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Java Forum / General / July 2006size occupied by a byte[]
Hi everybody, I seem to remember that there where some JVM that would "waste" space when allocating a byte[]. But I'm too sure about that anymore, so I've got a few questions... Does a boolean[] waste space? And is this defined in some specification? Does an byte[] waste space? And is this defined in some specification? How can I find this by myself by "hacking it"? (was thinking of running that in a virtual machine and examine the memory by placing some pattern in the byte[] but this seems very stupid). :) fab > Hi everybody, > [quoted text clipped - 4 lines] > Does a boolean[] waste space? And is this defined in some > specification? You may be thinking of the JVM specification. There are no special bytecode instructions for manipulating booleans. The instructions for reading/writing from/to boolean and byte arrays are the same. And in early Sun VMs a boolean array did indeed use 8 bits per element. This is however not mandated by the spec, a VM could very well use packed arrays, but I /think/ most VMs will go with bytesized booleans. The downside of packed arrays is that while you save some space, array access becomes quite expensive. If you are worried about the extra space use of boolean arrays, try using java.util.BitSet instead which is almost guaranteed to use a packed int/long implementation. It is not likely that a JVM would waste any space when allocating a byte array, since bytes are readily accesible on just about any processor, as opposed to single bits which require extra code to access. Regards, Daniel Sjöblom >> Hi everybody, >> [quoted text clipped - 9 lines] > processor, as opposed to single bits which require extra code to > access. Well, any object in Java has about 8 bytes of overhead, at least. Do a google search for "java memory object footprint," or similar, to get some ideas. Primitives are NOT objects and don't have this issue. But a single boolean could still take as much as 8 bytes, depending on the JVM (esp. on 64 bit systems). The best way to determine memory overhead is to get some of the test programs that the google search above will give you, and try them out. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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