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pron 2 hours ago  [ - ]

Yes, but it comes from Java having both runtime and compile-time types; it's harder to make the distinction in languages that don't have runtime types.

In Java, you can ask, `x instanceof T` (and this is a runtime test), which means, is x one of the values in the set of values allowed by T. `null instanceof Integer` is false, even though a variable of type `Integer` can be assigned a null. So you can think of `Integer x` as being `Integer|null x`, i.e. x can hold a null, even though `null instancof Integer` is false.

zoogeny an hour ago  [ - ]

I think I mostly got this, but just to test it, it would be like in Typescript where I might say:

    type Foo = { x: number; }
    type Bar = { x: number; y: number }
    type FooBar = Foo | Bar;
    function baz(x: FooBar) {
      if ('y' in x) {
        // compiler now knows x is a Bar
      }
    }
In this case, the variable `x` has a property that is determined by the compiler based on control flow. i.e. it isn't explicitly carried by the type of `x`.