给定一个字符串列表，创建一个具有长度和字符串的哈希表

Question

I am trying to understand a programming question where I am given a list of words for example, (apple, orange, car, can, fuzzy) I would get a hashmap with the length for the key and linkedlist of words for value. For example,

(3, {car, can})
(5, {apple, fuzzy})
(6, {orange})

How could I build this? I am relatively new to Java and only knows how to read string input and get the length of each string. But very unfamiliar with hashmaps. Could someone guide me in the right direction?

Answer 1

As Tim Biegeleisen said, here's an approach if you're using Java 8:

import java.util.List;
import java.util.Map;
import static java.util.stream.Collectors.*;

public class CollectByLength {
    public static void main(String[] args) {
        Map<Integer, List<String>> map = Stream.of("apple", "orange", "car", "can", "fuzzy")
                                        .collect(groupingBy(String::length));

        System.out.println(map); //prints {3=[car, can], 5=[apple, fuzzy], 6=[orange]}
    }
}

If you care about the List implementation for some reason, the above solution doesn't provide any guarantee on the list implementation. From the doc:

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List objects returned.

But it's also possible to specify the List implementation you need ( LinkedList here)

import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import static java.util.stream.Collectors.*;

public class CollectByLength {
    public static void main(String[] args) {
        Map<Integer, List<String>> map = Stream.of("apple", "orange", "car", "can", "fuzzy")
                                        .collect(groupingBy(String::length, toCollection(LinkedList::new)));

        System.out.println(map); //prints {3=[car, can], 5=[apple, fuzzy], 6=[orange]}
    }
}

Answer 2

Here is a fairly concise Java 7 approach:

List<String> words = Arrays.asList("apple", "orange", "car", "can", "fuzzy");
Map<Integer, List<String>> map = new HashMap<>();

for (String word : words) {
    List<String> wordList = map.get(word.length());
    if (wordList == null) {
        wordList = new ArrayList<String>();
        wordList.add(word);
        map.put(word.length(), wordList);
    }
    else {
        wordList.add(word);
    }
}

I will leave it up the experts to give an even leaner Java 8 solution.