concept functional interface in category software development

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Notice the close similarity between this imaginary collector and the Attribute interface introduced earlier for abstracting the water level value of containers. However, the actual Collector interface introduces one more level of indirection by having each method return an object that performs the corresponding function. This is in line with the rest of the stream framework and with the functional programming style by which it’s inspired. The return types for all four methods are functional interfaces, that is, interfaces that each have a single abstract method. Table 9.2 outlines the characteristics of these four interfaces.

Table 9.2. Functional interfaces mutable collectors use. They’re among the more than 40 functional interfaces in the java.util.function package. (view table figure)

Interface	Type of abstract method	Role
Supplier<S>	void → S	Provides the initial summary
BiConsumer<S,V>	(S, V) → void	Updates a summary with a value
BinaryOperator<S>	(S, S) → S	Merges two summaries
Function<S,R>	S → R	Converts a summary into a result