Assignment compatibility for references
Assignment compatibility for references doesn't involve range issues, as is the case with primitives. Rather, the reference to an object instantiated from agiven class can be assigned to (is assignment-compatible with):
- Any reference variable whose type is the same as the class from which the object was instantiated.
- Any reference variable whose type is a superclass of the class from which the object was instantiated.
- Any reference variable whose type is an interface that is implemented by the class from which the object was instantiated.
- Any reference variable whose type is an interface that is implemented by a superclass of the class from which the object was instantiated.
- A couple of other cases involving interfaces that extend other interfaces.
In this module, we are interested only in cases 1 and 2 above. We will be interested in the other cases in future modules involving interfaces.
Such an assignment does not require the use of a cast operator.
Type Object is completely generic
As mentioned earlier, a reference to any object can be assigned to a reference variable of the type Object , because the Object class is a superclass of every other class.
Converting reference types with a cast
Assignments of references, other than those listed above, require the use of a cast operator to purposely change the type of the reference.
However, it is not possible to perform a successful cast to convert the type of a reference to another type in all cases.
Generally, a cast can only be performed among reference types that fall on the same ancestral line of the class hierarchy, or on an ancestral line of aninterface hierarchy. For example, a reference cannot be successfully cast to the type of a sibling or a cousin in the class hierarchy.
Downcasting
When we cast a reference along the class hierarchy in a direction away from the root class Object toward the leaves , we often refer to it as a downcast .
While it is also possible to cast in the direction from the leaves to the root , this conversion happens automatically, and the use of a cast operator is not required.
Preview
A sample program is provided that illustrates much of the detail involved in type conversion, method invocation, and casting with respect to referencetypes.
Discussion and sample code
The program named Polymorph02 , shown in Listing 11 near the end of the module illustrates the use of the cast operator with references.
When you examine that program, you will see that two classes named A and C each extend the class named Object . Hence, we might say that they are siblings in the class hierarchy.
Another class named B extends the class named A . Thus, we might say that A is a child of Object , and B is a child of A .
The class named A
The definition of the class named A is shown in Listing 1 . This class implicitly extends the class named Object by default.