在 Java 中创建空地图的最佳方法
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Best way to create an empty map in Java
提问by JorgeO
I need to create an empty map.
我需要创建一个空地图。
if (fileParameters == null)
fileParameters = (HashMap<String, String>) Collections.EMPTY_MAP;
The problem is that the above code produces this warning: Type safety: Unchecked cast from Map to HashMap
问题是上面的代码产生了这个警告： Type safety: Unchecked cast from Map to HashMap
What is the best way to create this empty map?
创建此空地图的最佳方法是什么？
采纳答案by Jonik
1)If the Map can be immutable:
1)如果 Map 可以是不可变的：
Collections.emptyMap()
// or, in some cases:
Collections.<String, String>emptyMap()
You'll have to use the latter sometimes when the compiler cannot automatically figure out what kind of Map is needed (this is called type inference). For example, consider a method declared like this:
有时当编译器无法自动确定需要哪种 Map 时，您将不得不使用后者（这称为类型推断）。例如，考虑这样声明的方法：
public void foobar(Map<String, String> map){ ... }
When passing the empty Map directly to it, you have to be explicit about the type:
将空 Map 直接传递给它时，您必须明确说明类型：
foobar(Collections.emptyMap()); // doesn't compile
foobar(Collections.<String, String>emptyMap()); // works fine
2)If you need to be able to modify the Map, then for example:
2）如果您需要能够修改地图，那么例如：
new HashMap<String, String>()
(as tehblanx pointed out)
（正如tehblanx指出的那样）
Addendum: If your project uses Guava, you have the following alternatives:
附录：如果您的项目使用Guava，您有以下选择：
1)Immutable map:
1)不可变映射：
ImmutableMap.of()
// or:
ImmutableMap.<String, String>of()
Granted, no big benefits here compared to Collections.emptyMap(). From the Javadoc:
当然，与Collections.emptyMap(). 从 Javadoc：
This map behaves and performs comparably to
Collections.emptyMap(), and is preferable mainly for consistency and maintainability of your code.
此映射的行为和性能与 相当
Collections.emptyMap()，并且主要用于代码的一致性和可维护性。
2)Map that you can modify:
2）可以修改的地图：
Maps.newHashMap()
// or:
Maps.<String, String>newHashMap()
Mapscontains similar factory methods for instantiating other types of maps as well, such as TreeMapor LinkedHashMap.
Maps包含类似的工厂方法，用于实例化其他类型的地图，例如TreeMap或LinkedHashMap。
Update (2018): On Java 9or newer, the shortest code for creating an immutable empty map is:
更新（2018 年）：在Java 9或更新版本上，创建不可变空映射的最短代码是：
Map.of()
...using the new convenience factory methodsfrom JEP 269.
回答by AndreiM
If you need an instance of HashMap, the best way is:
如果你需要一个 HashMap 的实例，最好的方法是：
fileParameters = new HashMap<String,String>();
Since Map is an interface, you need to pick some class that instantiates it if you want to create an empty instance. HashMap seems as good as any other - so just use that.
由于 Map 是一个接口，如果你想创建一个空实例，你需要选择一些实例化它的类。HashMap 看起来和其他的一样好 - 所以就用它吧。
回答by basszero
回答by Peter ?tibrany
Either Collections.emptyMap(), or if type inference doesn't work in your case,Collections.<String, String>emptyMap()
或者Collections.emptyMap()，如果类型推断在您的情况下不起作用，Collections.<String, String>emptyMap()
回答by OscarRyz
回答by Vitalii Fedorenko
Since in many cases an empty map is used for null-safe design, you can utilize the nullToEmptyutility method:
由于在许多情况下空映射用于空安全设计，因此您可以使用nullToEmpty实用程序方法：
class MapUtils {
static <K,V> Map<K,V> nullToEmpty(Map<K,V> map) {
if (map != null) {
return map;
} else {
return Collections.<K,V>emptyMap(); // or guava ImmutableMap.of()
}
}
}
Similarly for sets:
同样对于集合：
class SetUtils {
static <T> Set<T> nullToEmpty(Set<T> set) {
if (set != null) {
return set;
} else {
return Collections.<T>emptySet();
}
}
}
and lists:
并列出：
class ListUtils {
static <T> List<T> nullToEmpty(List<T> list) {
if (list != null) {
return list;
} else {
return Collections.<T>emptyList();
}
}
}
