New features of Java8: Optional

I. Replace NULL with optional

1. Optional class

Java.util.optional<t> is a class that encapsulates Optional values.

When a variable is present, the optional class simply encapsulates the class. When a variable does not exist, the missing value is modeled as an "empty" optional object, returned by Method Optional.empty ().

The Optional.empty () method is a static factory method that returns a specific single instance of the Optional class.

a very important and practical semantic difference between using optional instead of NULL:

declaring a variable using optional<t> instead of a specific object, this statement is very clear that the loss of the occurrence of the variable here is allowed.

2. Several modes of applying optional

Create a optional object

--declaration of an empty optional

optional<car> Optcar = Optional.empty ();

--Create optional based on a non-null value

optional<car> Optcar = Optional.of (Car); If car is a null, this code throws nullpointexception immediately instead of waiting for an error to be returned when trying to access the property value of car.

--acceptable null optional (used when implementing a serialized domain model)

optional<car> Optcar = optional.ofnullable (Car); Creates a optional object that allows null values, and if car is null, then the resulting optional object is an empty object

extracting and converting values from a optional object using map

public class Person {//someone has a car, someone does not have a car private optional<car> cars; Public optional<car> Getcar () {return Car;}}

public class Car {//Some cars have insurance, some cars have no insurance private optional<insurance> Insurance; Public optional<insurance> getinsurance () {return Insurance;}}

public class Insurance {//insurance company must have a name, if there is an insurance company without a name is an error case private String name; Public String GetName () {return name;}}

optional<insurance> optinsurance = optional.ofnullable (Insurance);optional<string> name = Optinsurance.map (Insurance::getname)

optional objects with flatmap links

When using a stream, the Flatmap method takes a function as an argument, and the return value of the function is another stream. This method is applied to each function in the stream, resulting in the flow of a new stream. However, Flatmap replaces each newly generated stream with the contents of the stream. That is, each stream generated by the method is merged or flattened into a single stream.

 optional<person> optperson = optional.of (person);           Optional<String> name =                                                         optperson.map (Person::getCar)                                                                          .map (Car::getInsurance)   //Error code &NBSP;&NBSP;&NBSP;                                                                     .map (Insurance::getName)

Getcar returns an object of type optional<car>, so the first map gets the result optional<optional<car>> ; the object of type. So his call to getinsurance is illegal. The Getinsurance method is not supported.

Using Flatmap, the optional of the two-layered structure is a single-layer structure with car.

      Optional<String> name =                                                         optperson.flatmap (Person::getCar )                                                                         .flatmap (Car:: Getinsurance)                                                                         .map (Insurance::getName)                                                                          .orelse ("Unknown");  // If the optional result is empty, set the default value

default behavior and dereference optional objects

-Get () is the simplest and least secure method of these methods . If there is a variable, he directly returns the value of the encapsulated variable, otherwise throws nosuchelementexception;

--OrElse (T Other) allows you to provide a default value when the optional object does not contain a value;

--Orelseget (supplier< extends t> other) is a deferred call version of the OrElse method, Supplier method only in Optiona The call is executed only if the object does not contain a value. If creating a default value is a time-consuming effort, you should take it this way, or you need to be very sure that a method is called only when optional is empty, or you can consider the method (which has strict constraints);

--Orelsethrow (supplier<? extends x> Exceptionalsupplier) is very similar to the Get method when optional Throws an exception when the object is empty, but using Orelsethrow you can customize the exception you want to throw;

--Ifpresent (consumer< Super t>) allows you to execute a method passed in as a parameter when the value of the variable exists, otherwise no action is taken.

Two combination of optional objects

Public optional<insurance> nullsafefindcheapestinsurance (optional<person> person, Optional<Car> Car) {return Person.flatmap (P-Car.map (C-Findcheapestinsurance (P, c));}

use filter to reject specific values

Find the list of insurers for the person older than or equal to the Minage parameter public String getcarinsurancename (optional<person> person, int minage) {        Return Person.filter (P-p.getage () >= minage). FlatMap (Person::getcar). FlatMap (Car::getinsurance) . Map (Insurance::getname). OrElse ("Unknown");}

There are many similar methods for optional and stream interfaces .

methods of the optional class

Method	Describe
Empty	Returns an empty instance of optional
Filter	If the value exists and satisfies the provided predicate, the optional object containing the value is returned, otherwise an empty optional object is returned
FlatMap	If the value exists, the provided mapping function call is performed on the value, returning a value of type optional, otherwise returning an empty optional object
Get	If the value exists, it will be returned by the optional encapsulated value, or a Nosunchelementexception exception is thrown
Ifpresent	If a value exists, execute a method call that uses that value, or do nothing
IsPresent	Returns True if the value exists, otherwise false
Map	If the value exists, the provided mapping function call is provided for the value execution
Of	Returns the specified value after encapsulation with optional, or throws a Nullpointexception exception if the value is null
Ofnullable	Returns the specified value after encapsulation with optional, or returns an empty optional object if the value is null
OrElse	Returns a value if it is available, otherwise returns a default value
Orelseget	Returns a value if it is available, or returns a value generated by the specified supplier interface
Orelsethrow	Returns a value if it is returned, or throws an exception that is generated by the specified supplier interface

New features of Java8: Optional