people that do not want null variables. So before the set gets initialized, they can use the empty set.

Note: Below code is just an example (change it according to your use case):

private Set myset = Collections.emptySet();

void initSet() {
   myset = new HashSet();
}
void deleteSet() {
   myset = Collections.emptySet();
}

These methods offer a couple of advantages:

1. They're more concise because you don't need to explicitly type out the generic type of the collection - it's generally just inferred from the context of the method call.

2. They're more efficient because they don't bother creating new objects; they just re-use an existing empty and immutable object. This effect is generally very minor, but it's occasionally (well, rarely) important.

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