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You may find it useful to open another copy of this lesson in a separate
browser window. That will make it easier for you to scroll back and
forth among the different listings while you are reading about them.
This lesson explains various details regarding the use of array objects
in Java, and illustrates many of those details through the use of sample
code.
A sample program shows you three ways to emulate traditional two-dimensional
rectangular arrays, and also shows you how to create and use ragged arrays.
Discussion
and Sample Code
Array objects
A different syntax is required to create array objects than the syntax
normally used to create ordinary objects. Array objects are accessed
via references. Any of the methods of the Object class can
be invoked on a reference to an array object.
The indices of a Java array object
Array objects encapsulate a group of variables, which don't have individual
names. They are accessed using positive integer index values. The
integer indices of a Java array object always extend from 0 to (n-1)
where n is the length of the array encapsulated in the object.
Multidimensional arrays
All array objects in Java encapsulate one-dimensional arrays.
However, the component type of an array may itself be an array type.
This makes it possible to create array objects whose individual components
refer to other array objects. This is the mechanism for creating multi-dimensional
or ragged arrays in Java.
Such a structure of array objects can be thought of as a tree of array
objects, with the data being stored in the array objects that make up the
leaves of the tree.
Array types
When declaring a reference variable capable of referring to an array
object, the array type is declared by writing the name of an element type
followed by some number of empty pairs of square brackets []. This
is illustrated in Listing 1, which declares a reference variable named
v1,
capable of storing a reference to a two-dimensional array of type
int.
int[][] v1;
Listing 1
(Note that Listing 1 doesn't really declare a two-dimensional
array in the traditional sense of other programming languages. Rather,
it declares a reference variable capable of storing a reference to a one-dimensional
array object, which in turn is capable of storing references to one-dimensional
array objects of type int.)
Multiple pairs of square brackets are allowed
The components in an array object may refer to other array objects.
The number of bracket pairs used in the declaration of the reference variable
indicates the depth of array nesting (in the sense that array elements
can refer to other array objects). This is one of the ways that
Java implements the concept of traditional multi-dimensional arrays (I
will show you some other ways later in this lesson).
The code in Listing 1 shows two levels of nesting for the reference
variable of type int[][].
Length not part of variable declaration (Note that an array's length is not part of its type or reference
variable declaration.)
Ragged arrays (Note also that multi-dimensional arrays, when implemented in this
fashion, are not required to represent rectangles, cubes, etc. For
example, the number of elements in each row of a Java two-dimensional array
can be different. Some authors refer to this as a ragged array.)
Allowable types
The specified element type of an array may be any primitive or reference
type. Note, however, that all elements of the array must be of the same
type (consistent with the type-conversion rules discussed below).
Listing 2 shows the declaration of a reference variable capable of referring
to a three-dimensional array object of element type Button (Button
is one of the classes in the standard class library).
Button[][][] v2;
Listing 2
Rules of type conversion and assignment compatibility
apply
The normal rules of type conversion and assignment compatibility
apply when creating and populating array objects. For example, if
the specified type is the name of a non-abstract class, a null reference
or a reference to any object instantiated from that class or any subclass
of that class may be stored in the array element.
The generic class Object
For example, Listing 3 shows the declaration of a reference variable
capable of referring to a one-dimensional array object of element type
Object.
Since Object is the superclass of all other classes, this array
object is capable of storing references to objects instantiated from any
other class. (As we saw in the previous lesson, it is also capable
of storing a reference to any other array object as well.)
Object[] v3;
Listing 3
Primitive type conversions
Similarly, if the declared element type for the array object is one
of the primitive types, the elements of the array can be used to store
values of any primitive type that is assignment compatible with
the declared type (without the requirement for a cast).
For example, the code in Listing 4 shows the creation of a one-dimensional
array object capable of storing values of type int. The array
object has a length of 3 elements, and the object's reference is stored
in a reference variable named v1.
int[] v1;
v1 = new int[3];
byte x1 = 127;
short x2 = 16384;
int x3 = 32000;
v1[0] = x1;
v1[1] = x2;
v1[2] = x3;
Listing 4
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