Method References in Lambda - Java Lambda Stream

Description

Method References in Lambda
Demo Code

import java.time.LocalDate;
import java.time.chrono.IsoChronology;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.function.Supplier;

public class MethodReferencesTest {

  // The method transferElements copies elements from one collection to
  // another//ww  w.j  a  v  a 2s .  c  om

  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;
  }

}
/*
 * 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.
 */
Method References in Lambda - Java Lambda Stream

Description

Demo Code

Result

Related Tutorials
