For example

g(s.charAt(i));
g(s.charAt(i));
            

is always equivalent (regardless of g) to:

final char c = s.charAt(i);
g(c);
g(c);
            

Therefore, charAt is referentially transparent.

Bad news; new is a side-effect. Ever wanted to hide that constructor for your immutable type?

More bad news; Java makes it extremely difficult to be rid of impurity.^[6]