Arrays - Java - page 346

PITFALL: Arrays with a Class Base Type

The base type of an array can be of any type, including a class type. For example, sup-
pose Date is a class and consider the following:

Date[] holidayList = new Date[20];

This creates the 20 indexed variables holidayList[0] , holidayList[1] , ...,
holidayList[19] . It is important to note that this creates 20 indexed variables of type
Date . This does not create 20 objects of type Date . (The index variables are automat-
ically initialized to null , not to an object of the class Date .) Like any other variable of
type Date , the indexed variables require an invocation of a constructor using new to
create an object. One way to complete the initialization of the array holidayList is
as follows:

Date[] holidayList = new Date[20];
for ( int i = 0; i < holidayList.length; i++)
holidayList[i] = new Date();

If you omit the for loop (and do not do something else more or less equivalent), then
when you run your code, you will undoubtedly get an error message indicating a “null
pointer exception.” If you do not use new to create an object, an indexed variable like
holidayList[i] is just a variable that names no object and hence cannot be used as
the calling object for any method. Whenever you are using an array with a class base
type and you get an error message referring to a “null pointer exception,” it is likely
that your indexed variables do not name any objects and you need to add something
like the above for loop. ■

Array Parameters
You can use both array indexed variables and entire arrays as arguments to methods,
although they are different types of parameters. We first discuss array indexed variables
as arguments to methods.
An indexed variable can be an argument to a method in exactly the same way that
any variable of the array base type can be an argument. For example, suppose a pro-
gram contains the following declarations:

indexed
variable
arguments

double n = 0;
double [] a = new double [10];
int i;

If myMethod takes one argument of type double , then the following is legal:

myMethod(n);