Downcasting

Casting down the inheritance tree is known as downcasting.

• Downcasting requires the cast operator ().

Person p = new Employee();
Employee e = (Employee) p; // this works

The compiler lets us cast to any class in the hierarchy. * If the object in memory isn't actually what we cast its reference to according to the is-a relationship, our program will break at runtime with a ClassCastException.

Person p = new Person();   // Person in memory
Employee e = (Employee) p; // Compiles, but fails at runtime
                           // with ClassCastException

instanceof Operator

We can use the instanceof operator to check if the object in memory could be cast to another type. * If the test returns true, we can safely cast to that type.

Person p = new Employee();

if (p instanceof Employee) {
  Employee e = (Employee) p;
  e.getTitle();
}

Practice Exercise

instanceof can only use operands that are a reference type or null. A primitive like int x results in a compiler error.

Reference: https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.20.2

Drill

(If you did not finish VehicleApp, use the version provided in the ...solutions.vehicles package)

Polymorphism/com.example.polymorphism.labs.vehicles.VehicleApp * Change calculateVehicleRegistration to use a rate of 0.04 if the vehicle is-a Automobile or Truck, and 0.065 if the vehicle is-a Boat. Otherwise use 0.02.

(Solution: VehicleApp2.java)

---

Prev -- Up -- Next