文章目录

方法引用一共包含4中形式Reference to a static method && Reference to an instance method of a particular objectReference to an instance method of an arbitrary object of a particular typeReference to a constructor 参考文档 简单来说， 方法引用是针对接口来使用的。语法的一般形式为 xx::yy，jre会自动推断实现的方法、参数列表以及返回值。这也就要求了接口必须带有 @FunctionalInterface注解，或者符合 @FunctionalInterface规范。

方法引用一共包含4中形式

kind Example Reference to a static methodContainingClass::staticMethodNameReference to an instance method of a particular objectcontainingObject::instanceMethodNameReference to an instance method of an arbitrary object of a particular typeContainingType::methodNameReference to a constructorClassName::new

Reference to a static method && Reference to an instance method of a particular object

这两个用法相似，放在一起写。

public class MethodRef { public static void main(String[] args) { IMethod iMethod1 = MethodRef::staticMethod; IMethod iMethod2 = new MethodRef()::particularMethod; } public static void staticMethod(MethodRefTest test) { } public void particularMethod(MethodRefTest test) { } } //Consumer //@FunctionalInterface interface IMethod{ void method(MethodRefTest test); }

这两种写法分别相当于以staticMethod、particularMethod作为实现方法去生成一个匿名实现类。与下方代码等效

public class MethodRef { public static void main(String[] args) { IMethod iMethod1 = MethodRef::staticMethod; IMethod iMethod2 = new MethodRef()::particularMethod; IMethod iMethod3 = new IMethod() { @Override public void method(MethodRefTest test) { // TODO Auto-generated method stub MethodRef.staticMethod(test); } }; IMethod iMethod4 = new IMethod() { @Override public void method(MethodRefTest test) { // TODO Auto-generated method stub new MethodRef().particularMethod(test); } }; } public static void staticMethod(MethodRefTest test) { } public void particularMethod(MethodRefTest test) { } } //Consumer //@FunctionalInterface interface IMethod{ void method(MethodRefTest test); }

可以看出，这个语法的关键点只在于实现方法的参数列表与返回值，只要这两点一致，那么，不管方法写在了哪个类中，都可以使用。

Reference to an instance method of an arbitrary object of a particular type

关于这种写法，我一直没有找到什么太好的解释。只是从形式上看，它与第二种写法的主要区别就是，允许参数列表不同，jre会自动的向下寻找合适的方法来执行。由于推断的规则的原因，所使用的接口必须使用泛型。 见下例中method5

public class MethodRef { public static void main(String[] args) { IMethod iMethod1 = MethodRef::staticMethod; IMethod iMethod2 = new MethodRef()::particularMethod; IMethod iMethod3 = new IMethod() { @Override public void method(MethodRefTest test) { // TODO Auto-generated method stub MethodRef.staticMethod(test); } }; IMethod iMethod4 = new IMethod() { @Override public void method(MethodRefTest test) { // TODO Auto-generated method stub new MethodRef().particularMethod(test); } }; IMethod2<MethodRef> iMethod5 = MethodRef::noVar; } public static void staticMethod(MethodRefTest test) { } public void particularMethod(MethodRefTest test) { } public IMethod noVar() { return (p) -> {}; } } //Consumer //@FunctionalInterface interface IMethod{ void method(MethodRefTest test); } interface IMethod2<T>{ void method(T test); }

jre会自动推断出合适的方法来执行，但是，到底什么是合适，我目前还没有找到像样的解释。

Reference to a constructor

与上一种相似，也是必须要使用泛型。但是我目前只找到了适用于类似Supplier 接口的例子，感觉用处不是很大。 例（见下方代码iMethod6，iMethod7）

import java.util.function.Supplier; public class MethodRef { public static void main(String[] args) { IMethod iMethod1 = MethodRef::staticMethod; IMethod iMethod2 = new MethodRef()::particularMethod; IMethod iMethod3 = new IMethod() { @Override public void method(MethodRefTest test) { // TODO Auto-generated method stub MethodRef.staticMethod(test); } }; IMethod iMethod4 = new IMethod() { @Override public void method(MethodRefTest test) { // TODO Auto-generated method stub new MethodRef().particularMethod(test); } }; IMethod2<MethodRef> iMethod5 = MethodRef::noVar; IMethod3<MethodRef> iMethod6 = MethodRef::new; Supplier<MethodRef> iMethod7 = MethodRef::new; } public static void staticMethod(MethodRefTest test) { } public void particularMethod(MethodRefTest test) { } public IMethod noVar() { return (p) -> {}; } } //Consumer //@FunctionalInterface interface IMethod{ void method(MethodRefTest test); } interface IMethod2<T>{ void method(T test); } //形式与Supplier相似 interface IMethod3<T>{ T method(); }

参考文档

官方文档地址 https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html 参考帖子 Java 8 constructor method references Reference to an instance method of an arbitrary object of a particular type… not working with custom classes? 官方例子

/* * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * * - Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * * - Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * - Neither the name of Oracle or the names of its * contributors may be used to endorse or promote products derived * from this software without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ import java.util.List; import java.util.ArrayList; import java.util.Arrays; import java.util.Comparator; import java.util.Collection; import java.util.function.Supplier; import java.util.Set; import java.util.HashSet; import java.time.LocalDate; import java.time.chrono.IsoChronology; public class MethodReferencesTest { // The method transferElements copies elements from one collection to // another public static <T, SOURCE extends Collection<T>, DEST extends Collection<T>> DEST transferElements( SOURCE sourceCollection, Supplier<DEST> collectionFactory) { DEST result = collectionFactory.get(); for (T t : sourceCollection) { result.add(t); } return result; } public static void main(String... args) { List<Person> roster = Person.createRoster(); for (Person p : roster) { p.printPerson(); } Person[] rosterAsArray = roster.toArray(new Person[roster.size()]); class PersonAgeComparator implements Comparator<Person> { public int compare(Person a, Person b) { return a.getBirthday().compareTo(b.getBirthday()); } } // Without method reference Arrays.sort(rosterAsArray, new PersonAgeComparator()); // With lambda expression Arrays.sort(rosterAsArray, (Person a, Person b) -> { return a.getBirthday().compareTo(b.getBirthday()); } ); // With method reference Arrays.sort(rosterAsArray, Person::compareByAge); // Reference to an instance method of a particular object class ComparisonProvider { public int compareByName(Person a, Person b) { return a.getName().compareTo(b.getName()); } public int compareByAge(Person a, Person b) { return a.getBirthday().compareTo(b.getBirthday()); } } ComparisonProvider myComparisonProvider = new ComparisonProvider(); Arrays.sort(rosterAsArray, myComparisonProvider::compareByName); // Reference to an instance method // of an arbitrary object of a particular type String[] stringArray = { "Barbara", "James", "Mary", "John", "Patricia", "Robert", "Michael", "Linda" }; Arrays.sort(stringArray, String::compareToIgnoreCase); Set<Person> rosterSetLambda = transferElements(roster, () -> { return new HashSet<>(); }); Set<Person> rosterSet = transferElements( roster, HashSet::new); System.out.println("Printing rosterSet:"); rosterSet.stream().forEach(p -> p.printPerson()); } } class Person { public enum Sex { MALE, FEMALE } String name; LocalDate birthday; Sex gender; String emailAddress; Person(String nameArg, LocalDate birthdayArg, Sex genderArg, String emailArg) { name = nameArg; birthday = birthdayArg; gender = genderArg; emailAddress = emailArg; } public int getAge() { return birthday .until(IsoChronology.INSTANCE.dateNow()) .getYears(); } public void printPerson() { System.out.println(name + ", " + this.getAge()); } public Sex getGender() { return gender; } public String getName() { return name; } public String getEmailAddress() { return emailAddress; } public LocalDate getBirthday() { return birthday; } public static int compareByAge(Person a, Person b) { return a.birthday.compareTo(b.birthday); } public static List<Person> createRoster() { List<Person> roster = new ArrayList<>(); roster.add( new Person( "Fred", IsoChronology.INSTANCE.date(1980, 6, 20), Person.Sex.MALE, "fred@example.com")); roster.add( new Person( "Jane", IsoChronology.INSTANCE.date(1990, 7, 15), Person.Sex.FEMALE, "jane@example.com")); roster.add( new Person( "George", IsoChronology.INSTANCE.date(1991, 8, 13), Person.Sex.MALE, "george@example.com")); roster.add( new Person( "Bob", IsoChronology.INSTANCE.date(2000, 9, 12), Person.Sex.MALE, "bob@example.com")); return roster; } }