[英]Converting HashMap to Sorted ArrayList
I have a HashMap<String, Integer>
containing words along with their frequencies. 我有一个HashMap<String, Integer>
包含单词及其频率。 I need to now convert this HashMap
into an ArrayList
of just the words, discarding of the frequencies, but i also want the ArrayList
to be sorted in descending order of words by frequency. 我现在需要将此HashMap
转换为仅包含单词的ArrayList
,丢弃频率,但我还希望ArrayList
以单词的降序按频率排序 。
Does anyone know an efficient way to do this? 有人知道这样做的有效方法吗?
HashMap
has a convenient method called entrySet()
, which lets you access a collection of key-value pairs. HashMap
有一个方便的方法,称为entrySet()
,使您可以访问键-值对的集合。 You can use it to construct a List<Map.Entry<String,Integer>>
. 您可以使用它来构造List<Map.Entry<String,Integer>>
。
Now you have something you can sort. 现在,您可以进行排序了。 Use a sort method with a custom comparator, which orders entries with higher frequencies toward the beginning of the list. 使用带有自定义比较器的排序方法,该比较器将频率较高的条目排序到列表的开头。
With a sorted list in hand, all you need to do is walk it, and harvest the words which are now in the proper order. 手头有一个已排序的列表,您所要做的就是走动它,然后按正确的顺序收获单词。
List<Map.Entry<String,Integer>> entries = new ArrayList<Map.Entry<String,Integer>>(
freqMap.entrySet()
);
Collections.sort(
entries
, new Comparator<Map.Entry<String,Integer>>() {
public int compare(Map.Entry<String,Integer> a, Map.Entry<String,Integer> b) {
return Integer.compare(b.getValue(), a.getValue());
}
}
);
for (Map.Entry<String,Integer> e : entries) {
// This loop prints entries. You can use the same loop
// to get the keys from entries, and add it to your target list.
System.out.println(e.getKey()+":"+e.getValue());
}
When using Java 8 you can make use of the Stream API like follows: 使用Java 8时,您可以使用Stream API ,如下所示:
final Map<String, Integer> wordStats = new HashMap<>();
// some dummy data:
wordStats.put("twice", 2);
wordStats.put("thrice", 3);
wordStats.put("once", 1);
final List<String> sortedStats = wordStats.entrySet().stream()
.sorted(Comparator.comparing(Map.Entry::getValue, Comparator.reverseOrder()))
.map(Map.Entry::getKey)
.collect(Collectors.toList());
// or to specify the list implementation:
//.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
// Output
sortedStats.forEach(System.out::println);
Output: 输出:
thrice
twice
once
In Java 8 you can also do this shorter variation of what has been answered 在Java 8中,您还可以对已回答的内容进行这种简短的修改
Ascending order 升序
ArrayList<Map.Entry<String, Integer>> sorted = newArrayList<>(frequencies.entrySet());
sorted.sort(Comparator.comparingInt(Map.Entry::getValue));
Descending order 降序排列
ArrayList<Map.Entry<String, Integer>> sorted = new ArrayList<>(frequencies.entrySet());
sorted.sort(Collections.reverseOrder(Comparator.comparingInt(Map.Entry::getValue)));
You can: 您可以:
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