Cheatsheet: Java Functional Interfaces

Want a quick breakdown of Java's functional interfaces? Here's a great compilation of predicates, operators, and functions, as well as their uses.

Tony Tapper

Mar. 09, 18 · Presentation

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Looking at the alphabetical list of 43 functional interfaces in java.util.function is a bit overwhelming. Trying to learn and remember them all is going to be a challenge!

Luckily Joshua Bloch came to the rescue in his 3rd edition of Effective Java.

Item 44: Favor the use of standard functional interfaces.

I really recommend you read this!

It's unlikely that you will need to write your own, except in the cases that Prof. Bloch describes:

Will be often used and a descriptive name is helpful
Has a strong contract associated with it
Would benefit from custom default methods

My goal is to produce a cheat sheet based on his work for easy reference (or your next interview!).

Remember (covers 39 of the 43 functional interfaces)

Predicate, Unary Operator, BinaryOperator, Function, Supplier, and Consumer operate on reference types. Each has 3 variants, which operate on double, int, or long respectively
BiPredicate, BiFunction, BiConsumer accept two reference types as arguments
Function has six variants, which can accept a primitive and return a different primitive, and six (inc. BiFunction) that accept reference types and return a primitive

Summary

There are five basic functional interfaces that, by default, operate on a single reference type: Predicate, Unary Operator, Function, Supplier, Consumer and one which operates on two reference types — BinaryOperator
Each of the six basic types has three variants that accept a primitive: double, int, or long
Variants of the three basic types accept two arguments: BiPredicate, BiFunction, BiConsumer
Function has 6 variants that convert one of the primitives (double, int, long) to a different primitive.
Function and BiFunction each have 3 variants that take a reference type and return a primitive double, int, or long
Supplier has a variant that returns a boolean
BiConsumer has three variants that accept a reference type and a primitive: double, int, or long

Basic Types

PREDICATE	takes one (or two) argument(s) and returns a boolean (5 variants)
UNARY OPERATOR	result and the single argument types are the same (4 variants)
BINARY OPERATOR	result and both argument types are the same (4 variants)
FUNCTION	result and one (or two) argument(s) types are different (17 variants)
SUPPLIER	takes no arguments, returns a value ( 5 variants )
CONSUMER	takes one (or two) arguments and returns no value (8 variants)

Notation

If the interface accepts primitive arguments: prefixed Double, Int, Long, e.g. DoubleConsumer

If the interface produces a primitive result: prefixed ToDouble, ToInt, ToLong, e.g. ToDoubleFunction

If the interface both accepts and produces a primitive: prefixes combined e.g. IntToDoubleFunction

BiConsumer variants that accept an object type and a primitive are prefixed Obj + the primitive, e.g. ObjDoubleConsumer

Package Summary

https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html

Predicate

Predicate<T>	Represents a predicate (boolean-valued function) of one argument (reference type)
DoublePredicate	Accepts one double-valued argument
IntPredicate	Accepts one int-valued argument.
LongPredicate	Accepts one long-valued argument
BiPredicate<T,U>	Accepts two arguments (reference types)

Unary Operator

UnaryOperator<T>	Represents an operation on a single operand that produces a result of the same type as its operand (reference type)
DoubleUnaryOperator	Accepts single double-valued operand and produces a double-valued result
IntUnaryOperator	Accepts a single int-valued operand and produces an int-valued result
LongUnaryOperator	Accepts a single long-valued operand and produces a long-valued result

Binary Operator

BinaryOperator<T>	Represents an operation upon two operands of the same type, producing a result of the same type as the operands (reference type)
DoubleBinaryOperator	Accepts two double-valued operands and produces a double-valued result
IntBinaryOperator	Accepts two int-valued operands and produces an int-valued result
LongBinaryOperator	Accepts two long-valued operands and produces a long-valued result.

Function

Function<T,R>	Represents a function that accepts one argument and produces a result (reference type)
DoubleFunction<R>	Accepts a double-valued argument and produces a result
IntFunction<R>	Accepts an int-valued argument and produces a result
LongFunction<R>	Accepts a long-valued argument and produces a result
DoubleToIntFunction	Accepts a double-valued argument and produces an int-valued result
DoubleToLongFunction	Accepts a double-valued argument and produces a long-valued result
IntToDoubleFunction	Accepts an int-valued argument and produces a double-valued result
IntToLongFunction	Accepts an int-valued argument and produces a long-valued result
LongToIntFunction	Accepts a long-valued argument and produces an int-valued result
LongToDoubleFunction	Accepts a long-valued argument and produces a double-valued result.
ToDoubleFunction<T>	Accepts a reference type and produces an int-valued result
ToIntFunction<T>	Accepts a reference type and produces an int-valued result
ToLongFunction<T>	Accepts a reference type and produces a long-valued result.
BiFunction<T,U,R>	Represents a function that accepts two arguments and produces a result (reference type)
ToDoubleBiFunction<T,U>	Accepts two reference type arguments and produces a double-valued result
ToIntBiFunction<T,U>	Accepts two reference type arguments and produces an int-valued result
ToLongBiFunction<T,U>	Accepts two reference type arguments and produces a long-valued result

Supplier

Supplier<T>	Represents a supplier of results (reference type)
DoubleSupplier	A supplier of double-valued results
IntSupplier	A supplier of int-valued results
LongSupplier	A supplier of long-valued results
BooleanSupplier	A supplier of boolean-valued results

Consumer

Consumer<T>	Represents an operation that accepts a single (reference type) input argument and returns no result
DoubleConsumer	Accepts a single double-valued argument and returns no result
IntConsumer	Accepts a single int-valued argument and returns no result
LongConsumer	Accepts a single long-valued argument and returns no result
BiConsumer<T,U>	Represents an operation that accepts two (reference type) input arguments and returns no result
ObjDoubleConsumer<T>	Accepts an object-valued and a double-valued argument, and returns no result
ObjIntConsumer<T>	Accepts an object-valued and an int-valued argument, and returns no result
ObjLongConsumer<T>	Accepts an object-valued and a long-valued argument, and returns no result

Interface (computing) Java (programming language)

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