How to know if this Java's string.replaceAll is executed or not?

Question

if i have some string e.g:

String s = "This is a string";

And now i perform this action:

s = s.replaceAll("This","What");

It works, with s = "What is a string"

fine, but now if the expression was not a match:

s = s.replaceAll("junk","What");

s remains what it was before, i.e s = "This is a string";

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I want to know, without equating these strings, is there a way to know if s.replaceAll really performed some action or not??

Answer 1

would it be ok if you try

if(s.contains("junk")) 
  s.replaceAll("junk", "What"); 
else 
  //you know it hasn't executed!