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Java Data Structures - Contents

The Vector...


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    The java.util.Vector class is provided by the Java API, and is one of the most useful array based data storage classes I've ever seen. The information provided here is as far as JDK 1.2 goes, future versions may have other implementations; still, the functionality should remain the same. A vector, is a growing array; as more and more elements are added onto it, the array grows. There is also a possibility of making the array smaller.

    But wait a minute, all this time I've been saying that arrays can't grow or shrink, and it seems Java API has done it. Not quite. The java.util.Vector class doesn't exactly grow, or shrink. When it needs to do these operations, it simply allocates a new array (of appropriate size), and copies the contents of the old array into the new array. Thus, giving the impression that the array has changed size.

    All these memory operations can get quite expensive if a Vector is used in a wrong way. Since a Vector has a similar architecture to the array stack we've designed earlier, the best and fastest way to implement a Vector is to do stack operations. Usually, in programs, we need a general data storage class, and don't really care about the order in which things are stored or retrieved; that's where java.util.Vector comes in very useful.

    Using a Vector to simulate a queue is very expensive, since every time you insert or remove, the entire array has to be copied (not necessarily reallocated but still involves lots of useless work).

    Vector allows us to view it's insides using an Enumerator; a class to go through objects. It is very useful to first be able to look what you're looking for, and only later decide whether you'd like to remove it or not. A sample program that uses java.util.Vector for it's storage follows.

import java.util.*;

class pVectorTest{
    public static void main(String[] args){
        Vector v = new Vector(15);
        Integer j = null;
        int i;
            j = new Integer((int)(Math.random() * 100));
            System.out.println("addElement: " + j);
        System.out.println("size: "+v.size());
        System.out.println("capacity: "+v.capacity());

        Enumeration enum = v.elements();
            System.out.println("enum: "+(Integer)enum.nextElement());

        System.out.println("Done ;-)");

    The example above should be self evident (if you paid attention when I showed test programs for the previous data structures). The main key difference is that this one doesn't actually remove objects at the end; we just leave them inside. Removal can be accomplished very easily, and if you'll be doing anything cool with the class, you'll sure to look up the API specs.

    Printing is accomplished using an Enumerator; which we use to march through every element printing as we move along. We could also have done the same by doing a for loop, going from 0 to v.size(), doing a v.elementAt(int) every time through the loop. The output from the above program follows:

addElement: 9
addElement: 5
addElement: 54
addElement: 49
addElement: 60
addElement: 81
addElement: 8
addElement: 91
addElement: 76
addElement: 81
size: 10
capacity: 15
enum: 9
enum: 5
enum: 54
enum: 49
enum: 60
enum: 81
enum: 8
enum: 91
enum: 76
enum: 81
Done ;-)

    You should notice that when we print the size and capacity, they're different. The size is the current number of elements inside the Vector, and the capacity, is the maximum possible without reallocation.

    A trick you can try yourself when playing with the Vector is to have Vectors of Vectors (since Vector is also an Object, there shouldn't be any problems of doing it). Constructs like that can lead to some interesting data structures, and even more confusion. Just try inserting a Vector into a Vector ;-)

    I guess that covers the Vector class. If you need to know more about it, you're welcome to read the API specs for it. I also greatly encourage you to look at java.util.Vector source, and see for yourself what's going on inside that incredibly simple structure.

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