Method references

Function<String, Integer> parse   = Integer::parseInt;      // static
Supplier<Integer>         size    = list::size;             // bound instance
Function<String, Integer> length  = String::length;         // unbound instance
Supplier<List<String>>    factory = ArrayList::new;         // constructor

The unbound form is the one that surprises people: String::length becomes a one-argument function whose argument is the receiver.

Bound references capture eagerly

List<String> list = new ArrayList<>();
Supplier<Integer> s = list::size;
list = new ArrayList<>(List.of("a"));
s.get();   // 0 — bound to the original object, not the variable

The receiver is evaluated when the reference is created. A lambda () -> list.size() would not compile here, since list is not effectively final.

Ambiguity

Integer::parseInt could target Function<String,Integer> or ToIntFunction<String>; the target type decides. When both are plausible in context, the compiler reports ambiguity and an explicit lambda resolves it.