Instance Method Reference and Lambda Parameters

Question

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

Answer 1

I would give a couple of examples here, for those who find Oracle documentation a bit hard to take in. Imagine you need a reference to a Comparator instance:

.sorted(String::compareTo)

String::compareTo is identical to:

(String a, String b) -> a.compareTo(b);

Because, as Jon explained, a method reference will be transformed to a lambda that will expect 2 parameters. The actual arbitrary object passed in the stream as a first argument, and one more parameter(since Comparator expects int compare(T o1, T o2)). Another case:

.map(Employee::getSalary)

In this case map expects: Function. Function requires implementation of R apply(T var1) - a method with 1 argument. In this case the only parameter that will be passed to the lambda is the actual arbitrary object - instance on Employee.

To sum up - depending on the compile time context, method reference to arbitrary object will always be "transformed" into a lambda that expects that object as a first parameter + any number of parameters that the target method requires in the same corresponding order.