String replace method is not working [closed]

Answer 1

 str.replace(chr, chr1);

Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar. replace
But you are not storing that value. So you can use following code

str = str.replace(chr, chr1);

Answer 2

Please make sure for String you have to assign the value

 str = str.replace(chr, chr1);

Answer 3

Your problem is that you are not returning the result from str.replace. Strings are immutable

Answer 4

why you don't simply use:

String str = "THIS IS my TEST strinG";
String my_string = str.substring(0,1).toUpperCase() + str.substring(1).toLowercase();