Revert "Use Java collections"

[eed3si9n]

This reverts commit 52959c9.
Fixes #538

The use of java.util.HashMap causes java.util.ConcurrentModificationException on JDK 9 and JDK 10. This is likely because processType recursively end up calling processType while modifying typeCache.

[retronym]

Sorry about this, I didn't consider the re-entrancy of these caching methods.

s.c.m.HashMap.getOrElseUpdate accomodates such use cases:

  override def getOrElseUpdate(key: K, defaultValue: => V): V = {
    val hash = table.elemHashCode(key)
    val i = table.index(hash)
    val firstEntry = table.findEntry0(key, i)
    if (firstEntry != null) firstEntry.value
    else {
      val table0 = table.table
      val default = defaultValue
      // Avoid recomputing index if the `defaultValue()` hasn't triggered
      // a table resize.
      val newEntryIndex = if (table0 eq table.table) i else table.index(hash)
      val e = table.createNewEntry(key, default)
      // Repeat search
      // because evaluation of `default` can bring entry with `key`
      val secondEntry = table.findEntry0(key, newEntryIndex)
      if (secondEntry == null) table.addEntry0(e, newEntryIndex)
      else secondEntry.value = default
      default
    }
  }

Whereas the Java collections consider it a programming error and fail fast.

[retronym]

I believe @jvican is going to try reworking Zinc to compute the hashes directly without first creating this structure, so reverting is and waiting for that work seems like a good course of action.

[eed3si9n]

I think this mostly means that we need to add automated testing on JDK 10 sooner rather than later.

[jvican]

I believe @jvican is going to try reworking Zinc to compute the hashes directly without first creating this structure, so reverting is and waiting for that work seems like a good course of action.

This is indeed correct, I'm working on it.

[eed3si9n]

Thanks for the quick review guys.