Java SE 8 Lambdas and Streams - Java: How to Program Early Objects - page 747

Original strings: [Red, orange, Yellow, green, Blue, indigo, Violet]
strings in uppercase: [RED, ORANGE, YELLOW, GREEN, BLUE, INDIGO, VIOLET]
strings greater than m sorted ascending: [orange, Red, Violet, Yellow]
strings greater than m sorted descending: [Yellow, Violet, Red, orange]

Fig. 17.7 | Demonstrating lambdas and streams with an array of String s. (Part 2 of 2.)

17.5.1 Mapping String s to Uppercase Using a Method Reference
Lines 18-21 display the String s in uppercase letters. To do so, line 19 creates a
Stream<String> from the array strings , then line 20 calls Stream method map to map
each String to its uppercase version by calling String instance method toUpperCase .
String::toUpperCase is known as a method reference and is a shorthand notation for a
lambda expression—in this case, for a lambda expression like:

(String s) -> { return s.toUpperCase();}

or

s -> s.toUpperCase()

String::toUpperCase is a method reference for String instance method toUpperCase .
Figure 17.8 shows the four method reference types.

Lambda Description

String::toUpperCase

Method reference for an instance method of a class. Creates a one-
parameter lambda that invokes the instance method on the lambda's
argument and returns the method's result. Used in Fig. 17.7.

System.out::println

Method reference for an instance method that should be called on a spe-
cific object. Creates a one-parameter lambda that invokes the instance
method on the specified object—passing the lambda's argument to the
instance method—and returns the method's result. Used in Fig. 17.10.

Math::sqrt

Method reference for a static method of a class. Creates a one-parame-
ter lambda in which the lambda's argument is passed to the specified a
static method and the lambda returns the method's result.

TreeMap::new Constructor reference. Creates a lambda that invokes the no-argument

constructor of the specified class to create and initialize a new object of
that class. Used in Fig. 17.17.

Fig. 17.8 | Types of method references.

Stream method map receives as an argument an object that implements the functional
interface Function —the instance method reference String::toUpperCase is treated as a
lambda that implements interface Function . This interface's apply method receives one
parameter and returns a result—in this case, method apply receives a String and returns
the uppercase version of the String . Line 21 collects the results into a List<String> that
we output as a String .