The title pretty much sums it up. I have seen people construct an instance of Random globally, and use it in all of their code, and I have also seen people that construct an instance everytime they want to use Random.
My question is: When, if ever, should I construct a new instance of Random for generating random numbers?
Math.random() stores a Random instance in a RandomNumberGeneratorHolder, and calls it every time Math.random() is called.
My view: I should use a global Random() instance, because:
- Save the time of object creation. The no-arg constructor of Random() calls seedUniquifier(), which basically loops forever until it finds a new AtomicLong(), and raises it to the power of System.nanoTime(). Pretty expensive.
- I am currently using random numbers for a custom hashCode(). I overrided equals(), and now I am doing the same for hashCode(). The class that I am doing this on will be mostly used to store data in Collections, which heavily abuses hashCode(). Seeing the no-arg Random() constructor will take more time than a couple of multiplications that I am using to generate my hashcode, it will more than double the execution time. Not good.
I can't think of any more reasons, but if I should use a global Random instance, I can imagine java developers implementing Random with at instance field, in addition to the constructor for special cases. That tells me that I am wrong. Should I use a global Random instance?