Java 8 方法引用

Java 8 方法引用

There are four kinds of method references:

Kind	Example
Reference to a static method	ContainingClass::staticMethodName
Reference to an instance method of a particular object	containingObject::instanceMethodName
Reference to an instance method of an arbitrary object of a particular type	ContainingType::methodName
Reference to a constructor	ClassName::new

方法引用通过方法的名字来指向一个方法。
方法引用可以使语言的构造更紧凑简洁，减少冗余代码。
方法引用使用一对冒号 :: 。
下面，我们定义了 4 个方法作为例子来区分 Java 中 4 种不同方法的引用。

Person

``` import java.time.LocalDate; import java.time.chrono.IsoChronology; import java.util.ArrayList; import java.util.List;

public class Person {

@FunctionalInterface
public interface Supplier<T> {
    T get();
}

public enum Sex {
    MALE, FEMALE
}

String name;
LocalDate birthday;
Sex gender;
String emailAddress;

public Person() {
    super();
}

Person(String nameArg, LocalDate birthdayArg, Sex genderArg, String emailArg) {
    name = nameArg;
    birthday = birthdayArg;
    gender = genderArg;
    emailAddress = emailArg;
}

public static Person create(Supplier<Person> supplier) {
    return supplier.get();
}

public int getAge() {
    return birthday.until(IsoChronology.INSTANCE.dateNow()).getYears();
}

public void printPerson() {
    System.out.println(name + ", " + this.getAge());
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public LocalDate getBirthday() {
    return birthday;
}

public void setBirthday(LocalDate birthday) {
    this.birthday = birthday;
}

public Sex getGender() {
    return gender;
}

public void setGender(Sex gender) {
    this.gender = gender;
}

public String getEmailAddress() {
    return emailAddress;
}

public void setEmailAddress(String emailAddress) {
    this.emailAddress = emailAddress;
}

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;
}

@Override
public String toString() {
    return "[" + name + ", " + birthday + "]";
}

}

</div>

```java
import java.time.LocalDate;
import java.util.Arrays;
import java.util.List;

public class ComparisonProvider {

    public int compareByName(Person a, Person b) {
        return a.getName().compareTo(b.getName());
    }

    public static int compareByAge(Person a, Person b) {
        return a.getBirthday().compareTo(b.getBirthday());
    }

    public static void main(String[] args) {
        List<Person> roster = Person.createRoster();
        Person[] rosterAsArray = roster.toArray(new Person[roster.size()]);

        // 特定对象的实例方法引用: containingObject::instanceMethodName
        ComparisonProvider myComparisonProvider = new ComparisonProvider();
        System.out.println("排序前:" + Arrays.toString(rosterAsArray));
        Arrays.sort(rosterAsArray, myComparisonProvider::compareByName);
        System.out.println("排序后:" + Arrays.toString(rosterAsArray));

        // 静态方法引用：ContainingClass::staticMethodName
        System.out.println("排序前:" + Arrays.toString(rosterAsArray));
        Arrays.sort(rosterAsArray, ComparisonProvider::compareByAge);
        System.out.println("排序后:" + Arrays.toString(rosterAsArray));

        // 特定类型的任意对象的实例方法的引用: ContainingType::methodName
        String[] stringArray = { "Barbara", "James", "Mary", "John", "Patricia", "Robert", "Michael", "Linda" };
        System.out.println("排序前:" + Arrays.toString(stringArray));
        Arrays.sort(stringArray, String::compareToIgnoreCase);
        System.out.println("排序后:" + Arrays.toString(stringArray));

        // 构造器引用: ClassName::new
        Person newPerson = Person.create(Person::new);
        newPerson.setName("xiaocao");
        newPerson.setBirthday(LocalDate.of(1995, 8, 20));
        System.out.println(newPerson);
    }
}

点击查看结果

``` 排序前:[[Fred, 1980-06-20], [Jane, 1990-07-15], [George, 1991-08-13], [Bob, 2000-09-12]] 排序后:[[Bob, 2000-09-12], [Fred, 1980-06-20], [George, 1991-08-13], [Jane, 1990-07-15]] 排序前:[[Bob, 2000-09-12], [Fred, 1980-06-20], [George, 1991-08-13], [Jane, 1990-07-15]] 排序后:[[Fred, 1980-06-20], [Jane, 1990-07-15], [George, 1991-08-13], [Bob, 2000-09-12]] 排序前:[Barbara, James, Mary, John, Patricia, Robert, Michael, Linda] 排序后:[Barbara, James, John, Linda, Mary, Michael, Patricia, Robert] [xiaocao, 1995-08-20] ```

参考链接

• https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html
• https://blog.csdn.net/yitian_66/article/details/81010434