Instance Method Reference and Lambda Parameters

I am having trouble understanding the syntax for a method reference, where there are two parameters a and b, and the reference is to a method of a on b.

For example I understand how

Arrays.sort(personArray, comparators::compareByName);

is equivalent to

Arrays.sort(personArray, (o1, o2) -> comparators.compareByName(o1, o2));

because in that case the lambda parameters match the method call parameters (o1, o2).

Howevever for this lambda

stream.sorted((o1, o2) -> o1.compareToIgnoreCase(o2));

my IDE tells me that is equivalent to:

stream.sorted(String::compareToIgnoreCase);

and I am not finding a rule for replacing that syntax: a.method(b) with a method reference.

For example, what if there are three or more parameters to the lambda? Is that legal? Does the first parameter become the method target, and the remaining become the parameters?

Jon Skeet
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quotationmark

I think you're looking for JLS section 15.13.3, which includes:

If the form is ReferenceType :: [TypeArguments] Identifier, the body of the invocation method similarly has the effect of a method invocation expression for a compile-time declaration which is the compile-time declaration of the method reference expression. Run-time evaluation of the method invocation expression is as specified in §15.12.4.3, §15.12.4.4, and §15.12.4.5, where:

  • The invocation mode is derived from the compile-time declaration as specified in §15.12.3.

  • If the compile-time declaration is an instance method, then the target reference is the first formal parameter of the invocation method. Otherwise, there is no target reference.

  • If the compile-time declaration is an instance method, then the arguments to the method invocation expression (if any) are the second and subsequent formal parameters of the invocation method. Otherwise, the arguments to the method invocation expression are the formal parameters of the invocation method.

Note the last two bullets, basically.

For example, what if there are three or more parameters to the lambda? Is that legal? Does the first parameter become the method target, and the remaining become the parameters?

Yup :)

people

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