I am confused. If I calculate
System.out.println(0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1);
Then I get a result of 0.9999999999999999. But if I calculate
Double sum = DoubleStream.builder().add(0.1).add(0.1).add(0.1).add(0.1).add(0.1).add(0.1).add(0.1).add(0.1).add(0.1).add(0.1).build().sum();
System.out.println(sum);
Then I get a result of 1.0.
Why is there a difference?
The Javadoc of double java.util.stream.DoubleStream.sum() answers your question:
In particular, this method may be implemented using compensated summation or other technique to reduce the error bound in the numerical sum compared to a simple summation of double values.
In other words, the implementation of sum() doesn't have to use simple summation of double values (which can have accuracy issues, as you noticed in your first snippet), and therefore may return a more accurate result.
EDIT: Note that even though using DoubleStream's sum() appears to give a more accurate result, this is an implementation detail, so it's not guaranteed by the Javadoc. Besides, simple double addition is more efficient, since it doesn't have the overhead of constructing a DoubleStream. You have to decide if you prefer potential better accuracy or performance.