Immutability of Strings means you cannot modify any String object.

Notice the above example for the method toLowerCase. This method returns a new String, which is the lower case version of the object that invoked the method.

String greeting = "Hi World!";
greeting.toLowerCase();
System.out.println(greeting);

Run Output:

Hi World!

The object greeting did not change. To change the value of greeting, you need to assign the return value of the method to the object greeting.

greeting = greeting.toLowerCase();
System.out.println(greeting);

Run Output:

hi world!