String replaceAll not working with string object [duplicate]

Question

I have used below string operation but not able to understand why 1st case is not working.

Case - 1 :

String myString = new String("old String");
System.out.println(myString);
myString.replaceAll( "old", "new" );
System.out.println( myString );

//Output
old String
old String

Case - 2 :

String myString = new String("old String" );
System.out.println(myString);
myString = new String("new String");
System.out.println(myString);

//Output
old String
new String

Why case - 1 is working but case - 2 is not working?

Answer 1

String.replaceAll() returns a new string with the specified replacement. You need to do:

myString = myString.replaceAll( "old", "new" );

Answer 2

replaceAll method returns a String object. You must assign this back to myString or to another variable as strings are immutable.