Although this declaration establishes the fact that month_days is an array variable, no
array actually exists. In fact, the value of month_days is set to null, which represents an array
with no value. To link month_days with an actual, physical array of integers, you must allocate
one using new and assign it to month_days. new is a special operator that allocates memory.
You will look more closely at new in a later chapter, but you need to use it now to allocate
memory for arrays. The general form of new as it applies to one-dimensional arrays appears
as follows:
array-var = new type[size];
Here, type specifies the type of data being allocated, size specifies the number of elements in
the array, and array-var is the array variable that is linked to the array. That is, to use new to
allocate an array, you must specify the type and number of elements to allocate. The elements
in the array allocated by new will automatically be initialized to zero. This example allocates
a 12-element array of integers and links them to month_days.
month_days = new int[12];
After this statement executes, month_days will refer to an array of 12 integers. Further, all
elements in the array will be initialized to zero.
Let's review: Obtaining an array is a two-step process. First, you must declare a variable of the
desired array type. Second, you must allocate the memory that will hold the array, using
new, and assign it to the array variable. Thus, in Java all arrays are dynamically allocated. If
the concept of dynamic allocation is unfamiliar to you, don't worry. It will be described
at length later in this topic.
Once you have allocated an array, you can access a specific element in the array by
specifying its index within square brackets. All array indexes start at zero. For example,
this statement assigns the value 28 to the second element of month_days.
month_days[1] = 28;
The next line displays the value stored at index 3.
System.out.println(month_days[3]);
Putting together all the pieces, here is a program that creates an array of the number
of days in each month.
// Demonstrate a one-dimensional array.
class Array {
public static void main(String args[]) {
int month_days[];
month_days = new int[12];
month_days[0] = 31;
month_days[1] = 28;
month_days[2] = 31;
month_days[3] = 30;
month_days[4] = 31;
month_days[5] = 30;
month_days[6] = 31;
Search WWH :
