Existing Java implementation for a map with a list of values?

Question

A common type of datastructure I might use is a

   Map<String,List<MyObject>> myMap...

The common operation I'd do is to add an object to the list given a matching key. For example,

  myMap.addValueToList("Key", myObject)

Implementing this is not very tricky, but it is a bit ugly (for this example, assume that myObject contains the key):

for (MyObject myObject : myObjects) {
   List<MyObject> newList = newArrayList();
   if (myMap.contains(myObject.key)) {
     newList = myMap.get(myObject.key);
   }
   newList.add(myObject);
   myMap.put(myObject.key, newList);
}

I do this kind of operation more often than I'd like. Last time, I wrote my own class for this functionality, but I wonder if there isn't some kind of implementation available in one of the commonly used libraries such as apache.

You wrote your own class? I once wrote a single static method for this. Your larger code block would suffice for this, although you should create a new ArrayList() only when the key is not in the map, and you only need the put operation in this case, too. — arne.b
– arne.b, Commented May 20, 2014 at 19:32

amit · Accepted Answer · 2014-05-20 19:29:00Z

3

This is called a MultiMap, and there is an existing implementation of it in Apache commons: MultiMap

answered May 20, 2014 at 19:29

amit

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Juvanis · Accepted Answer · 2014-05-20 19:28:25Z

2

I think Guava MultiMap will save your time.

http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html

And here is a post explaining its usage with examples:

http://tomjefferys.blogspot.nl/2011/09/multimaps-google-guava.html

answered May 20, 2014 at 19:28

Juvanis

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1 Comment

Jeremy Over a year ago

Exactly what I wanted and very simple interface. Thanks.

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