Java8: (Optional)

1.Optional:

returnUser.map (U-u.getorders ()). OrElse (Collections.emptylist () )//It avoids our previous practices like Java 8.if(User.ispresent ()) {returnuser.get (). getorders ();}Else{  returncollections.emptylist ();} Map can be infinitely cascaded, such as deep, to get the uppercase form of the user namereturnUser.map (U-u.getusername ()). Map (name-name.touppercase ()). OrElse (NULL)            ;//If this is the case before, the following is the wording:User User = ....if(User! =NULL) {String name=User.getusername (); if(Name! =NULL){    returnname.touppercase (); }Else{    return NULL; }}Else{  return NULL;} try not to call the Optional.get () method directly when using Optional, Optional.ispresent ()
should be considered as a private method and should depend on other methods such as Optional.orelse (), Optional.orelseget (), Optional.map ().

 Public Static voidMain (string[] args) {//Create a optional instance, or you can return it by means of a method. optional<string> name = Optional.of ("Sanaulla"); //Create an optional instance with no value, for example, a value of ' null 'Optional empty = optional.ofnullable (NULL); //The Ispresent method is used to check whether the optional instance has a value.     if(Name.ispresent ()) {//call Get () to return the optional value. System.out.println (Name.get ()); }    Try {      //call Get () on the optional instance to throw the nosuchelementexception. System.out.println (Empty.get ()); } Catch(Nosuchelementexception ex) {System.out.println (Ex.getmessage ()); }    //The Ifpresent method accepts the lambda expression parameter. //If the optional value is not empty, the lambda expression processes and executes the action on it. Name.ifpresent (value){System.out.println ("The length of the value is:" +value.length ());    }); //if the value OrElse method returns an optional instance, the incoming error message is returned. System.out.println (Empty.orelse ("There is no value present!")); System.out.println (Name.orelse ("There is some value!")); //Orelseget is similar to OrElse, except that the default value is passed in. //Orelseget accepts a lambda expression to generate a default value. System.out.println (Empty.orelseget (), "Default Value")); System.out.println (Name.orelseget ()"Default Value")); Try {      //Orelsethrow is similar to the OrElse method, except that the return value is the difference. //Orelsethrow thrown by an incoming lambda expression/method generates an exception. Empty.orelsethrow (valueabsentexception::New); } Catch(Throwable ex) {System.out.println (Ex.getmessage ()); }    //The Map method modifies the default value of the Optonal instance by passing in the lambda expression. //the lambda expression return value is wrapped as an optional instance. optional<string> uppername = Name.map ((value)value.touppercase ()); System.out.println (Uppername.orelse ("No value found")); //Flatmap is very similar to map (Funtion), except for the return value of a lambda expression. //the lambda expression return value of the map method can be any type, but the return value is wrapped as a optional instance. //However, the lambda return value of the Flatmap method is always optional type. Uppername = Name.flatmap ((value)Optional.of (Value.touppercase ())); System.out.println (Uppername.orelse ("No value found")); //The filter method checks whether the Optiona value satisfies the given condition. //returns an empty optional if the return optional instance value is satisfied. optional<string> longname = Name.filter ((value) Value.length () > 6); System.out.println (Longname.orelse ("The name is less than 6 characters")); //Another example, the optional value does not meet the given criteria. optional<string> anothername = Optional.of ("Sana"); Optional<String> shortname = Anothername.filter ((value), Value.length () > 6); System.out.println (Shortname.orelse ("The name is less than 6 characters")); }

Java8: (Optional)