This method passes a constant literal String of length 1 as a parameter to a method, that exposes a similar method that takes a char. It is simpler and more expedient to handle one character, rather than a String.

Instead of making calls like: 
String myString = ... 
if (myString.indexOf("e") != -1) {
int i = myString.lastIndexOf("e");
System.out.println(myString + ":" + i); //the Java compiler will use a StringBuilder internally here [builder.append(":")]
...
return myString.replace("m","z");
}
Replace the single letter Strings with their char equivalents like so:
String myString = ... 
if (myString.indexOf(‘e‘) != -1) {
int i = myString.lastIndexOf(‘e‘);
System.out.println(myString + ‘:‘ + i); //the Java compiler will use a StringBuilder internally here [builder.append(‘:‘)]
...
return myString.replace(‘m‘,‘z‘);
}

Performance - Method passes constant String of length 1 to character overridden method