[LUCENE-1369] Eliminate unnecessary uses of Hashtable and Vector - ASF JIRA

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Lucene uses Vector, Hashtable and Enumeration when it doesn't need to. Changing to ArrayList and HashMap may provide better performance.

There are a few places Vector shows up in the API. IMHO, List should have been used for parameters and return values.

There are a few distinct usages of these classes:

internal but with ArrayList or HashMap would do as well. These can simply be replaced.
internal and synchronization is required. Either leave as is or use a collections synchronization wrapper.
As a parameter to a method where List or Map would do as well. For contrib, just replace. For core, deprecate current and add new method signature.
Generated by JavaCC. (All *.jj files.) Nothing to be done here.
As a base class. Not sure what to do here. (Only applies to SegmentInfos extends Vector, but it is not used in a safe manner in all places. Perhaps, implements List would be better.)
As a return value from a package protected method, but synchronization is not used. Change return type.
As a return value to a final method. Change to List or Map.

In using a Vector the following iteration pattern is frequently used.
for (int i = 0; i < v.size(); i++) {
Object o = v.elementAt;
}

This is an indication that synchronization is unimportant. The list could change during iteration.