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:

<         


        

Answer 1

You can think of a lambda as a method that doesn't belong to a class, that can be passed around like a piece of data.

Or, with a very small mental shift, you can think of it as an object with only one method.

There is a set of interfaces marked as functional interfaces. This is mentioned in the Javadoc as:

Functional Interface:

This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

That's a technical way of saying that because they have only one method, and they're marked as functional, the compiler can treat a lambda as an object with that interface. It knows to use the lambda as the implementation of the one method in the interface.

So you can do:

Comparator byNameComparator = (f1,f2) -> f1.name().compareTo(f2.name());
Predicate isVip = cust -> cust.orderCount > 20;
Callable getResult = () -> queue.getItem();
Function multiply = (a,b) -> a * b;

... and so on.

And wherever a parameter's type is a functional interface, you can instead use a lambda directly, so given:

 public void sort(Comparator cmp);

... you can call it as:

 foo.sort( (a,b) -> a.name().compareTo(b.name()));