What is the difference between (a=b)and (a==b) in java?

Answer 1

a=b means you are TELLING system that a is equal to b
a==b means you are ASKING the system wether a is equal to b or not?

Answer 2

a=b is called assignment where the value of b is assigned to a.

a==b is a condition checking or you can call it a comparison operator which checks whether the value of a and b are equal or not.

Edit: the == is also used in Java with reference variables to check whether both of them are pointing to the same reference or not.