0.7 Xna0112-type conversion, casting, and assignment compatibility  (Page 5/7)

This case violates the rule

In this case, the method named m is defined in the class named B , which is two levels down from the class named Object .

When the reference to the object of the class B was assigned to the reference variable of type Object , the type of the reference was automatically converted to type Object .

Therefore, because the reference is type Object , it cannot be used directly to call the method named m .

The solution is a downcast

In this case, the solution to the problem is a downcast.

The code in Listing 6 shows an attempt to solve the problem by casting the reference down the hierarchy to type A .

Still doesn't solve the problem

However, this still doesn't solve the problem, and the result is another compiler error.

Again, it was necessary to convert the statement into a comment in order to cause the program to compile.

What is the problem here?

The problem with this approach is that the downcast simply didn't go far enough down the inheritance hierarchy.

The class named A does not contain a definition of the method named m . Neither does it inherit the method named m . The method named m is defined in class B, which is a subclass of A .

Therefore, a reference of type A is no more useful than a reference of type Object insofar as calling the method named m is concerned.

The real solution

The solution to the problem is shown in Listing 7 .

The code in Listing 7 casts (temporarily converts) the reference value contained in the Object variable named var to type B .

The method named m is defined in the class named B . Therefore, a reference of type B can be used to call the method.

The code in Listing 7 compiles and executes successfully. This causes the method named m to execute, producing the following output on the computer screen:

A few odds and ends

Before leaving this topic, let's look at a few more issues.

The code in Listing 8 declares and populates a new variable of type B.

The code in Listing 8 uses a cast to:

• Convert the contents of the Object variable to type B
• Assign the converted reference to the new reference variable of type B .

A legal operation

This is a legal operation. In this class hierarchy, the reference to the object of the class B can be assigned to a reference variable of the types B, A , or Object .

Cannot be assigned to type C

However, the reference to the object of the class B cannot be assigned to a reference variable of any other type, including type C . An attempt to do so is shown in Listing 9 .