Sorting With a Comparator Written on One Line

One call, one lambda, and the pieces the compiler filled in without being told.

java

invoices.sort((a, b) -> Long.compare(a.pence(), b.pence()));

What was written, and what the compiler worked out
Written	Inferred
`(a, b)`	Both are `Invoice`, from `List<Invoice>.sort(Comparator<? super Invoice>)`
`->`	That this is the body of `compare`, the only abstract method on `Comparator`
`Long.compare(...)`	That the `int` it produces is the return value

Nothing was left out that the compiler could not supply. sort declares its parameter as a Comparator, Comparator has exactly one abstract method, and that method takes two Invoice values and returns an int.

What to notice: the lambda has no type written anywhere. Its type came from the place it was passed to, which is why an identical lambda handed to a different method would be a different interface entirely.

Java
Java Streams
Sorting Algorithms