When a bound method reference is captured, we should do a null check on the result of the receiver:

    Sam mr = foo::bar;

The expression "foo" is evaluated immediately, and if it is null, we should throw NPE.

Relevant spec:

If the expression is a method reference that begins with an ExpressionName or a Primary, this subexpression is first evaluated. [jsr335-15.28.2-30-A]
If the subexpression evaluation completes abruptly, then the method reference evaluation completes abruptly for the same reason. [jsr335-15.28.2-30-A1]
If the result of evaluation of the subexpression is null, then a NullPointerException is thrown. [jsr335-15.28.2-30-A2]



Test case below:

import java.util.function.*;

public class Foo {
  public static void main(String[] args) {
    String s = null;
    Supplier<Boolean> ss = s::isEmpty;
  }
}