instanceof determine if a subtypes is its super-type.

'instanceof' is used to determine if an object is a sub-type of a super-class.

If d is Double, then compiler definitely knows it is not a Date. so it will give a compiling error;

        String s = "ddd";
        System.out.println(""+ s instanceof String); 
        
        Double d = 1d;
        System.out.println(d instanceof java.util.Date);