Chapter 4: Functional Programming

Question 1

Supplier

Answer 1

When we want to generate or supply values with no input.

Method name is get()

Question 2

Consumer + BiConsumer

Answer 2

Do something with a parameter or two parameters but have no input.

Method name is accept.

Question 3

Predicate + BiPredicate

Answer 3

Returns a boolean value (can be used in filtering and matching.

method name is test()
Can join predicates using .and()
Can negate predicate using .negate()

Question 4

Function + BiFunction

Answer 4

Takes one parameter & returns another type. (Similar for Bifunction but taking two params and returning a type).

Method name is apply

Question 5

UnaryOperator + BinaryOperator

Answer 5

Transforms one or two values into a single one of the same type.

Method name is apply

Question 6

Optional.ofNullable

Answer 6

Returns optional.empty() if value is null

Question 7

Optional.orElse()

Answer 7

.orElse returns something else of SAME type
.orElseGet takes a supplier to return something of the same type
.orElseThrow takes a supplier to throw an exception

Question 8

Stream min/max

Answer 8

Pass in a comparator, returns optional as no min/max could be found

Question 9

Stream findAny

Answer 9

Finds any value (faster than findFirst if you’re using parallelStreams)

Question 10

Stream reduce

Answer 10

Combines a stream into a single object.
T reduce (T identity, BinaryOperator  accumulator)
Identity is the initial state/what is returned on empty stream and accumulator specifies how to merge with the next value.

T reduce (BinaryOperator accumulator)
We can omit the identity if we want, but then it will return optional

<u> U reduce (identity, BiFunction, BinaryOperator)
Can be used for parralel streams
Or if we want to apply a function to an object in our stream before accumulating them.</u>

Question 11

Stream collect

Answer 11

R collect (Supplier, BiConsumer, BiConsumer)
Supplier supplies a new object or collection which will store the value. This is a single container.
BiConsumer accumulates values in each container.
Other BiConsumer merges the results across all the containers

Note can also use a Collector to collect values.

Question 12

Stream intermediate operations

Answer 12

limit(int max size) + skip(int n). Watch out for where .limit is used, x number of items needs to have passed that point.

flatMap(), flattens a stream of collections to a stream of objects. E.g. Stream> –> Stream

sorted() either uses natural order, or you can pass in a comparator

peek() takes in a consumer which allows us to. perform a Stream operation without changing the stream. peek musn’t change the state of the Stream or there will be an error! This can be used for printing out values in the stream, for example.

Question 13

Generating a new Stream

Answer 13

Stream.generate(Supplier) or Stream.iteratore(start, function)

we need to limit this otherwise it will be an infinite stream..

Question 14

Primitive streams

Answer 14

IntStream, LongStream, DoubleStream.

Creating a new stream using IntStream.empty() or .of(…)

IntStream.range(lower, upper) generates values, upper is exclusive
IntStream.rangeClosed(lower, upper) generates values, upper is inclusive.

Question 15

Mapping between primitive types of Stream

Answer 15

For each type of primitive stream you can map to another using the mapToX() method. E.g. mapToDouble converts to DoubleStream or mapToObj() maps to a standard stream. For each of these you need to provide a mapping function to indicate how to map to that data type. If you’re just mapping to itself e.g. using map() then you need a _UnaryOperator e.g. DoubleUnaryOperator. Otherwise you need a _To_Function e.g. LongToIntFunction. If you’re mapping to normal stream you need a _Function e.g. DoubleFunction. Note you may need to cast if you’re going from a larger to a smaller type so it will compile!

Question 16

Primitive optionals

Answer 16

Optional_ e.g. OptionalDouble. You call .getAsDouble instead of get. orElseGet() takes a DoubleSupplier instead of a standard supplier.
.avg() will return Optional_. Note that IntStream avg() will return OptionalDouble!

Question 17

Summarising statistics

Answer 17

.summaryStatistics() returns _SummaryStatics() e.g. IntSummaryStatistics(). Which can give you the max, min and count

Question 18

Functional Interfaces for primatives

Answer 18

Suppliers methods are getAsX() e.g. BooleanSupplier getAsBoolean.
Operators e.g. DoubleBinaryOperator methods as applyAsDouble.

Question 19

Lazy evaluation of Streams

Answer 19

Streams are lazily evaluated, so don’t get evaluated until there is a terminal operation. So if you add something to the underlying data which is in the stream (e.g. add to a list) it will be evaluated by the stream at the terminal operation

Question 20

Chaining optionals

Answer 20

If you want to convert from one optional type to another, you need to use flatmap to ensure the optional stays at the top level (otherwise you may end up with Optional>.

Question 21

Collectors partitioningBy

Answer 21

Takes in a Predicate and returns Map>.

If we don’t want the values collected to a List, we can pass in a Collector to collect it into another collection

Question 22

Collectors toMap

Answer 22

toMap(Function for key, Function for value). Specifies how you want to map your keys and values. If we try to Map duplicate Keys we will throw an IllegalStateException. There is also an overload where you can pass in a merge function as a 3rd parameter, which will allow you to indicate how to merge values if there are any duplicates.

Question 23

Iterator in streams

Answer 23

Needs to be .iterate(1, x, ++x) otherwise you will get an infinite iteration of 1’s if it is (1, x, x++)