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

The interface Comparator is a Functional Interface, this means this interface can only contains one abstract method.

Then you can use lambda expression to define the implementation of this abstract method, basically (d1,d2) -> d1.getWeight() - d2.getWeight(); is the implementation of the abstract method int compare(T o1, T o2);.

As a functionnal Interface contains only one abstract method, you can use lambda expression to define the implementation of such interface