TreeMap with no unique key

I use a TreeMap class for store messages information with their priority in my application. I've used a treeMap class for do it because this class order automatically the element based on the key value, for example i have this situation :

enum Priority { HIGH, MEDIUM, LOW }
TreeMap<Priority,String> tMap = new Tree开发者_Go百科Map<Priority,String>();

I use the key (priority of the message) for automatically order messages based on the priority severity, but the problem is that in TreeMap the key is unique then if i try to insert two messages with the same priority the first is overwritten ....

How can i change this behaviour and disable unique constraint on TreeMap?

Is there a class like TreeMap that allow to put the same Key for multiple element ?

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How can i change this behaviour and disable unique constraint on TreeMap?

You can't. The uniqueness of keys is a fundamental invariant of the Map interface.

Is there a class like TreeMap that allow to put the same Key for multiple element ?

You can implement this as a Map<Priority,List<String>> and manage the lists yourself. This is a good option if (for example) you want to process the messages for a given priority in fifo order.

Alternatively, you can use a MultiMap class; e.g. from Apache commons collections or Guava.

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Check out the TreeMultimap class in the Google Guava library.

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You might want another collection type altogether. But if you're intent on using TreeMap, then:

1) Consider using a more complex Priority class. Perhaps create a priority type that has both the base priority (HIGH) and a unique number that's incremented every time you get a new one. Then implement equals, hash, Comparable, etc. using that extra value.

2) For each priority, the value can be a Collection. Retrieve the value for the given priority, and append the new value on the end of the retrieved collection. At that point, though, using a TreeMap is a bit of overkill.

Also, have a look at Apache Commons Collections.

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You may want to use another structure rather than a treemap.

A map is a map and the behavior is what it is, so in a map you cannot have the same key twice by definition.