Recently I was solving an interesting bug that came down to comparing two Double variables with equals method. It looks innocent, what can be wrong with something like firstDouble.equals(secondDouble)?

The problem here is with how doubles are stored. To fit them into 64bytes (usually) they are rounded.

See the example below:

Double firstDouble = 0d;
for (int i = 1; i <= 42; i++) {
 firstDouble += 0.1;
}
Double secondDouble = 0.1 * 42;
System.out.println(firstDouble); // 4.200000000000001
System.out.println(secondDouble); // 4.2
System.out.println(firstDouble.equals(secondDouble)); // false

This inaccuracy is caused by rounding errors.

We need to use a different approach to compare those doubles.

#java #double comparison in java #double comparison #comparisons #double