cast : Java Glossary

cast : Java Glossary

2007-07-06 by Roedy Green ©1996-2008 Canadian Mind Products
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cast

There are two useful kinds of casts.

Primitive or converting casts e.g.
```
// primitive converting cast
short s = (short) someInt;
```
would convert to a short integer.
Mollifying casts assure Java that a general reference is actually something more specific, a subclass of that general type. E.g.
```
// mollifying cast
Dalmatian d = (Dalmatian) dog;
```
assures Java that the dogRef pointer really points to a Dalmatian. Java checks that this is indeed so before proceeding. Casting objects is a misleading terminology. Any actual object has a definite class, set when it was instantiated. Nothing can change that during the lifetime of the object. The object it self is unchanged when you cast its reference.
In the days before generics, casts were most needed after you retrieved an element from a Collection. The collection just gave you a generic Object, but you knew better what it was, since you knew what sort of Objects you put into the collection.
If a mollifying downcast fails, because the object is not really what you tried to cast it to, you will get a ClassCastException. Upcasts always succeed. Upcasts are always unnecessay.
Before you cast, you can use instanceof, or one of its brothers, to check if a cast will work.

conversion
downcasting
gotchas
instanceof
upcasting

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