Function interface in Java 8

It's hard for me to classify why lambda expressions can be assigned to some functional interfaces instead of others For an example, use some functional interfaces from the metrics Library:

Gauge<Double> foo = () -> { return null; };
RatioGauge bar = () -> { return null; };

The second statement has a compilation error (in eclipse):

As far as I know, ratiogauge is a functional interface Did I miss anything?

Solution

An abstract class (even if it has only one abstract method) is not a functional interface Only one interface can be one

From JLS 9.8:

The original idea was to let the abstact class be expressed as lambda; They are called "Sam types" and represent "single abstract methods" This is a difficult problem to solve This thread talk about why; Basically, the constructor of the base class makes it difficult