Why is there no direct implemention of Bag in java collection framework?

Question

I can't figure out why JCF (Java Collection Framework) does't have a Bag implementation(to allow duplicates and not maintain order). Bag performance would be much better than current Collection implementations in JCF.

• I know how to implement Bag in Java.
• I know Bag is available in Apache commons.
• I know there are other implementations that can be used as a Bag but there is so much work to do in other implementations compared to a Bag.

Why has the Java Collections framework not provided direct implementations like this?

Answer 1

Posting my comment as an answer since it answers this question best.

From the bug report filed here :

There isn't a lot of enthusiasm among the maintainers of the Collection framework to design and implement these interfaces/classes. I personally can't recall having needed one. It would be more likely that a popular package developed outside the JDK would be imported into the JDK after having proved its worth in the real world.

The need for having support for Bags is valid today.

Guava has support for it. Also GS-Collections.

Answer 2

Currently, bag violates the collections contract. Many methods are in conflict with the current collections rules.

"Bag is a Collection that counts the number of times an object appears in the collection. Suppose you have a Bag that contains {a, a, b, c}. Calling getCount(Object) on a would return 2, while calling uniqueSet() would return {a, b, c}.

Note that this interface violates the Collection contract. The behavior specified in many of these methods is not the same as the behavior specified by Collection. The noncompliant methods are clearly marked with "(Violation)" in their summary line. A future version of this class will specify the same behavior as Collection, which unfortunately will break backwards compatibility with this version."

 boolean add(java.lang.Object o)       (Violation) Add the given object to the bag and keep a count.   boolean removeAll(java.util.Collection c)       (Violation) Remove all elements represented in the given collection, respecting cardinality. 

Please see the link for more information: HERE