Question

Problem: Double minus another double is returning unaccurate results.

Usual solution: Use big decimal to perform calculations.

I can’t use this because it’s in the middle of a game loop, can only set big decimal with a value when it’s created with new, within a game loop I’m running out of memory.

What solution?

Here’s the code:

public void UpdateStatsBar(double WorldPopulation, double WorldDead)
{
    Number = mOriginalPopulation - WorldPopulation;
}

The kind of numbers I’m using go into the billions and go tens of digits deep (averages for stats) example: 1432,323,234.234215232425322342

Answer 1

Assuming you can’t have a decimal number of people live or dead, I would use long.

Can you provide an example of the numbers you are using which are not producing accurate results?

BTW: If you use whole numbers less than nine thousand trillion, even double will produce accurate results.

BTW2: If you create new BigDecimal like mad but don’t retain them you will hurt performance but you won’t run out of memory. If you are running out of memory its not due to use BigDecimal. I suggest you use a memory profiler to find the true cause of the problem.

An example

BigDecimal bd = BigDecimal.ZERO;
long l = 0;
double d = 0;
while (true) {
    for (int i = 0; i < 100000000; i++) {
        bd = bd.add(BigDecimal.ONE);
        l++;
        d++;
    }
    System.out.println("counting bd: " + bd + " l: " + l + " d:" + d);
}

prints number counting with BigDecimal, long and double. The format for double is different but the value is the same.

counting bd: 100000000 l: 100000000 d:1.0E8
counting bd: 200000000 l: 200000000 d:2.0E8
counting bd: 300000000 l: 300000000 d:3.0E8
... deleted ...
counting bd: 1900000000 l: 1900000000 d:1.9E9
counting bd: 2000000000 l: 2000000000 d:2.0E9
counting bd: 2100000000 l: 2100000000 d:2.1E9
counting bd: 2200000000 l: 2200000000 d:2.2E9
... deleted ...

Even with -mx2m limiting the heap to 2 MB it doesn’t run out of memory.