Java Learning Notes-method references

Method Reference (Methods Reference)

A lambda expression was recorded in the previous article, which can create an anonymous method. when a lambda expression simply calls an existing method, you can take a method reference (the attribute that JDK8 has). As follows:

1  Public classPerson {2 3      Public enumSex {4 MALE, FEMALE5     }6 7 String name;8 localdate birthday;9 Sex gender;Ten String EmailAddress; One     intAge ; A  -      Public intGetage () { -         returnAge ; the     } -      -      Publiclocaldate Getbirthday () { -         returnbirthday; +     }     -  +      Public Static intComparebyage (Person A, person B) { A         returnA.birthday.compareto (b.birthday); at     } -}

Suppose you need to sort a group of people by age, you can pass the people array to the Array.Sort method in the following way, with the implemented comparer:

1person[] Rosterasarray = Roster.toarray (Newperson[roster.size ()]);2 3 classPersonagecomparatorImplementsComparator<person> {4      Public intCompare (person A, person B) {5         returna.getbirthday (). CompareTo (B.getbirthday ());6     }7 }8         9Arrays.sort (Rosterasarray,NewPersonagecomparator ());

Of course, you can use the Personagecomparator implementation as a lambda expression, as follows:

1  Arrays.sort (Rosterasarray, (A, B), A.birthday.compareto (B.birthday));

The person class already contains a comparison comparebyage based on age, which only needs to be called directly in the lambda expression body :

1 (A, B), Person.comparebyage (A, b));

Because a lambda expression invokes a method that already exists, you can use a method reference instead of a lambda expression, as follows:

1 person::comparebyage);

Among them, Person::comparebyage and (A, B), Person.comparebyage (A, b) are equivalent. (1) Its parameters are copied in Comparator<person>.compare, i.e.(Person, Person)；（2）body将调用Person.compareByAge。

Method Reference Category

There are 4 methods to refer to, as follows:

• Referencing static methods, such as Containingclass::staticmethodname;
• Referencing instance methods, such as Containingobject::instancemethodname;
• Methods that refer to special types of objects, such as Containingtype::methodname;
• Reference constructors, such as Classname::new.

(1) referencing static methods

The example above is a static method reference.

(2) Reference instance method

That is, the method is referenced by an instance of the class, as follows:

1 classComparisonprovider {2      Public intComparebyname (Person A, person B) {3         returna.getname (). CompareTo (B.getname ());4     }5         6      Public intComparebyage (Person A, person B) {7         returna.getbirthday (). CompareTo (B.getbirthday ());8     }9 }TenComparisonprovider Mycomparisonprovider =NewComparisonprovider (); OneArrays.sort (Rosterasarray, mycomparisonprovider::comparebyname);

(3) Methods for referencing special types of objects

Take string for example:

1 string[] Stringarray = {"Barbara", "James", "Mary", "John",2     "Patricia", "Robert", " Michael "," Linda " }; 3 arrays.sort (Stringarray, string::comparetoignorecase);

(4) Reference constructor

Suppose transferelements implements a copy element from one collection to another, as follows:

1  Public Static<t, SOURCEextendsCollection<t>, DESTextendsCollection<t>>2 DEST transferelements (3 SOURCE sourcecollection,4Supplier<dest>collectionfactory) {5         6DEST result =collectionfactory.get ();7          for(T t:sourcecollection) {8 Result.add (t);9         }Ten         returnresult; One}

Where supplier contains a GET method that returns only an empty collection object, and does not require any parameters, it can be implemented using a lambda expression, as follows:

1 set<person> rostersetlambda =2     returnnew hashset<> ();});

You can take the method of referencing the constructor at this point, as follows:

set<person> Rosterset = transferelements (Roster, HashSet::new); // or set<person> Rosterset = transferelements (roster, hashset<person>::new);

Summarize

• In the case of lambda expressions, when just invoking an existing method, it can be implemented by means of a method reference, which the compiler will translate itself

Reference

Method references

Java Learning Notes-method references