When to use casting in java?

Answer 1

Casting must be done when converting an object from a "wider" or "parent" object to a "narrower" or "child" object. As an example, you do not need to cast from a byte to an int, but you must cast from an int to a byte; the compiler requires the cast so that the developer confirms that they are okay with a possible loss of data. Similarly, given the classes Vehicle, Car, and Truck, where Car and Truck are children of Vehicle, no cast is required to convert either a Car or Truck to a Vehicle, but the language requires a cast to convert a Vehicle to a Car or Truck.

In this case, if the Vehicle is cast to a Car, but is actually a Truck, the cast will fail and a runtime exception will be thrown. By casting a Vehicle to a Car, the developer acknowledges that they already know the Vehicle is a Car, and the compiler honors that arrangement. The Virtual Machine expects the Vehicle to be a Car, but if it is actually a Truck, the exception is thrown.

The objective of casting is to allow conversion between semi-compatible types of data where some loss of data may occur, but the developer has logically proven that the loss will never occur or is acceptable. You cannot cast a Car to an int, for example, because there is no possibility that a Car will ever be an int. The compiler will advise you of this fact and refuse to compile the code.