Java String.indexOf and empty Strings

Question

I\'m curious why the String.indexOf is returning a 0 (instead of -1) when asking for the index of an empty string within a string.

The Javadocs only say this method

Answer 1

Using an algebraic approach, "" is the neutral element of string concatenation: x + "" == x and "" + x == x (although + is non commutative here).

Then it must also be:

x.indexOf ( y ) == i and i != -1 
<==> x.substring ( 0, i ) + y + x.substring ( i + y.length () ) == x

when y = "", this holds if i == 0 and x.substring ( 0, 0 ) == "". I didn't design Java, but I guess mathematicians participated in it...