I want to add duplicate elements on hashmap
so:
put("name1", 1);
put("name1", 3);
put("name1", 3);
put("name2", 1);
put("name2", 3);
how i can do that?
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what is the benefit of doing this? what are you trying to accomplish, maybe there is a better way of doing this if you give some more deatilsant– ant2011-01-29 17:50:50 +00:00Commented Jan 29, 2011 at 17:50
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6 Answers
Use a Map<String, List<Integer>> i.e. you map a string to a list of integers.
So, in this case, name1 would map to a list of [1,3,3].
Obviously you'd have to write your own put method, in which you add the int to the list. Example:
put(String s, int i){
List<Integer> list = map.get(s);
if(list == null){
list = new ArrayList<Integer>();
map.put(s, list);
}
list.add(i);
}
Comments
The ListMultimap interface from Guava may meet your requirements. It allows duplicate keys and duplicate key/value pairs.
ListMultimap<String, Integer> m =
ArrayListMultimap.create();
m.put("name1", 1);
m.put("name1", 3);
m.put("name1", 3);
m.put("name2", 1);
m.put("name2", 3);
System.out.println(m.get("name1")); // => [1, 3, 3]
System.out.println(m.get("name2")); // => [1, 3]
Also do you really need to preserve duplicate key/value pairs? If not then a HashMultimap may be sufficient (and more efficient.) If you insert the same entries into a HashMultimap you get:
System.out.println(m.get("name1")); // => [1, 3]
System.out.println(m.get("name2")); // => [1, 3]
Comments
Your idea violates the contract of the Map interface:
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
It'd understandably be confusing to the map when you ask:
map.get("name1")
It wouldn't know which value to get.
I'd use dogbane's solution of mapping each key to a list of Integers. In your example, you have possible duplicate values. If you don't want duplicate values (i.e. for "name1" there'd be only one 3 in the resulting list), you could instead make it a Map of Strings to Sets of Integers.
Comments
You should use Google Collection's Multimap data structure.
A collection similar to a Map, but which may associate multiple values with a single key. If you call put(K, V) twice, with the same key but different values, the multimap contains mappings from the key to both values.
This is exactly what you are trying to achieve. No need to re-invent the wheel by writing your custom Map operations in my opinion. Also you may find this tutorial on MultiMap useful.
4 Comments
CoolBeans
Right. Or you can use HashMultimap
finnw
HashMultimap does allow duplicate keys but not duplicate key/value pairs. If you insert the example entries into a HashMultimap you end up with only one name1=3 entry.CoolBeans
Right. Okay I went back and closely looked at the OP's data .. he/she does have a duplicate key/value pair ... at first I thought he/she was just needing duplicate keys. Thanks for pointing it out :-)
org.apache.commons.collections.map.MultiValueMap is a more suitable choice.It can put one key to multiple values.
public Object put(Object key, Object value) {
boolean result = false;
Collection coll = getCollection(key);
if (coll == null) {
coll = createCollection(1);
result = coll.add(value);
if (coll.size() > 0) {
// only add if non-zero size to maintain class state
getMap().put(key, coll);
result = false;
}
} else {
result = coll.add(value);
}
return (result ? value : null);
}
