44. Use a standard functional interface

By using the ones provided as standard, you can increase the ease of use and benefit from the various methods already built in.
There are 43 functional interfaces included in java.util.function.
The core is ```UnaryOperator ``, ``BinaryOperator , Predicate , Function , Six of Consumer <T> , Supplier <T> ``. (6)
Each of the above six has a functional interface for handling primitive types of int, `` `long, and double```. (18)
There are 6 Functions with SrcToResultFunction (LongToIntFunction```, etc.). There are three SrcToObjFunctions (such as `DoubleToObjFunction```). (9)
There are nine that take two arguments. (9)
There is something called BooleanSupplier. (1)
Do not use objects that box primitive types in a functional interface that does not support primitive types. Performance deteriorates.
You need to write a functional interface yourself in the following cases.
When you need something that is not provided as standard, such as a functional interface that takes three arguments.
If there are features such as being commonly used and benefiting from the description, having a strongly relevant contract (** not sure **), and benefiting from the default method. (``` Comparator `` `is given as an example)
When writing a functional interface, be sure to add @FunctionalInterface. With this, if you write it incorrectly, it will cause a compile error.
Methods that take a functional interface as an argument should not be overridden. (Item52)

Item 44: Favor the use of standard functional interfaces