Background or Issue:
It's no sence to invoke an existing method by a lambda expression，so do it with method's name.
Introduction with Example
1 public class MethodReferencesTest { 2 3 // The method transferElements copies elements from one 4 // collection to another 5 6 public static <T, SOURCE extends Collection<T>, DEST extends Collection<T>> 7 DEST transferElements( 8 SOURCE sourceCollection, 9 Supplier<DEST> collectionFactory){ 10 11 DEST result = collectionFactory.get(); 12 for (T t : sourceCollection) { 13 result.add(t); 14 } 15 return result; 16 } 17 18 public static void main(String[] args) { 19 20 List<Person> roster = Person.createRoster(); 21 22 for (Person person : roster) { 23 person.printPerson(); 24 } 25 26 Person[] rosterAsArray = roster.toArray(new Person[roster.size()]); 27 28 29 class PersonAgeComparator implements Comparator <Person>{ 30 @Override 31 public int compare(Person o1, Person o2) { 32 return o1.getBirthday().compareTo(o2.getBirthday()); 33 } 34 } 35 36 // Without method reference 37 Arrays.sort(rosterAsArray, new PersonAgeComparator()); 38 39 // With lambda expression 40 // Notice that the interface Comparator is a functional interface. 41 // Therefore, use a lambda expression instead of defining 42 // and then creating a new instance of a class that implements Comparator 43 Arrays.sort(rosterAsArray, 44 (a, b) -> a.getBirthday().compareTo(b.getBirthday())); 45 46 // Invokes an existing method with lambda expression 47 // This method to compare the birth dates of two Person instances 48 // already exists as Person.compareByAge. Invoke this method instead 49 // in the body of the lambda expression 50 Arrays.sort(rosterAsArray, 51 (a, b) -> Person.compareByAge(a, b)); 52 53 // 1. Reference to a Static Method 54 Arrays.sort(rosterAsArray, Person::compareByAge); 55 56 // 2. Reference to an instance method of a particular object 57 class ComparisionProvider{ 58 public int compareByName(Person a, Person b){ 59 return a.getName().compareTo(b.getName()); 60 } 61 62 public int compareByAge(Person a, Person b){ 63 return a.getBirthday().compareTo(b.getBirthday()); 64 } 65 } 66 ComparisionProvider myComparisionProvider = new ComparisionProvider(); 67 Arrays.sort(rosterAsArray, myComparisionProvider::compareByAge); 68 69 // 3. Reference to an instance method 70 // of an arbitrary object of a particular type 71 String[] stringArray = { "Barbara", "James", "Mary", "John", 72 "Patricia", "Robert", "Michael", "Linda" }; 73 Arrays.sort(stringArray, String::compareToIgnoreCase); 74 75 // the equivalent lambda expression for the method inference 76 // String::compareToIgnoreCase 77 Arrays.sort(stringArray, (a, b) -> a.compareToIgnoreCase(b)); 78 79 // 4. Reference to a Constructor 80 // You can reference a constructor in the same way 81 // as a static method by using the name new. 82 Set<Person> rosterSetLambda = 83 transferElements(roster, () -> new HashSet<>()); 84 85 // use a constructor reference in place of the lambda expression 86 // The Java compiler infers that you want to create a HashSet 87 // collection that contains elements of type Person 88 Set<Person> rosterSet = transferElements( 89 roster, HashSet::new); 90 91 System.out.println("Printing rosterSet: "); 92 rosterSet.stream().forEach(p -> p.printPerson()); 93 } 94 }