从Arraylist< String>中获取恰好发生三遍的字符串. [英] Get the Strings that occur exactly three times from Arraylist<String>

问题描述

我有一个ArrayList，其中包含一些具有重复项和三次出现的值的值，我想将那些三次发生的值专门收集到另一个ArrayList中，例如

I have an ArrayList which contains some values with duplicates and elements that occur thrice, I want to collect those values that occur thrice specifically into another ArrayList like

Arraylist<String> strings;   //contains all strings that are duplicates and that occur thrice

在这里，我只想获取在另一个数组列表中三次出现的字符串.

Here, I want to get only the Strings that occur thrice in another array list.

Arraylist<String> thrice;    //contains only elements that occur three times.

目前，我有一个

Currently, I have a solution for dealing with duplicates but I cannot extend this for only getting strings that occur thrice, this please help me to find out.

推荐答案

关于您试图解决的两个问题的通用实用程序将使用

A generic utility of what you're trying to acheieve in both the questions would be using Collections.frequency as :

/**
 * @param input the list as your input
 * @param n number of occurrence (duplicates:2 , triplets:3 etc..)
 * @param <T> (type of elements)
 * @return elements with such conditional occurrent in a Set
 */
static <T> Set<T> findElementsWithNOccurrence(List<T> input, int n) {
    return input.stream()
            .filter(a -> Collections.frequency(input, a) == n) // filter by number of occurrences
            .collect(Collectors.toSet()); // collecting to a final set (one representation of each)
}

注意 :这将是一种 O(n^2) 方法，因为它使用了Collections.frequency，该方法再次遍历整个集合以获取频率.但建议针对您要的内容采用一种更具可读性和通用性的方法.另外，这有意将最终输出收集到Set，因为List毕竟可以再次具有重复项.

Note: This would be an O(n^2) approach since its using Collections.frequency which iterates over the entire collection again to get the frequency. But proposed for a more readable and generic approach towards what you're looking for. Also, this intentionally collects final output to a Set, since a List can again have duplicates after all.

或者，您可以使用该方法计算Java-8中元素的频率，然后遍历创建的Map可以根据需要进行过滤，并在同一迭代中收集输出:

Alternatively, you can use the method to count the frequency of elements in Java-8 and iterate over the entries of the Map created thereby to process filtering as desired and collect the output in the same iteration :

/**
 * @param input the list as your input
 * @param n     number of occurrence (duplicates :2 , triplets :3 etc)
 * @param <T>   (type of elements)
 * @return elements in a set
 */
static <T> Set<T> findElementsWithNOccurrence(List<T> input, int n) {
    return input.stream() // Stream<T>
            .collect(Collectors.groupingBy(Function.identity(), 
                    Collectors.counting())) // Map<T, Long>
            .entrySet() // Set<Map.Entry<T,Long>>
            .stream() // Stream<Map.Entry<T,Long>>
            .filter(e -> e.getValue() == n) // filtered with frequency 'n'
            .map(Map.Entry::getKey) // Stream<T>
            .collect(Collectors.toSet()); // collect to Set
}

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