Effective Java

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ITEM 55: RETURN OPTIONALS JUDICIOUSLY
253
An 
Optional
is an object that has to be allocated and initialized, and reading the
value out of the optional requires an extra indirection. This makes optionals
inappropriate for use in some performance-critical situations. Whether a particular
method falls into this category can only be determined by careful measurement
(Item 67).
Returning an optional that contains a boxed primitive type is prohibitively
expensive compared to returning a primitive type because the optional has two
levels of boxing instead of zero. Therefore, the library designers saw fit to provide
analogues of 
Optional
for the primitive types 
int
, 
long
, and 
double
. These
optional types are 
OptionalInt
, 
OptionalLong
, and 
OptionalDouble
. They
contain most, but not all, of the methods on 
Optional
. Therefore, 
you should
never return an optional of a boxed primitive type,
with the possible exception
of the “minor primitive types,” 
Boolean
, 
Byte
, 
Character
, 
Short
, and 
Float
.
Thus far, we have discussed returning optionals and processing them after they
are returned. We have not discussed other possible uses, and that is because most
other uses of optionals are suspect. For example, you should never use optionals as
map values. If you do, you have two ways of expressing a key’s logical absence
from the map: either the key can be absent from the map, or it can be present and
map to an empty optional. This represents needless complexity with great
potential for confusion and errors. More generally,