Casting
When you assign an object reference to a variable, or pass it to a function, the type of the reference and the expected type should be compatible. As explained in Inheritance, a derived object is always compatible with its ancestor. Let’s try this:
class Base { int x } class Derived(Base) { int y }
Derived d = new Derived Base b = d ; Legal because Base is ancestor of Derived b.x = 4 print(d.x) ; Prints 4 Derived d2 = b ; Compilation error: incompatible types!
The last assignment is illegal because b has type Base, which is not compatible with Derived. However, b really points to an object of type Derived, so it should be possible to get the ‘original’ object back. You can do this with a cast. A cast looks like a function call, using the name of the class to cast to as the name of the function:
Derived d = new Derived d.y = 5 Base b = d Derived d2 = Derived(b) ; Cast b to Derived print(d2.y) ; Prints 5 Base b2 = new Base d2 = Derived(b2) ; This cast fails and returns 0 print(d2) ; Prints 0
A cast checks if the object is really an instance of the class to cast to. If so, it returns a reference with the desired type. Otherwise, it returns a null reference. You can use this to query the type of object that a reference actually points to.
Because all classes ultimately descend from the internal Object class, you can use Object as a common type to store different types of objects in an array, for example. With a cast, you can later test what type of object an element in the array really contains.
For debugging purposes, you can also print the value of an object reference, which will print the class name and the reference count of the object:
Object obj = new Derived print(obj) ; Prints Derived (1)
| Note: With a plug-in parameter, you can also determine the currently selected class by comparing it to a class with the == operator or the != operator. See also Writing plug-ins. |
Next: Plug-in parameters
See Also
Classes
Inheritance