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这个练习的重点是计算列表中单词的出现次数，并将列表中出现的第二次、第四次、第六次等的单词添加到结果列表中。到目前为止，我只能计算每个单词的出现次数，并且只能将计数为偶数的单词添加到结果列表中。例如，如果我遍历一个列表，第一次获得“a”count = 1，第二次获得“a”count = 2 并将“a”添加到列表中，第三次获得“a”，count = 3 和第四次"a" count = 4 并将 "a" 添加到列表中，结果将是 ["a", "a"]

public static List<String> onlyEvenWordsList(List<String> words) {
    Map<String, Integer> wordsWithCount = new HashMap<>();
    List<String> onlyEvenWords = new ArrayList<>();

    for (String word : words) {
        Integer count = wordsWithCount.get(word);
        if (count == null) {
            count = 0;
        }
        wordsWithCount.put(word, count + 1);
        //System.out.println(wordsWithCount);
            
        }
    for(Map.Entry<String, Integer> entry: wordsWithCount.entrySet()) {
        if(entry.getValue() % 2 == 0){
            onlyEvenWords.add(entry.getKey());
        }
    }

    return onlyEvenWords;
}

因此，例如，给定的输入和所需的输出：

System.out.println(onlyEvenWordsList(Arrays.asList("eggs", "bacon", "SPAM", "ham", "SPAM", "SPAM"))); // [SPAM]
System.out.println(onlyEvenWordsList(Arrays.asList("a", "a", "b", "b", "c", "a", "c", "a"))); // [a, b, c, a]
System.out.println(onlyEvenWordsList(Arrays.asList("a", "a", "a"))); // [a]
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