具有重复键的Java映射[duplicate]

$ cat YourMap.java
public class YourMap extends HashMap> {
public void put(String key, Integer number) {
List current = get(key);
if (current == null) {
current = new ArrayList();
super.put(key, current);
}
current.add(number);
}

    public static void main(String args[]) {        YourMap m = new YourMap();        m.put("a", 1);        m.put("a", 2);        m.put("b", 3);        for(Map.Entry e : m.entrySet()) { System.out.println(e.getKey() + " -> " + e.getValue());        }    }}$ java mapb -> [3]a -> [1, 2]