在java集合中计数事件的优雅方式 [英] Elegant way of counting occurrences in a java collection

问题描述

给定一个具有可能重复项的对象集合，我最终得到每个对象的出现次数。我通过初始化一个空的 Map ，然后遍历 Collection 并将对象映射到它的计数（增加每次地图已经包含对象时计数）。

  public Map< Object，Integer> countOccurrences（Collection< Object> list）{
 Map< Object，Integer> occurrenceMap = new HashMap< Object，Integer>（）; 
 
 for（Object obj：list）{
 Integer numOccurrence = occurrenceMap.get（obj）; 
 if（numOccurrence == null）{
 //第一个计数
 occurrenceMap.put（obj，1）; 
} else {
 occurrenceMap.put（obj，numOccurrence ++）; 
} 
} 
 return occurrenceMap; 
} 
  

对于计数事件的简单逻辑，这看起来太冗长。有更优雅/更短的方式这样做吗？

解决方案

签出

a href =http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained> Guava的Multiset 。

不幸的是，它没有一个addAll（迭代迭代）函数，而是一个简单的循环调用add（E

我的错误，确实如此有一个addAll方法 - 因为它必须实现Collection。

Given a collection of objects with possible duplicates, I'd like end up with a count of occurrences per object. I do it by initializing an empty Map, then iterating through the Collection and mapping the object to its count (incrementing the count each time the map already contains the object).

 public Map<Object, Integer> countOccurrences(Collection<Object> list){
      Map<Object, Integer> occurrenceMap = new HashMap<Object, Integer>();

      for(Object obj: list){
           Integer numOccurrence = occurrenceMap.get(obj);
           if(numOccurrence == null){
                //first count
                occurrenceMap.put(obj, 1);
           } else{
                occurrenceMap.put(obj, numOccurrence++);
           }
      }
      return occurrenceMap;
 }

This looks too verbose for a simple logic of counting occurrences. Is there a more elegant/shorter way of doing this? I'm open to a completely different algorithm or a java language specific feature that allows for a shorter code.

解决方案

Check out Guava's Multiset. Pretty much exactly what you're looking for.

Unfortunately it doesn't have an addAll(Iterable iterable) function, but a simple loop over your collection calling add(E e) is easy enough.

EDIT

My mistake, it does indeed have an addAll method - as it must, since it implements Collection.

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