Java™ How To Program (Early Objects), Tenth Edition

17.5.1 Mapping Strings to Uppercase Using a Method Reference

Lines 18–21 display the Strings 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:

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(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.

Fig. 17.8 | Types of method references.

Stream method map receives ...