Identity-sensitive operations include reference equality (==), identity hash-based, or synchronization.

The Optional class is a value-based class such as LocalDateTime, therefore identity-sensitive operations should be avoided.

For example, let's test the equality of two Optional classes via ==:

Book book = new Book();
Optional<Book> op1 = Optional.of(book);
Optional<Book> op2 = Optional.of(book);

// Avoid
// op1 == op2 => false, expected true
if (op1 == op2) {
  System.out.println("op1 is equal with op2, (via ==)");
} else {
  System.out.println("op1 is not equal with op2, (via ==)");
}

This will give the following output:

op1 is not equal with op2, (via ==)

Since op1 and op2 are not references to the same object, they are not equal so don't conform with the== implementation.

To compare the values, we need to rely on equals(), as follows:

// Prefer
if (op1.equals(op2)) {
  System.out.println("op1 is equal with op2, (via equals())");
} else {
  System.out.println("op1 is not equal with op2, (via equals())");
}

This will give the following output:

op1 is equal with op2, (via equals())

In the context of the identity-sensitive operations, never do something like this (think that Optional is a value-based class and such classes should not be used for locking—for more details, see https://rules.sonarsource.com/java/tag/java8/RSPEC-3436):

Optional<Book> book = Optional.of(new Book());
synchronized(book) {
  ...
}