Why can a lambda expression be used as a Comparator?

Question

In the book OCP Study Guide there is this example about a Comparator that can be initialized in two ways. The first is via an anonymous class like this:

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Answer 1

Lambda expression is just a short-hand for a functional interface (an interface with just one function), you don't need to write new/function name just write parameter list in ( yourParameterListHere ) and then -> and after this write what to do/return (i.e function body). you can also write it with { } like

Comparator byWeight = (d1,d2) -> { d1.getWeight() - d2.getWeight(); }