为什么我在创建Map时会收到IllegalArgumentException？ [英] Why am I getting an IllegalArgumentException when creating a Map?

问题描述

我正在尝试创建城市和温度的地图，但它正在抛出 IllegalArgumentException 。这就是我正在做的事情：

I'm trying to create a map of cities and temperature, but it's throwing an IllegalArgumentException. Here's what I'm doing:

Map<String, Integer> tempMap = Map.of("London", 13, "Paris", 17, "Amsterdam", 13, 
                                      "Madrid", 21, "Rome", 19, "London", 13, "Bonn", 14,
                                      "Moscow", 7, "Barcelona", 20, "Berlin", 15);

如果我逐个添加它们就没有问题：

If I add them one by one there's no problem:

Map<String, Integer> tempMap = new Hashmap<>(); // or LinkedHashMap
tempMap.put("London", 13);
tempMap.put("Madrid", 21);
tempMap.put("Moscow", 7);
// etc.

为什么会这样？内容是否应该相同？

Why does this happen? Aren't the contents supposed to be the same?

推荐答案

为什么会发生这种情况？

Why does this happen?

因为实例化中有一个重复的密钥：London。 <$ href=\"http://download.java.net/java/jdk9/docs/api/java/util/Map.html#immutable\" rel=\"noreferrer\">不可变的静态工厂 地图和设置不允许重复（如果地图条目的密钥重复，则地图条目重复） - 不在创建时间 - 因此一点也不。限制表现为抛出 IllegalArgumentException 。

Because you have a duplicate key in your instantiation: "London". The immutable static factories for Map and Set do not allow duplicates (a map entry is duplicate if its key is duplicate) - not during creation time - therefore not at all. The restriction is manifested by a thrown IllegalArgumentException.

虽然从技术上讲你没有做任何不相容的事情，但是图书馆认为这是一个（可能是复制粘贴）错误。你为什么要添加一个项目只是为了稍后覆盖几行？

Although technically you're not doing anything incompatible, the writers of the library assumed that it's a (probably copy-paste) mistake. Why would you add an item just to override it a few lines later?

这让我想到...

如果我逐个添加它们没有问题

If I add them one by one there's no problem

这就是你的想法，只有你可能没有意识到你的地图将包含少于你输入的1个条目。重复条目会覆盖前一个条目（最后一个胜利规则）。当因此发生错误时，会出现很多问号。出于这个原因，故障快速方法有其优点（虽然我不会提倡它只是更好）。

That's what you think, only you might not have realized that your map will contain 1 entry less than you put in it. The duplicate entry overrides the previous one ("last one wins" rule). When a bug will happen because of this there are going to be a lot of question marks thrown around. For this reason, the fail-fast method has its advantages (though I won't advocate it's just plain better).

作为提示，在创建地图时，如果你稍微格式化它会更容易看到它的内容：

As a tip, when creating the map it's easier to see its content if you format it a bit:

Map<String, Integer> tempMap = Map.of(
        "London",    13,
        "Paris",     17,
        "Amsterdam", 13,
        "Madrid",    21,
        "Rome",      19,
        "London",    13, // !
        "Bonn",      14,
        "Moscow",     7,
        "Barcelona", 20,
        "Berlin",    15
);

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