Difference between method reference and lambda [duplicate]

Question

I would expected that both put operations throw a NullPointerException in the following code, but actually the lambda expression works fine while just the method reference throws a NPE.

public static void main(String... args) {
    Object object = null;
    Map<String, FuncInterface> map = new HashMap<>();

    map.put("key1", () -> object.notify());    // works
    map.put("key2", object::notify);           // throws NPE
}

@FunctionalInterface
private interface FuncInterface {
    public void someAction();
}

What is the difference?

Answer 1

The lambda is evaluated when it's called: if you called map.get("key1").someAction() you would get a NPE.

The method reference is evaluated at creation time, i.e. when you first write object::notify, which throws a NPE straight away.

In particular, the JLS 15.13.3 states:

Evaluation of a method reference expression is distinct from invocation of the method itself.
First, if the method reference expression begins with an ExpressionName or a Primary, this subexpression is evaluated. If the subexpression evaluates to null, a NullPointerException is raised, and the method reference expression completes abruptly.