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copied!<p>A object or array reference occupies one 32 bit word (4 bytes) on a 32 bit JVM or Davlik VM. A <code>null</code> takes the same space as a reference. (It has to, because a null has to fit in a reference-typed slot; i.e. instance field, local variable, etc.)</p> <p>On the other hand, an object occupies a minimum of 2 32 bit words (8 bytes), and an array occupies a minimum of 3 32 bit words (12 bytes). The actual size depends on the number and kinds of fields for an object, and on the number and kind of elements for an array.</p> <hr> <p>For a 64 bit JVM, the size of a reference is 64 bits, unless you have configured the JVM to use compressed pointers:</p> <blockquote> <p>-XX:+UseCompressedOops Enables the use of compressed pointers (object references represented as 32 bit offsets instead of 64-bit pointers) for optimized 64-bit performance with Java heap sizes less than 32gb.</p> </blockquote> <hr> <p>This is the nub of your question, I think.</p> <blockquote> <p>Before determining the size of the hash table, I wanted to know how much memory would it consume in order not to exagerate.</p> </blockquote> <p>If you allocate a <code>HashMap</code> or <code>Hashtable</code> with a large initial size, the majority of the space will be occupied by the hash array. This is an array of references, so the size will be <code>3 + initialSize</code> 32 bit words. It is unlikely that this will be significant ... unless you get your size estimate drastically wrong.</p> <p>However, I think you are probably worrying unnecessarily about performance. If you are storing objects in a default allocated <code>HashMap</code> or <code>Hashtable</code>, the class will automatically resize the hash table as it gets larger. So, provided that your objects have a decent hash function (not too slow, not hashing everything to a small number of values) the hash table should not be a direct CPU performance concern.</p>

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