Map Sum Pairs

https://www.lintcode.com/problem/map-sum-pairs/description

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

class MapSum {
    private Map<String, Integer> map = new HashMap<>();

    /**
     * Initialize your data structure here.
     */
    public MapSum() {

    }

    public void insert(String key, int val) {
        map.put(key, val);
    }

    public int sum(String prefix) {
        int sum = 0;
        Iterator<Map.Entry<String, Integer>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, Integer> next = iterator.next();
            if (next.getKey().startsWith(prefix)) {
                sum += next.getValue();
            }
        }
        return sum;
    }
}

/**
 * Your MapSum object will be instantiated and called as such:
 * MapSum obj = new MapSum();
 * obj.insert(key,val);
 * int param_2 = obj.sum(prefix);
 */