2.5 Java1610: polymorphism, type conversion, casting, etc.  (Page 5/6)

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 get the program to compile.

What is the problem here?

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

The class named A neither defines nor inherits the method named m() . The method named m() is defined in class B , which is a subclass of class 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 down 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 couple more issues. The code in Listing 8 declares and populates a new variable of type B .

The code in Listing 8 also 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 the type C . An attempt to do so is shown in Listing 9 .

The code in Listing 9 attempts to cast the reference to type C and assign it to a reference variable of type C .

A runtime error

Although the program will compile, it won't execute. An attempt to execute the statement in Listing 9 results in a ClassCastException at runtime. As a result, it was necessary to convert the statement into a comment in order to execute the program.

Another failed attempt

Similarly, an attempt to cast the reference to type B and assign it to a reference variable of type C , as shown in Listing 10 , won't compile.

The problem here is that the class C is not a superclass of the class named B . Therefore, a reference of type B is not assignment compatible with a reference variable of type C .

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

Summary

This module discusses type conversion for both primitive and reference types.

A value of a particular type may be assignment compatible with variables of other types.

If the type of a value is not assignment compatible with a variable of a given type, it may be possible to perform a cast on the value to change its type and assign it to the variable as the new type. For primitive types, this will often result in the loss of information.