Preface

We have summarized the functional interface of Java SE 8 and the primitive specialization of Stream. The purpose is to prepare for the Java Gold exam.

What is primitive specialization?

In java, primitives cannot be given to type arguments such as T, R of R apply (T t) of Function \ <T, R>. In functional interfaces and Streams, some classes are individually defined so that int, long, and double can be operated like generics (without boxing in the wrapper class). This is called a type with primitive specialization. If you remember only int, there is no difference between long and double.

Organize functional interfaces

[java.util.function] 1 full-function interface

Basic class name	Specialization to int	Specialization to long	Specialization to double	Basic method name
Function<T,R>	IntFunction<R>	LongFunction<R>	DoubleFunction<R>	apply()
Function<T,R>	IntToDoubleFunction	LongToDoubleFunction	DoubleToIntFunction	apply()
Function<T,R>	IntToLongFunction	LongToIntFunction	DoubleToLongFunction	apply()
Function<T,R>	ToIntFunction<T>	ToLongFunction<T>	ToDoubleFunction<T>	apply()
UnaryOperator<T>	IntUnaryOperator	LongUnaryOperator	DoubleUnaryOperator	apply()
BiFunction<T,U,R>	ToIntBiFunction<T,U>	ToLongBiFunction<T,U>	ToDoubleBiFunction<T,U>	apply()
BinaryOperator<T>	IntBinaryOperator	LongBinaryOperator	DoubleBinaryOperator	apply()
Predicate<T>	IntPredicate	LongPredicate	DoublePredicate	test()
BiPredicate<T,U>	-	-	-	test()
Consumer<T>	IntConsumer	LongConsumer	DoubleConsumer	accept()
BiConsumer<T,U>	ObjIntConsumer<T>	ObjLongConsumer<T>	ObjDoubleConsumer<T>	accept()
Supplier<T>	IntSupplier	LongSupplier	DoubleSupplier	get()
BooleanSupplier	-	-	-	getAsBoolean()

point

--The primitive-specialized functional interface appears as an argument to the primitive-specialized Stream method.

e.g.
[IntStream.filter ()] Take IntPredicate as an argument of 3.

--The method name of the functional interface should be ** "basic method name As type name" ** only if the return value is a primitive.

e.g.
--R apply (T t) → Return value to int → int applyAsInt (T t) --T get () → Return value to double → double getAsDouble ()

--The target of specialization is basically only int, long, double, and BooleanSupplier is special. ――Is BooleanSupplier supposed to be used when writing tests and assertions? -"Bad float, this package is for 3 people"

--Function primitive specialization corresponds to each of the following patterns. --Take a primitive argument and return a primitive --Take an object argument and return a primitive --Take a primitive argument and return an object -Is there IntToIntFunction in the 2nd and 3rd rows of the table? It looks like it will be IntUnaryOperator.

--In accept () of ObjIntConsumer corresponding to BiConsumer, argument 1 is an object and argument 2 is an int. This is used as an accumulator for [IntStream.collect ()] 2.

[java.util.stream] Organize primitive specializations in 0

type

The following 3 classes are primitive specialized types from Stream \ .

IntStream
LongStream
DoubleStream

Here, we will call it a primitive specialization stream.

Primitive specialized stream-specific termination operations

--sum (): Take the sum. --avage (): Take the arithmetic mean. The return value is OptionalDouble. --max (): Since the order is self-explanatory, it takes no arguments unlike Stream \ .max (Comparator). --min (): Since the order is self-explanatory, it does not take an argument unlike Stream \ .min (Comparator).

Mutual conversion between Stream \ and primitive specialized streams

From a primitive specialization stream

mapToXxx () method

this	Return value	Method name	argument
IntStream	LongStream	mapToLong *1	IntToLongFunction
IntStream	DoubleStream	mapToDouble *1	IntToDoubleFunction
IntStream	Stream<T>	mapToObj *2	IntFunction<? extends T>
LongStream	IntStream	mapToInt	LongToIntFunction
LongStream	DoubleStream	mapToDouble *1	LongToDoubleFunction
LongStream	Stream<T>	mapToObj *2	LongFunction<? extends T>
DoubleStream	IntStream	mapToInt	DoubleToIntFunction
DoubleStream	LongStream	mapToLong	DoubleToLongFunction
DoubleStream	Stream<T>	mapToObj *2	DoubleFunction<? extends T>

* 1 A simple upcast can be done with the asXxx () method.

this	Return value	Method name	argument
IntStream	DoubleStream	asDoubleStream	-
IntStream	LongStream	asLongStream	-
LongStream	DoubleStream	asDoubleStream	-

* 2 Simple boxing can be done with the boxed () method

this	Return value	Method name	argument
IntStream	Stream<Integer>	boxed	-
LongStream	Stream<Long>	boxed	-
DoubleStream	Stream<Double>	boxed	-

From Stream \ to a primitive specialized stream

mapToXxx () method

this	Return value	Method name	argument
Stream<T>	IntStream()	mapToInt	ToIntFunction<? super T>
Stream<T>	LongStream()	mapToLong	ToLongFunction<? super T>
Stream<T>	DoubleStream()	mapToDouble	ToDoubleFunction<? super T>

flatMapToXxx () method

A combination of flattening and primitive specialization. ** The one who seems to be in the exam **.

this	Return value	Method name	argument
Stream<T>	IntStream()	flatMapToInt	Function<? super T,? extends IntStream>
Stream<T>	LongStream()	flatMapToLong	Function<? super T,? extends LongStream>
Stream<T>	DoubleStream()	flatMapToDouble	Function<? super T,? extends DoubleStream>

Please let me know if there is something to add.

Java Primitive Specialization Summary

Preface

What is primitive specialization?

Organize functional interfaces

[java.util.function] 1 full-function interface

point

[java.util.stream] Organize primitive specializations in 0

type

Primitive specialized stream-specific termination operations

Mutual conversion between Stream \ and primitive specialized streams

From a primitive specialization stream

mapToXxx () method

* 1 A simple upcast can be done with the asXxx () method.

* 2 Simple boxing can be done with the boxed () method

From Stream \ to a primitive specialized stream

mapToXxx () method

flatMapToXxx () method