Polymorphism and Abstract Classes - Absolute Java - page 501

PITFALL: Downcasting

It is the responsibility of the programmer to use downcasting only in situations where
it makes sense. The compiler makes no checks to see if downcasting is reasonable.
However, if you use downcasting in a situation in which it does not make sense, you
will usually get a run-time error message.

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TIP: Checking to See Whether Downcasting Is Legitimate ★

You can use the instanceof operator to test whether or not downcasting is sensible.
Downcasting to a specific type is reasonable if the object being cast is an instance of
that type, which is exactly what the instanceof operator tests for.
The instanceof operator checks whether an object is of the type given as its second
argument. The syntax is
Object instanceof Class_Name

instanceof

which returns true if Object is of type Class_Name ; otherwise it returns false . So,
the following will return true if someObject is of type DiscountSale :

someObject instanceof DiscountSale

Note that because every object of every descendent class of DiscountSale is also
of type DiscountSale , this expression will return true if someObject is an instance
of any descendent class of DiscountSale .
So, if you want to type cast to DiscountSale , then you can make the casts safer
as follows:

DiscountSale ds = new DiscountSale();
if (someObject instanceof DiscountSale)
{
ds = (DiscountSale)someObject;
System.out.println("ds was changed to " + someObject);
}
else
System.out.println("ds was not changed.");
someObject might be, for example, a variable of type Sale or of type Object. ■