Why can you make a lambda referencing an instance method by using a class reference?

Question

String[] foo = {"one","two", "three"};
Arrays.sort(foo, (a,b) -> a.compareTo(b));

And

String[] foo = {"one","two", "three"};
Arrays.sort(foo, String::compareTo);

are equivalent.

Why can I reference compareTo via String when compareTo is an instance method?

Reference to an Instance Method of an Arbitrary Object of a Particular Type — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Jun 12, 2015 at 17:06
Because that's how Java handles method references: by adding the receiver as the first argument of the resulting lambda. — Louis Wasserman
– Louis Wasserman, Commented Jun 12, 2015 at 19:07

ZhongYu · Accepted Answer · 2015-06-12 17:06:14Z

4

Think of this example

Function<String, String> func;  

func = string -> string.toUpperCase();

func = String::toUpperCase;

Think of an instance method as a static method with an extra this parameter, for the purpose of deducing method reference.

    static String toUpperCase(String this)

answered Jun 12, 2015 at 17:06

ZhongYu

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