Enthuware

DoubleUnaryOperator is a function that accepts a Double value and returns a Double and so can be used here.

Since we are assigning this lambda expression to a DoubleFunction<DoubleUnaryOperator>, it implies that we are trying to using this lambda expression to implement a DoubleFunction that converts a Double into a DoubleUnaryOperator.

Now, in this question we are using a DoubleStream and so, its map method expects a an operation that can accept a Double value and onvert it to some other Double value. (Note that the resulting stream, therefore, will also be of type DoubleStream).