如何通过Java中的键对Map值进行排序？ [英] How to sort Map values by key in Java?

问题描述

我有一个包含键和值的字符串的Map。

数据如下：

question1，1
question9，1

question2，4

question5，2

我想根据键对地图进行排序。所以，最后，我会有 question1，question2，question3 ....等等。

最终，我试图从这张地图中取出两个字符串。

• 第一个字符串：问题（按顺序1 ..10）第二个字符串：Answers（按照与问题相同的顺序）

现在我拥有以下内容：

  Iterator it = paramMap.entrySet（）。iterator（）; （it.hasNext（））{
 Map.Entry pairs =（Map.Entry）it.next（）; 
 questionAnswers + = pairs.getKey（）+，; 
} 
  

这让我得到了字符串中的问题，但它们并没有按顺序排列。 / b>

解决方案

简短回答

使用 TreeMap 。这正是它的目的。

如果将此地图传递给您并且您无法确定类型，则可以执行以下操作：

 的SortedSet<字符串> keys = new TreeSet<>（map.keySet（））; 
 for（String key：keys）{
 String value = map.get（key）; 
 //做某事
} 
  

这将在自然中迭代整个地图

更长的答案

从技术上讲，你可以使用任何实现 SortedMap 的东西，但除了极少数情况下，这相当于 TreeMap ，就像使用 Map 实现通常等于 HashMap 。

如果你的键是一个复杂的类型，它没有实现Comparable，或者你不想使用自然顺序，那么 TreeMap 和 TreeSet 有额外的构造函数，可以让你传入 Comparator ：

  //作为演示内联放置，但不一定是lambda表达式
比较器< Foo>比较器=（Foo o1，Foo o2） - > {
 ... 
} 
 
 SortedSet< Foo> keys = new TreeSet<>（比较器）; 
 keys.addAll（map.keySet（））; 
  

记住使用 TreeMap 或 TreeSet ，它将具有与 HashMap 或 HashSet 不同的性能特性。粗略地说，查找或插入元素的操作将从 O（1）到 O（Log（N））。

在 HashMap 中，从1000个项目移动到10,000个并不会真正影响查找元素的时间，而是对于 TreeMap 查找时间将减慢大约3倍（假设Log ₂ ）。对于每个元素查找，从1000移动到100,000将会慢6倍。

I have a Map that has strings for both keys and values.

Data is like following:

"question1", "1"
"question9", "1"
"question2", "4"
"question5", "2"

I want to sort the map based on its keys. So, in the end, I will have question1, question2, question3....and so on.

Eventually, I am trying to get two strings out of this Map.

• First String: Questions ( in order 1 ..10)
• Second String: Answers (in the same order as the question)

Right now I have the following:

Iterator it = paramMap.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry pairs = (Map.Entry) it.next();
    questionAnswers += pairs.getKey() + ",";
}

This gets me the questions in a string but they are not in order.

解决方案

Short answer

Use a TreeMap. This is precisely what it's for.

If this map is passed to you and you cannot determine the type, then you can do the following:

SortedSet<String> keys = new TreeSet<>(map.keySet());
for (String key : keys) { 
   String value = map.get(key);
   // do something
}

This will iterate across the map in natural order of the keys.

---

Longer answer

Technically, you can use anything that implements SortedMap, but except for rare cases this amounts to TreeMap, just as using a Map implementation typically amounts to HashMap.

For cases where your keys are a complex type that doesn't implement Comparable or you don't want to use the natural order then TreeMap and TreeSet have additional constructors that let you pass in a Comparator:

// placed inline for the demonstration, but doesn't have to be a lambda expression
Comparator<Foo> comparator = (Foo o1, Foo o2) -> {
        ...
    }

SortedSet<Foo> keys = new TreeSet<>(comparator);
keys.addAll(map.keySet());

Remember when using a TreeMap or TreeSet that it will have different performance characteristics than HashMap or HashSet. Roughly speaking operations that find or insert an element will go from O(1) to O(Log(N)).

In a HashMap, moving from 1000 items to 10,000 doesn't really affect your time to lookup an element, but for a TreeMap the lookup time will be about 3 times slower (assuming Log₂). Moving from 1000 to 100,000 will be about 6 times slower for every element lookup.

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