In Java: Get array of values of a map sorted by the map's keys

I have a hashmap, I wish to get an array of the values in this hashmap, but I wish for the array to be sorted by the keys in the hashmap.

For instance if the map looks like this:

• <2,obj1>
• <4,obj2>
• <0,obj3>
• <10,obj4开发者_开发技巧>
• <5,obj5>
• <1,obj6>
• <15,obj7>
• <3,obj8>

I want the array to be: [obj3,obj6,obj1,obj8,obj2,obj5,obj4,obj7]

The faster the better. Is there a built in way to do this?

Sounds like you want a SortedMap, specifically a TreeMap. You can use the fact that the map maintains a sorted list of keys to generate your output array, e.g. by using SortedMap.values() or by iterating over the entries in the map.

I always like when I can answer in the form of a passing unit test:

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

import junit.framework.TestCase;

public class SortedMapValuesTest extends TestCase {
    public void testSortedMapValues() throws Exception {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        map.put(1, 200);
        map.put(2, 300);
        map.put(3, 100);
        List<Integer> list = getSortedMapValues(map);
        assertEquals("[200, 300, 100]", list.toString());
    }

    private <K extends Comparable<K>, V> List<V> getSortedMapValues(Map<K, V> map) {
        ArrayList<K> keys = new ArrayList<K>(map.keySet());
        ArrayList<V> values = new ArrayList<V>(keys.size());
        Collections.sort(keys);
        for (K key : keys)
            values.add(map.get(key));
        return values;
    }
}