Detect whether lambda is a method reference

Question

Is there a way to determine whether a given Java lambda object is a method reference or a "CallSite-specific" lambda:

boolean isMethodReference(Object lambda)

Positive example:

assertTrue(isMethodReference(Object::toString));

Negative example with "CallSite-specific" lambda:

long valueFromCallSite = System.currentTimeMillis();
Consumer<Object> lambda = o -> {
    if (valueFromCallSite % 2 == 0) {
        o.toString();
    } else {
        o.hashCode();
    }
};
assertFalse(isMethodReference(lambda));

Answer 1

A heuristic approach for isMethodReference(lambda) was proposed in "Determine if a lambda expression is stateless or stateful in Java":

boolean isMethodReference(Object lambda) {
    return lambda.getClass().getDeclaredFields().length == 0;
}

It’s only a heuristic because it relies on unspecified behavior and thus is JDK implementation-specific and even might break in a future version.