Effective Java

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ITEM 44: FAVOR THE USE OF STANDARD FUNCTIONAL INTERFACES
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There are nine additional variants of the 
Function
interface, for use when the
result type is primitive. The source and result types always differ, because a func-
tion from a type to itself is a 
UnaryOperator
. If both the source and result types are
primitive, prefix 
Function
with 
Src
To
Result
, for example 
LongToIntFunction
(six variants). If the source is a primitive and the result is an object reference, prefix
Function
with
ToObj
, for example 
DoubleToObjFunction
(three variants). 
There are two-argument versions of the three basic functional interfaces for
which it makes sense to have them: 
BiPredicate
, 
BiFunction
,
and 
BiConsumer
. There are also 
BiFunction
variants returning the three
relevant primitive types: 
ToIntBiFunction
, 
ToLongBiFunction
, and
ToDoubleBiFunction
. There are two-argument variants of 
Consumer
that
take one object reference and one primitive type: 
ObjDoubleConsumer
,
ObjIntConsumer
, and 
ObjLongConsumer
. In total, there are nine two-
argument versions of the basic interfaces.
Finally, there is the 
BooleanSupplier
interface, a variant of 
Supplier
that
returns 
boolean
values. This is the only explicit mention of the 
boolean
type in
any of the standard functional interface names, but 
boolean
return values are
supported via 
Predicate
and its four variant forms. The 
BooleanSupplier
interface and the forty-two interfaces described in the previous paragraphs
account for all forty-three standard functional interfaces. Admittedly, this is a lot
to swallow, and not terribly orthogonal. On the other hand, the bulk of the
functional interfaces that you’ll need have been written for you and their names
are regular enough that you shouldn’t have too much trouble coming up with one
when you need it.
Most of the standard functional interfaces exist only to provide support for
primitive types.