String is Immutable

String is the most used immutable class in Java. If new Java developers had to deal with a mutable String they may get very frustrated and overwhelmed by the complexity of the object model and give up on Java not long after starting.

String s = "nothing special";
s.toUppercase();

This does not modify the variable s at all. Instead, a new String object is created with all the characters of s changed to upper case. Since the new object is not assigned to anything, it is simply loses all references (as the toUpperCase method has completed) and is eventually garbage collected. On the other hand, you could write it like this:

String s = "nothing special";
s = s.toUppercase();

In this example, s is now in all upper case letters, but it is not the same object as the one that was instantiated in the first line. The new String object that is returned from toUpperCase becomes assigned to s, and the original object loses its last reference and gets garbage collected.

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