7.6 Ap0060: self-assessment, more on arrays  (Page 7/9)

Would require reconstructing the array object

Therefore, to convert an array object containing int values to an array object containing double values would require reconstructing the array object and allocating twice as much storage space foreach element in the array.

Restriction doesn't apply to arrays of references

As you have seen from previous questions, such a casting restriction does not apply to arrays containing references to objects. This may be because the amountof storage required to store a reference to an object is the same, regardless of the type of the object. Therefore, the allowable casts that you have seen in theprevious questions did not require any change to the size of the array. All that changed was some supplemental information regarding the type of objects to whichthe elements in the array refer.

Back to Question 11

Answer 10

C. 1.0 2.0

Explanation 10

Assigning array reference to variable of type Object

A reference to an array can be assigned to a non-array reference of the class named Object , as in the following statement extracted from the program, where A is a reference to an array object of type double .

Note that there are no square brackets anywhere in the above statement. Thus, the reference to the array object is not being assigned to an array reference ofthe type Object[] . Rather, it is being assigned to an ordinary reference variable of the type Object .

Downcasting to an array type

Once the array reference has been assigned to the ordinary reference variable of the type Object , that reference variable can be downcast and used to access the individual elements in the array as illustrated in thefollowing fragment. Note the empty square brackets in the syntax of the cast operator (double[]) .

Placement of parentheses is critical

Note also that due to precedence issues, the placement of both sets of parentheses is critical in the above code fragment. You must downcast thereference variable before applying the index to that variable.

Back to Question 10

Answer 9

C. 1 2

Explanation 9

General array casting rule

The general rule for casting array references (for arrays whose declared type is the name of a class or an interface) is:

A reference to an array object can be cast to another array type if the elements of the referenced array are of a type that can be cast to the typeof the elements of the specified array type.

Old rules apply here also

Thus, the general rules covering conversion and casting up and down the inheritance hierarchy and among classes that implement the same interfaces alsoapply to the casting of references to array objects.

A reference to an object can be cast down the inheritance hierarchy to the actual class of the object. Therefore, an array reference can also be cast downthe inheritance hierarchy to the declared class for the array object.

This program declares a reference to, creates, and populates an array of the class type Subclass . This reference is assigned to an array reference of a type that is a superclass of the actual class type of the array. Then the superclassreference is downcast to the actual class type of the array and assigned to a different reference variable. This third reference variable is used tosuccessfully access and display the contents of the elements in the array.