The key point to realise is that the value of a floating point number can be worked out in two different ways, that aren't in general equal.
- There's the value that the bits in the floating point number give the exact binary representation of.
- There's the "decimal display value" of a floating point number, which is the number with the least decimal places that is closer to that floating point number than any other number.
To understand the difference, consider the number whose exponent is 10001011 and whose significand is 1.11111010001111111111111. This is the exact binary representation of 8099.99951171875. But the decimal value 8099.9995 has fewer decimal places, and is closer to this floating point number than to any other floating point number. Therefore, 8099.9995 is the value that will be displayed when you print out that number.
Note that this particular floating point number is the next lowest one after 8100.
Now consider 8099.99975. It's slightly closer to 8099.99951171875 than it is to 8100. Therefore, to represent it in single precision floating point, Java will pick the floating point number which is the exact binary representation of 8099.99951171875. If you try to print it, you'll see 8099.9995.
Lastly, when you do 8099.9995 + 0.00025 in single precision floating point, the numbers involved are the exact binary representations of 8099.99951171875 and 0.0002499999827705323696136474609375. But because the latter is slightly more than 1/2^12, the result of addition will be closer to 8100 than to 8099.99951171875, and so it will be rounded up, not down at the end, making it 8100.