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# reference type transition

change **only the nubmer of usable members**! **only between parent-child relation** NOT siblings

type transition of primitive type, the value is changed. ex) (int)3.6 => 3

diminishing the number of usable members is okay but vice versa increasing the number of usable memberw is see is NOT recommended.

**basically, reference type transition is  allowed.**

**The important thing is what object(members) reference type var is indicating . Figure out the real object that reference type var is indicating. The reference type variable must NOT be more than the number of object's usable members.**

```java
class Ex7_7 {
    public static void main(String args[]) {
        Car c = new Car();//the object is Car type!!!
        FireEngine fe = (FireEngine)c;//even ClassCast is proceeded and compiler does not catch issue
        fe.water();//there is execution error! because in Car class, there is NO water() method!
    }
}
```