This is not immutability, final only prevents re-assignment, not mutation: final ArrayList<String> foo = new ArrayList<>(); foo.add("Foo"); // Compiles

This is not immutability, final only prevents re-assignment, not mutation: final ArrayList<String> foo = new ArrayList<>(); foo.add("Foo"); // Compiles and runs - this is mutation foo = new ArrayList<>(); // Compiler error - this is re-assignment In your example, both Strings are immutable, since Java Strings are always immutable.