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POAllocated Memory for Hashtable.put()
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copied!<p>So I was reading Peter Norvig's IAQ (infrequently asked questions - <a href="http://www.norvig.com/java-iaq.html" rel="nofollow">link</a>) and stumbled upon this:</p> <blockquote> <p>You might be surprised to find that an Object takes 16 bytes, or 4 words, in the Sun JDK VM. This breaks down as follows: There is a two-word header, where one word is a pointer to the object's class, and the other points to the instance variables. Even though Object has no instance variables, Java still allocates one word for the variables. Finally, there is a "handle", which is another pointer to the two-word header. Sun says that this extra level of indirection makes garbage collection simpler. (There have been high performance Lisp and Smalltalk garbage collectors that do not use the extra level for at least 15 years. I have heard but have not confirmed that the Microsoft JVM does not have the extra level of indirection.)</p> <p>An empty new String() takes 40 bytes, or 10 words: 3 words of pointer overhead, 3 words for the instance variables (the start index, end index, and character array), and 4 words for the empty char array. Creating a substring of an existing string takes "only" 6 words, because the char array is shared. Putting an Integer key and Integer value into a Hashtable takes 64 bytes (in addition to the four bytes that were pre-allocated in the Hashtable array): I'll let you work out why.</p> </blockquote> <p>So well I obviously tried, but I can't figure it out. In the following I only count words: A Hashtable put creates one Hashtable$Entry: 3 (overhead) + 4 variables (3 references which I assume are 1 word + 1 int). I further assume that he means that the Integers are newly allocated (so not cached by the Integer class or already exist) which comes to 2* (3 [overhead] + 1 [1 int value]).</p> <p>So in the end we end up with.. 15 words or 60bytes. So what I first thought was that the Entry as a inner class needs a reference to its outer object, but alas it's static so that doesn't make much sense (sure we have to store a pointer to the parent class, but I'd think that information is stored in the class header by the VM).</p> <p>Just idle curiosity and I'm well aware that all this depends to a good bit on the actual JVM implementation (and on a 64bit version the results would be different), but still I don't like questions I can't answer :)</p> <p>Edit: Just to make this a bit clearer: While I'm well aware that more compact structures can get us some performance benefits, I agree that in general worrying about a few bytes here or there is a waste of time. I surely wouldn't stop using a Hashtable just because of a few bytes overhead here or there just like I wouldn't use plain char arrays instead of Strings (or start using C). This is purely of academic interest to learn a bit more about the insides of Java/the JVM :)</p>

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