Java：列表中元素的数量
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Java: count of elements in a list
提问by Saliha Uzel
Possible Duplicate:
How to count occurrence of an element in a List
可能的重复：
如何计算列表中元素的出现次数
I have a list like List<String> A={12, 12, 14, 16, 16}. How can I find the number of elements distinctly as
我有一个像List<String> A={12, 12, 14, 16, 16}. 我怎样才能清楚地找到元素的数量
12->2
14->1
16->2
by using a function like countElements(A,"12")or A.count("12")? Is there a library or a function?
通过使用像countElements(A,"12")或这样的函数A.count("12")？有库或函数吗？
回答by Jigar Joshi
Just iterate through each and maintain a
只需遍历每个并维护一个
Map<Integer, Integer> numberToFrequencyMap;
回答by Bhesh Gurung
You can also utililize the method Collections.frequencyif you need the frequency of only some of the elements individually.
Collections.frequency如果您只需要个别元素的频率，您也可以使用该方法。
回答by Rohit Jain
Take a look at Apache CommonsCollectionUtils#getCardinalityMap
看一眼 Apache CommonsCollectionUtils#getCardinalityMap
It returns a Map<Element, Integer>with frequency of each element in your list.
它返回Map<Element, Integer>列表中每个元素的频率。
List<String> list = {"12", "12", "14", "16", "16"};
Map<String, Integer> frequencyMapping = CollectionUtils.getCardinalityMap(list);
Also, you have a CollectionUtils#cardinalityif you want to fetch count for a specific element.
此外，CollectionUtils#cardinality如果您想获取特定元素的计数，您还有一个。
回答by Louis Wasserman
If you can use third-party dependencies, Guavahas a collection type for this called Multiset:
如果您可以使用第三方依赖项，Guava有一个名为 的集合类型Multiset：
Multiset<String> multiset = HashMultiset.create(list);
multiset.count("foo"); // number of occurrences of foo
multiset.elementSet(); // returns the distinct strings in the multiset as a Set
multiset.entrySet(); // returns a Set<Multiset.Entry<String>> that you can
// iterate over to get the strings and their counts at the same time
(Disclosure: I contribute to Guava.)
（披露：我为番石榴做出了贡献。）
回答by Yogendra Singh
Iterate your numbers, maintain the count in a Mapas below:
迭代您的数字，将计数保持在 a 中Map，如下所示：
List<Integer> myNumbers= Arrays.asList(12, 12, 14, 16, 16);
Map<Integer, Integer> countMap = new HashMap<Integer, Integer>();
for(int i=0; i<myNumbers.size(); i++){
Integer myNum = myNumbers.get(i);
if(countMap.get(myNum)!= null){
Integer currentCount = countMap.get(myNum);
currentCount = currentCount.intValue()+1;
countMap.put(myNum,currentCount);
}else{
countMap.put(myNum,1);
}
}
Set<Integer> keys = countMap.keySet();
for(Integer num: keys){
System.out.println("Number "+num.intValue()+" count "+countMap.get(num).intValue());
}
