Difference between java 9 OR and java 8 orElseGet

Question

Java9 has added .or method to Optional. How it is different for existing .orElseGet ?

checkUserInMemory(userId).or(() -> checkUserInDB(userId));

Answer 1

Primarily the return type of both varies.

Optional.orElseGet

• The call to orElseGet returns the object T itself.

• throws NullPointerException if no value is present and the supplying function is null

• Use case: To fetch the value deterministically based on the supplied function.

Optional.or

• The Optional.or returns an Optional describing the value, otherwise returns an Optional produced by the supplying function

• throws NullPointerException if the supplying function is null or if the supplying function produces a null result.

• Use case: To chain a series of Optionals based on the result of the supplied function. Sample - How do I concisely write a || b where a and b are Optional values?