Project
From previous discussions on this subject, the general concensus seems to be that we should start a new project, [collections15], rather than trying to shoehorn generics into [collections], which is required to maintain backwards compatibility with older versions of the JDK. It is clear, however, that there would be great similarities between the codebases of [collections] and [collections15]. Should we aim to ultimately develop the two in parallel, just try to keep the public APIs similar (they can't be kept identical - see MultiMap below) or allow the two projects to drift off in different directions over time? Furthermore, if the codebases do remain very similar, what's the best way to handle the tracking of bugs which affect both projects?
MultiMap
Most of the interfaces defined in [collections] can be trivially adapted to generics. The only exception is MultiMap. MultiMap violates the Map interface, as after calling put(key, value) on a map, get(key) doesn't return value, but a Collection containing the value. This works fine for the current Map interface, since Object get(Object key) can legitimately return a Collection instance rather than the original Object. It's not possible to continue with the current behaviour with generics. The map interface's get method now has the signature V get(K key), and so we simply cannot return a collection of type Collection<V>. It seems to me that we'll have to settle for a Collection<V> getValues(K key) method, but the question is, what happens to V get(K key)? I can think of three obvious behaviours:
1. V get(K key) should throw an UnsupportedOperationException.
2. V get(K key) returns one of the values that is mapped to the specified key. The choice of exactly which value is left to the implementation.
3. V get(K key) should return the latest value to be mapped against the specified key.
I'm not keen on either of the first two options, as they both violate the Map interface. The first because Map doesn't specify that V get(K key) is an optional operation, and the second because it doesn't guarantee that get(key) will return value after put(key, value) has been called. The third option looks good to me, but it would pretty much require that the values for each key must be stored in a list, so that V get(K key) always returns the latest value mapped, even taking into account any calls to V remove(K key, Object item).
Also, on a more practical level, since I'm new to the Apache development scene, I obviously can't setup a project or commit any code. What's the best way to get things started?
Chris
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