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Reference :

• mkyong

• reduce is an aggregating method that aggregates the output into a single value
• stream class has 3 variant of reduce method for which return types of each is different
  • Optional reduce(BinaryOperator accumulator);
  • T reduce(T identity, BinaryOperator<T> accumulator);
  • <U> U reduce(U identity,BiFunction<U, ? super T> accumulator,BinaryOperator<U> combiner);

Example :

Example :

• Java 8 Stream’s map method is intermediate operation and consumes single element forom input Stream and produces single element to output Stream.
• It simply used to convert Stream of one type to another.
•  signature of Stream’s map method.
  • <R> Stream<R> map(Function<? super T,? extends R>mapper)
• Types of map method in Stream class
  • map()
  • mapToInt()
  • mapToLong()
  • mapToDouble()

• Types of Java 8 binary Operator
  • BinaryOperator<T>
  • DoubleBinaryOperator
  • IntBinaryOperator
  • LongBinaryOperator

Example Reference :

• mkyoung

• Types of UnitaryOperator
  • UnaryOperator<T>
  • LongUnaryOperator
  • IntUnaryOperator
  • DoubleUnaryOperator

Example Reference :

• mkyoung

• Types of function Interface
  • Function<T,R>
  • BiFunction<T,U,R>
  • DoubleFunction<R>
  • IntFunction<R>
  • LongFunction<R>
  • DoubleToIntFunction
  • DoubleToLongFunction
  • IntToDoubleFunction
  • IntToLongFunction
  • LongToDoubleFunction
  • LongToIntFunction
  • ToDoubleFunction<T>
  • ToIntFunction<T>
  • ToLongFunction<T>
  • ToDoubleBiFunction<T,U>
  • ToIntBiFunction<T,U>
  • ToLongBiFunction<T,U>

Example Reference :

• mkyoung

• chaining of consumers is possible
  • default Consumer<T> andThen(Consumer<? super T> after)
• Types of consumers
  • BiConsumer
  • Consumer
  • DoubleConsumer
  • IntConsumer
  • LongConsumer
  • ObjDoubleConsumer<T>
  • ObjIntConsumer<T>
  • ObjLongConsumer<T>

• Stream's generate method returns an infinite sequential stream where supplier generates each element.
• Following are the primitive Specializations for Supplier interface in java.util.function package.
  • IntSupplier  – Represents a supplier of int-valued results. Having one method getAsInt().
  • LongSupplier – Represents a supplier of long-valued results. Having one method getAsLong().
  • DoubleSupplier – Represents a supplier of double-valued results. Having one method getAsDouble().
  • BooleanSupplier  – Represents a supplier of boolean-valued results. Having one method getAsBoolean().

Syntax :

• Let’s say you want to generate 5 random numbers between 0 to 10.
• Stream's generate method returns an infinite sequential stream where supplier generates each element.

Example :

Example: Returns a supplier

Example: Factory

• findFirst and findAny are shortcircuiting methods, they are used to stop the parallel processing and return an optional.

javadocs

• Java 8 has introduced a new class Optional in java.util package. It can help in writing a neat code without using too many null checks. 
• By using Optional, we can specify alternate values to return or alternate code to run.
• This makes the code more readable because the facts which were hidden are now visible to the developer.

Practical User :

• We can easily make a null check on findFirst and find any using orElse method of optional
  • T orElse(T other)

Methods

• T orElse(T other)
  • Checks if an optional is null, if yes return other else return value
  • This value is a single value returned via findFirst() or findAny() of steam method
• T get()
  • If a value is present in this {@code Optional}, returns the value,
  • otherwise throws NoSuchElementException
  • Hence isPresent() method value should be used before get
  • This method is similar to orElse but no null check or alternative value hear
• boolean isPresent()
  • Return true if there is a value present, otherwise returns false