Why adding these two double does not give correct answer? [duplicate]

Answer 1

That is because some numbers -- such as 0.1 -- cannot be represented exactly in binary floating-point.

Consider reading the following article:

• What Every Computer Scientist Should Know About Floating-Point Arithmetic

Answer 2

You are seeing a rounding error. See How to resolve a Java Rounding Double issue

To resolve it you can change to BigDecimal instead of double as mentioned in the accepted answer to the linked question.