// String.replace produces a new String. It does not modify the original.
// Since String is immutable, no method can modify the original.

String aa = "peal";
String bb = aa.replace( 'a', 'e' );

out.println ( aa + " " + bb );
// Prints "peal peel" not "peel peel"
// Newbies often expect String aa to be changed too.