如何通过Java中的关键字对Map值进行排序 [英] how to sort Map values by key in Java

问题描述

我有一个java中的Map，它们都有字符串。

数据如下：<question1，1>"question9，1> <question2，4><question5，2>

基于其键的地图。所以最后我会有 question1，question2，question3 ....一个等等。

最终我试图从这张地图中获取两个字符串。第一个字符串：问题（按顺序1 ..10）和第二个字符串：答案（与问题相同的顺序）。

现在我有以下内容：

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

这让我得到一个字符串中的问题，但它们不是顺序的。

解决方案

简短答案

使用 TreeMap 。这正是它的原因。

如果此地图传递给您，您无法确定类型，则可以执行以下操作：

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

这将在自然中迭代地图

---

更长的答案

从技术上讲，您可以使用实现 SortedMap 的任何东西，但除了罕见的情况，这相当于 TreeMap ，就像使用映射实现通常等于 HashMap 。

对于您的密钥是不实现Comparable的复杂类型或您不想使用自然顺序的情况， TreeMap 和 TreeSet 有额外的构造函数，让您传递一个比较器：

  //放置在示范中，但不一定是匿名类
比较器&Foo> comparator = new Comparator&Foo>（）{
 public int compare（Foo o1，Foo o2）{
 ... 
} 
} 
 SortedSet&Foo& keys = new TreeSet< Foo>（比较器）; 
 keys.addAll（map.keySet（））; 
  

记住使用 TreeMap 或 TreeSet ，它将具有不同于 HashMap 或 HashSet 的性能特征。粗略地说，找到或插入元素的操作将从 O（1）转到 O（Log（N））。

在$ code> HashMap 中，从1000个项目移动到10,000不会真正影响您查找元素的时间，但对于 TreeMap 查找时间将减慢约3倍（假设为Log ₂ ）。从1000到100,000的每个元素查找速度将慢6倍。

I have a Map in java that has strings for both .

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....an so on.

Eventually I am trying to get two strings out of this Map. First String: Questions ( in order 1 ..10) and Second String: Answers (in same order as 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 its 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<String>(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 an anonymous class
Comparator<Foo> comparator = new Comparator<Foo>() {
  public int compare(Foo o1, Foo o2) {
    ...
  }
}
SortedSet<Foo> keys = new TreeSet<Foo>(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.

这篇关于如何通过Java中的关键字对Map值进行排序的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！