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javamemorydoubleheap-memory

Float or double type in terms of storage and memory

I am parsing data where precision is not my main concern. I often get java.lang.OutOfMemoryError even if I use maximum Java heap size. So my main concern here is memory usage, and java heap space. Should I use double or float data type?

Solution

  • If your memory usage is related to a huge amount (many millions) of floating-point numbers (which can be verified with a decent memory profiler), then you're most probably storing them in some data structures like arrays or lists.

    Recommendations (I guess, you are already following most of them...):

    • Prefer float over double if number range and precision are sufficient, as that consumes only half the size.
    • Do not use the java.lang.Float or java.lang.Double classes for storage, as they hav a considerable memory overhead compared to the naked scalar values.
    • Be sure to use arrays, not collections like java.util.List, as they store boxed java.lang.Float instances instead of the naked numbers.

    But above that, have a decent memory profiler show you which instances occupy most of your memory. Maybe there are other memory consumers besides the float/double data.

    EDIT:

    The OP's recent comment "I consistently get OOM exceptions because I use a great number of ArrayLists with numbers" makes it clear. ArrayList<Float> wastes a lot of memory when compared to float[] (Stephen C gave detailed numbers in his answer), but gives the benefit of dynamic resizing.

    So, I see the following possibilities:

    • If you can tell the array size from the beginning, then immediately use float[] arrays.
    • If you need the dynamic size while initializing instances, use ArrayList<Float> while building one object (when size still increases), and then copy the contents to a float[] array for long-term storage. Then the wasteful ArrayLists exist only for a limited timespan.
    • If you need dynamic sizes over the whole lifespan of your data, create your own FloatArrayList class based on a float[] array, resembling the ArrayList<Float> as far as your code needs it (that can range from a very shallow implementation up to a full-featured List, maybe based on AbstractList).