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    <blockquote> <p>Am I wrong to assume a TreeMap's array's initial size should be able to be set? </p> </blockquote> <p>Yes. A <code>TreeMap</code> doesn't have an array. A <code>TreeMap</code> uses binary nodes with 2 children.</p> <p>If you are suggesting that the number of children in a tree node should be a parameter, then you need to figure out how that impacts on search time. And I think that it turns the search time from <code>O(log2N)</code> to <code>O(log2M * log2(N/M))</code> where <code>N</code> is the number elements and <code>M</code> is the average number of node children. (And I'm making some optimistic assumptions ...) That's not a "win".</p> <blockquote> <p>Is there a different reason that it is so slow?</p> </blockquote> <p>Yes. The reason that a (large) <code>TreeMap</code> is slow relative to a (large) <code>HashMap</code> under optimal circumstances is that lookup using a balanced binary tree requires looking at <code>log2N</code> tree nodes. By constrast, in an optimal <code>HashMap</code> (good load factor and no collision hotspots) a lookup involves 1 hashcode calculation and looking at <code>O(1)</code> hashchain nodes.</p> <p>Notes:</p> <ol> <li><code>TreeMap</code> uses a binary tree organization that gives balanced trees, so <code>O(log2N)</code> is the worst case lookup time.</li> <li><code>HashMap</code> performance depends on the collision rate of the hash function and key space. In the worst case where all keys end up on the same hash chain, a <code>HashMap</code> has <code>O(N)</code> lookup.</li> </ol>
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