Answer Posted / raju

We know that arrays and Vectors in Java are different kinds
of data structures for storing information in a
random-access fashion. In Java, an object-oriented language,
the two structures are implimented as classes. This
presentation will demonstrate some of the principle
differences between Java arrays and Java Vectors. It will
also provide some examples of the cool methods that come
with them.

Arrays

Arrays in Java are static lists declared to store a certain
number of a certain kind of variables. It stores these
values at specific, numbered locations in a list that starts
at zero and goes to N-1:

myArray Index Data
0 14
1 79
2 32
3 92
4 102

This array can be instantiated in Java with either the
shorthand syntax:

int[] myArray = new int[5];
int myArray[] = new int[5];

or an explicit method call to a Java "Array" class object:

int[] myArray = Array.newInstance[int, 5];

When an array is instantiated, the array's constructor
method allocates 5 integer-sized memory slots. The
allocation is static- the array size cannot change after it
is created. In this sense, it is analagous to a C or C++
style array. But the resemblence doesn't end here. Java
Arrays, like C arrays, have undefined contents until they
are initialized. One can set the contents of an array at the
time of instantiation with the simple syntax:

int[] myArray = {14,34,21,42,22};

We have already seen that, in addition to the built-in
C-style array operations syntax there Java provides
additional classes for manipulating arrays. This represents
a desire on the Java authors' part to both a) retain a
grounding in C to attract those programmers already familiar
with the popular language, and b) provide true
object-oriented support for array operations. This is true
for array referencing, as well. Various array-handling
objects exist in the class library java.lang. This provides
interface functions for various array operations For
example, one could refer to the individual array as in C:

fourth_item = myArray[3];

or, one could use the explicit method call:

fourth_item = myArray[3];

Java arrays can also be passed as parameters to functions.
Java permits either a copy of the original array or a
pointer to be sent.
2D-arrays are also supported. This is a statically-allocated
array of array pointers:

int[][] 2D_array = new int[x_length][y_length];

Finally, note that vectors of objects can exist. This does
not mean, however, that the new objects are instantiated
when the array is declared. Arrays take up a fixed size in
memory, and the size of an object isn't defined when it is
instantiated. Therefore, the objects must be instantiated in
a seperate command. Therefore, to create an array of 5
objects, the following code could be used:

object_list = new Object[5];
object_list[0] = new Object();
object_list[1] = new Object();
.
.
.
Vectors

The key difference between Arrays and Vectors in Java is
that Vectors are dynamically-allocated. They aren't declared
to contain a type of variable; instead, each Vector contains
a dynamic list of references to other objects. The Vector
class is found in the java.util package, and extends
java.util.Abstractlist.

The big advantage of using Vectors is that the size of the
vector can change as needed. Vectors handle these changes
through the "capacity" and "capacityIncrement" fields. When
a Vector is instantiated, it declares an object array of
size initialCapacity. Whenever this array fills, the vector
increases the total buffer size by the value
capacityIncrement. Thus, a Vector represents a compromise.
It doesn't allocate each object dynamically, which would
make access of middle list-items exceedingly slow; but it
does allow for growth in the size of the list. The default
constructor, Vector(), creates an empty Vector of initial
capacity zero that doubles in size whenever it fills.

Vectors can be filled using its method addElement(Object
obj). This method appends the given object to the end of the
Vector. Because the Vector stores pointers to the objects,
and not the objects themselves, these Vector items could be
any kind of object. Moreover, several different kinds of
objects could exist in the same Vector. Vector methods allow
all sorts of nifty ways to manipulate data. For example, one
can reference a list item not just by its index, but also by
name. Elements can be removed from or added to the middle of
the list:

myVector.insertElementAt(my_object, 5);
success_flag = myVector.remove(my_object);

So you're thinking, "GREAT! But if vectors are so wonderful,
why even bother with arrays?"
Comparative Advantages of Arrays and Vectors

Because Vectors are dynamically-allocated, they offer a
relatively memory-efficient way of handling lists whose size
could change drastically. The line buffer of a text editor,
for example, could be anywhere from zero to thousands of
entries in size. A Vector might be a good choice for such an
arrangement. Moreover, Vectors have some useful member
functions that make coding simpler (see the insertElementAt
and remove methods above). But because the Vector class is
based on an array of object references, these methods are
generally no more efficient than array-based algorithms. The
insert method must perform multiple "swap" operations just
as an array 'insert' algorithm would. Thus, Vectors are
easier to use than arrays for most applications, but they do
not offer all the performance advantages of fully-dynamic
storage.

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695

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2286

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