Java immutable strings confusion

Question

If Strings are immutable in Java, then how can we write as:

String s = new String();
s = s + \"abc\";

Answer 1

Immutable Classes are those whose methods can change their fields, for example:

Foo f = new Foo("a");
f.setField("b"); // Now, you are changing the field of class Foo

but in immutable classes, e.g. String, you cannot change the object once you create it, but of course, you can reassign the reference to another object. For example:

String s = "Hello";
s.substring(0,1); // s is still "Hello"
s = s.substring(0,1); // s is now referring to another object whose value is "H"