获取Hashmap前3个键

Question

I have a hashmap storing the number of occurrences of character in a text. I am trying to print out the top 3 occurrences, but it is printing incorrectly.

int max = 1000000000;
for (int i = 1; i <= 3; i++) {
    for (Character key : list.keySet()) {
        if (list.get(key) < max) {
            max = list.get(key);
            System.out.println(i + ": " + key + " " + list.get(key));
            break;
        }
    }
}

Answer 1

With Java 8 you can use the code below(*):

List<Entry<Character, Integer>> top3 = map.entrySet().stream()
                                    .sorted(comparing(Entry::getValue, reverseOrder()))
                                    .limit(3)
                                    .collect(toList());

_{(*) with the following imports:
import static java.util.Comparator.comparing;
import static java.util.Comparator.reverseOrder;
import static java.util.stream.Collectors.toList;}

Answer 2

You could modify your program to this form:

for (int i = 1; i <= 3; i++) {
    int max = -1;
    Character maxKey = 'a';
    for (Character key : list.keySet()) {
        if (list.get(key) > max) {
            max = list.get(key);
            maxKey = key;
        }
    }
    System.out.println(i + ": " + maxKey + " " + max );
    list.remove(maxKey);
}

Answer 3

You need to sort your entries by number of occurences and get the top 3:

List<Entry<Character, Integer>> entryList = new ArrayList<>(list.entrySet());
Collections.sort(entryList, new Comparator<Entry<Character, Integer>>(){
    @Override
    public int compare(Entry<Character, Integer> e1, Entry<Character, Integer> e2) {
        return e2.getValue() - e1.getValue(); // descending order
    }
});

// now let's get the top 3
List<Character> top3 = new ArrayList<>(3);
for(Entry<Character, Integer> e : entryList) {
    top3.add(e.getValue());
    if(top3.size() == 3) {
        break;
    }
}

Answer 4

Here's a solution using Java 8 streams, based on the one provided by @assylias. It performs the complete task of collecting character counts from a String into a Map and selecting the top 3 entries.

import java.util.ArrayList;
import static java.util.Comparator.*;
import java.util.List;
import java.util.Map.Entry;
import static java.util.stream.Collectors.*;

public class Stream {

    public static void main(final String[] args) {
        final String text = "hello stackoverflow, let's count these character occurrences!";
        final char[] charArray = text.toCharArray();
        final List<Character> characters = new ArrayList<Character>(text.length());
        for (final char c : charArray) {
            characters.add(c);
        }

        final List<Entry<Character, Long>> top3 = characters.stream()
                .collect(groupingBy(Character::charValue, counting()))
                .entrySet().stream()
                .sorted(comparing(Entry::getValue, reverseOrder())).limit(3).collect(toList());

        System.out.println(top3);
    }
}

Output:

[e=8, c=7,  =6]