Reference types - Data Structures and Problem Solving Using Java - page 38

Arrays are indexed
starting at zero. The
number of items
stored in the array is
obtained by the
length field. No
parentheses are
used.

array a can always be obtained by a.length . Note that there are no parentheses.
A typical array loop would use

for( int i = 0; i < a.length; i++ )

2.4.1 declaration, assignment, and methods

An array is an object, so when the array declaration

int [ ] array1;

is given, no memory is yet allocated to store the array. array1 is simply a name
(reference) for an array, and at this point is null . To have 100 int s, for exam-
ple, we use new :

To allocate an array,
use new .

array1 = new int [ 100 ];

Now array1 references an array of 100 int s.
There are other ways to declare arrays. For instance, in some contexts

int [ ] array2 = new int [ 100 ];

is acceptable. Also, initializer lists can be used, as in C or C++, to specify ini-
tial values. In the next example, an array of four int s is allocated and then ref-
erenced by array3 .

int [ ] array3 = { 3, 4, 10, 6 };

The brackets can go either before or after the array name. Placing them before
makes it easier to see that the name is an array type, so that is the style used
here. Declaring an array of reference types (rather than primitive types) uses
the same syntax. Note, however, that when we allocate an array of reference
types, each reference initially stores a null reference. Each also must be set to
reference a constructed object. For instance, an array of five buttons is con-
structed as

Button [ ] arrayOfButtons;
arrayOfButtons = new Button [ 5 ];
for( int i = 0; i < arrayOfButtons.length; i++ )
arrayOfButtons[ i ] = new Button( );

Figure 2.4 illustrates the use of arrays in Java. The program in Figure 2.4
repeatedly chooses numbers between 1 and 100, inclusive. The output is the
number of times that each number has occurred. The import directive at line 1
will be discussed in Section 3.8.1.