Strings are immutable then how the replace() works?

Question

I was trying to replace the first letter/char of a string by it's last one and last one by it's first one. E.g. abcd => dbca. Strings are immutable in Java then how can we explain the behavior of this program? Please have a look at final output. str1 has no char 'a' but in final output it appears unexpectedly.. how? //The argument of frontBack() is String "abcd".

 public static void frontBack(String str) {
   String first= ""+str.charAt(0);
   System.out.println("first char is "+first);
   String last = ""+str.charAt(str.length()-1);
   System.out.println("last char is "+last);
   String str1;
   str1 = str.replace(""+str.charAt(0),last);
   System.out.println("String str1 is => "+str1);
   String str2 ;
   str2 = str1.replace(""+str1.charAt(str1.length()-1),first);
   System.out.println("String str2 is derived from str1(dbcd) which has no 'a' but o/p is =>  "+str2);    
  }
 /* Have a look at output:
                        first char is a
                        last char is d
                        String str1 is => dbcd
                        String str2 is derived from str1 i.e. "dbcd" which has no 'a' in it but o/p is =>  abca*/

Answer 1

Strings are immutable in Java then how can we explain the behavior of this program?

str1 = str.replace(""+str.charAt(0),last);

This method takes a String which is immutable and creates a new String which is immutable. Immutable doesn't mean the String cannot be created.

Note: if you want to manipulate some text you can use a StringBuilder which is mutable. You can create this from a String and you can create a new String from it. This is often, but not always used, sometimes a char[] is used directly for performance reasons.

the new string which is created has no char 'a' but in final output it appears unexpectedly.

This is where using a your debugger would help. You would see that

String first= ""+str.charAt(0); // first = "a"

and later when you do

str2 = str1.replace(""+str1.charAt(str1.length()-1),first);

this is the same as

str2 = str1.replace("d", "a");

so it should be no surprise that the ds are replaced by as

I am using replace() not replaceAll()

From the Javadoc for String.replace(CharSequence, CharSequence)

Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence.