I have a HashMap<String, Integer> containing words along with their frequencies. 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.
Does anyone know an efficient way to do this?
HashMap has a convenient method called entrySet(), which lets you access a collection of key-value pairs. You can use it to construct a List<Map.Entry<String,Integer>>.
Now you have something you can sort. Use a sort method with a custom comparator, which orders entries with higher frequencies toward the beginning of the list.
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.
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:
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:
thrice
twice
once
final?In Java 8 you can also do this shorter variation of what has been answered
Ascending order
ArrayList<Map.Entry<String, Integer>> sorted = newArrayList<>(frequencies.entrySet());
sorted.sort(Comparator.comparingInt(Map.Entry::getValue));
Descending order
ArrayList<Map.Entry<String, Integer>> sorted = new ArrayList<>(frequencies.entrySet());
sorted.sort(Collections.reverseOrder(Comparator.comparingInt(Map.Entry::getValue)));
You can:
Wordthat holds theStringand the frequency and implements theComparableinterface comparingWordobjects by their frequency. Then populate anArrayList<Word>from theHashMapand then callCollections.sort(yourArrayList)Map<String, Integer>I've used anArrayList<MyClass>, whereMyClasscontains theString wordandint/double frequency. You can then use theCollections.sort(yourList, new Comparator<MyClass>(){ @Override public int compare(MyClass mc1, MyClass mc2){ ... } });to sort itHashMapinto aTreeMap, sorting it using acomparatorthen iterating over the elements and loading into anArrayList?