2.12 Java1624: array objects, part 2  (Page 2/15)

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 leavesof 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 bysome 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 .

(Note that Listing 1 doesn't really declare a two-dimensional array in the traditional sense of other programming languages. Rather, it declares areference variable capable of storing a reference to a one-dimensional array object, which in turn is capable of storing references to one-dimensionalarray 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 variableindicates 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 module).

The code in Listing 1 shows two levels of nesting for the reference variable of type

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 ofelements 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).

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 specifiedtype 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 storedin 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 ofelement 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 module, it is also capable of storing a reference to any other array object as well.)