UpCasting and DownCasting

Upcasting

  • An object of a sub class can be referred by its super class automatically.

  • You can cast an object implicitly to a super class type is Called UpCasting.

  • If this were not the case polymorphism wouldn't be possible.

  • When up casting primitives from left to right, automatic conversion done as below:-

    • byte -> short -> int -> long -> float -> double
    int i = 10;
    long j = 15; //Correct. Up casting or implicit casting
    byte c1 = i; //Incorrect. Compile time error " Type Mismatch".
    byte c2 = (byte) i ; //Correct. Down casting or explicit casting is required.

Downcasting

  • Explicit casting is to be done by the super class.

  • Which is performed manually by the developers.

UpCasting vs DownCasting:-

upcasting and downcasting
Vehicle v1 = new Car(); //Right.upcasting or implicit casting

Vehicle v2 = new Vehicle();

Car c0 = v1; //Wrong. compile time error "Type Mismatch".
//Explicit or down casting is required

Car c1 = (Car)v1; // Right. down casting or explicit casting.

// v1 has knowledge of Car due to line1

Car c2 = (Car)v2; //Wrong. Runtime exception ClassCastException
//v2 has no knowledge of Car.

Bus b1 = new Alto(); //Wrong. compile time error "Type Mismatch"

Car c3 = new Alto(); //Right.upcasting or implicit casting

Car c4 = (Alto)v1; //Wrong. Runtime exception ClassCastException

Object o = v1; //v1 can only be upcast to its parent or

Car c5 = (Car)v1; //v1 can be down cast to Car due to line 1.