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Java Data Structures - Contents

Arrays...

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    One of the most basic data structures, is an array. An array is just a number of items, of same type, stored in linear order, one after another. Arrays have a set limit on their size, they can't grow beyond that limit. Arrays usually tend to be easier to work with and generally more efficient than other structural approaches to organizing data; way better than a no formal structure approach.

    For example, lets say you wanted to have 100 numbers. You can always resort to having 100 different variables, but that would be a pain. Instead, you can use the clean notation of an array to create, and later manipulate those 100 numbers. For example, to create an array to hold 100 numbers you would do something like this:

int[] myArray;
myArray = new int[100];

or

int[] myArray = new int[100];

or

int myArray[] = new int[100];

    The three notations above do exactly the same thing. The first declares an array, and then it creates an array by doing a new. The second example shows that it can all be one in one line. And the third example shows that Java holds the backwards compatibility with C++, where the array declaration is: int myArray[]; instead of int[] myArray;. To us, these notations are exactly the same. I do however prefer to use the Java one.

    Working with arrays is also simple, think of them as just a line of variables, we can address the 5th element (counting from 0, so, it's actually the 6th element) by simply doing:

int i = myArray[5];

    The code above will set integer 'i' to the value of the 5th (counting from 0) element of the array. Similarly, we can set an array value. For example, to set the 50th element (counting from 0), to the value of 'i' we'd do something like:

myArray[50] = i;

    As you can see, arrays are fairly simple. The best and most convenient way to manipulate arrays is using loops. For example, lets say we wanted to make an array from 1 to 100, to hold numbers from 1 to 100 respectively, and later add seven to every element inside that array. This can be done very easily using two loops. (actually, it can be done in one loop, but I am trying to separate the problem into two)

int i;
for(i=0;i<100;i++)
    myArray[i] = i;
for(i=0;i<100;i++)
    myArray[i] = myArray[i] + 7;

    In Java, we don't need to remember the size of the array as in C/C++. Here, we have the length variable in every array, and we can check it's length whenever we need it. So to print out any array named: myArray, we'd do something like:

for(int i = 0;i<myArray.length;i++)
    System.out.println(myArray[i]);

    This will work, given the objects inside the myArray are printable, (have a corresponding toString() method), or are of primitive type.

    One of the major limitations on arrays is that they're fixed in size. They can't grow or shrink according to need. If you have an array of 100 max elements, it will not be able to store 101 elements. Similarly, if you have less elements, then the unused space is being wasted (doing nothing).

    Java API provides data storage classes, which implement an array for their storage. As an example, take the java.util.Vector class (JDK 1.2), it can grow, shrink, and do some quite useful things. The way it does it is by reallocating a new array every time you want to do some of these operations, and later copying the old array into the new array. It can be quite fast for small sizes, but when you're talking about several megabyte arrays, and every time you'd like to add one more number (or object) you might need to reallocate the entire array; that can get quite slow. Later, we will look at other data structures where we won't be overly concerned with the amount of the data and how often we need to resize.

    Even in simplest situations, arrays are powerful storage constructs. Sometimes, however, we'd like to have more than just a plain vanilla array.


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