问题

I need to enter key-value pairs into a data structure that allows me to retrieve them in ascending order of the key--BUT their may be many keys of the same value.

Thus, if the kv-pairs were {10-a, 10-b, 9-c, 8-d, 8-e, 8-f, 4-g, 4-h, 2-i} I would need to retrieve the values in the order: a, b ,c, d, e, f, g, h, i. Are there any data structures in the JAVA API that supports this?

I tried using a TreeMap because it kept them in order which allowed me to use TreeMap.lastKey() to retrieve the highest current key, but I didn't know that it overwrote any duplicate keys that were already in the map. I need something that doesn't overwrite (similar to a HASH), but also allows me to retrieve them in a sorted order--does this exist?

Thank you!

回答1:

Unfortunately you probably will not find a structure that supports multiple values of the same keys. As Dilum said, there are several implementations of "MultiMap" or "Multi-Valued Maps" that would work well.

In addition to Guava's TreeMultiMap, there's also the Spring Framework's MultiValueMap and Apache Common's MultiValueMap.

An example of the Spring implementation would be:

import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;


public class MultiValueMapExample {

    public static void main(String[] args) {
        // 10-a, 10-b, 9-c, 8-d, 8-e, 8-f, 4-g, 4-h, 2-i
        MultiValueMap<Integer, String> map = new LinkedMultiValueMap<Integer, String>();
        map.add(10, "a");
        map.add(10, "b");
        map.add(9, "c");
        map.add(8, "d");
        map.add(8, "e");
        map.add(8, "f");
        map.add(8, "g");
        map.add(4, "h");
        map.add(2, "i");

        System.out.println(map.toString());
        // {10=[a, b], 9=[c], 8=[d, e, f, g], 4=[h], 2=[i]}
    }
}

You could use this by adding Spring-Core via the following Maven dependency:

<dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>3.1.1.RELEASE</version>
        </dependency>

If you need help getting any of these libs in your project, feel free to comment / contact me.

Update 1

Turns out there's not a convenient way to filter / sort from the raw API's. I've included a simple filter function below that should do the trick.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;


public class MultiValueMapExample {

    public static void main(String[] args) {
        // 10-a, 10-b, 9-c, 8-d, 8-e, 8-f, 4-g, 4-h, 2-i
        MultiValueMap<Integer, String> map = new LinkedMultiValueMap<Integer, String>();
        map.add(8, "g");
        map.add(4, "h");
        map.add(10, "a");
        map.add(10, "b");
        map.add(9, "c");
        map.add(8, "d");
        map.add(8, "e");
        map.add(8, "f");

        map.add(2, "i");

        System.out.println(map.toString());
        // {8=[g, d, e, f], 4=[h], 10=[a, b], 9=[c], 2=[i]}

        MultiValueMap<Integer, String> filteredMap = filter(5, map);
        System.out.println( filteredMap.toString() );
        // {10=[a, b], 9=[c], 8=[g, d, e, f], 4=[h], 2=[i]}

    }

    public static MultiValueMap<Integer, String> filter(int numberOfResults, MultiValueMap<Integer, String> map){
        MultiValueMap<Integer, String> result = new LinkedMultiValueMap<Integer, String>();

        List<Integer> keys = new ArrayList<Integer>(map.keySet());
        Collections.sort(keys, Collections.reverseOrder());

        for(Integer key : keys){
            if( result.size() <= numberOfResults ){
                result.put(key, map.get(key));
            }else{
                break;
            }
        }

        return result;

    }
}