Java Lessons!

Arrays

Arrays are generally effective means of storing groups of variables. An array is a group of variables that share the same name and are ordered sequentially from zero to one less than the number of variables in the array. The number of variables that can be stored in an array is called the array’s dimension. Each variable in the array is called an element of the array.

Creating Arrays
There are three steps to creating an array, declaring it, allocating it and initializing it.
Declaring Arrays
Like other variables in Java, an array must have a specific type like byte, int, String or double. Only variables of the appropriate type can be stored in an array. You cannot have an array that will store both ints and Strings, for instance.
Like all other variables in Java an array must be declared. When you declare an array variable you suffix the type with [] to indicate that this variable is an array. Here are some examples:
int[] k;
float[] yt;
String[] names;

In other words you declare an array like you’d declare any other variable except you append brackets to the end of the variable type.Allocating Arrays
Declaring an array merely says what it is. It does not create the array. To actually create the array (or any other object) use the new operator. When we create an array we need to tell the compiler how many elements will be stored in it. Here’s how we’d create the variables declared above: newk = new int[3];
yt = new float[7];
names = new String[50];

The numbers in the brackets specify the dimension of the array; i.e. how many slots it has to hold values. With the dimensions above k can hold three ints, yt can hold seven floats and names can hold fifty Strings.Initializing Arrays
Individual elements of the array are referenced by the array name and by an integer which represents their position in the array. The numbers we use to identify them are called subscripts or indexes into the array. Subscripts are consecutive integers beginning with 0. Thus the array k above has elements k[0], k[1], and k[2]. Since we started counting at zero there is no k[3], and trying to access it will generate an ArrayIndexOutOfBoundsException. subscripts indexes k k[0] k[1] k[2] k[3]ArrayIndexOutOfBoundsExceptionYou can use array elements wherever you’d use a similarly typed variable that wasn’t part of an array.
Here’s how we’d store values in the arrays we’ve been working with:
k[0] = 2;
k[1] = 5;
k[2] = -2;
yt[6] = 7.5f;
names[4] = “Fred”;
This step is called initializing the array or, more precisely, initializing the elements of the array. Sometimes the phrase “initializing the array” would be reserved for when we initialize all the elements of the array.
For even medium sized arrays, it’s unwieldy to specify each element individually. It is often helpful to use for loops to initialize the array. For instance here is a loop that fills an array with the squares of the numbers from 0 to 100.
float[] squares = new float[101];
for (int i=0; i
Shortcuts
We can declare and allocate an array at the same time like this:
int[] k = new int[3];
float[] yt = new float[7];
String[] names = new String[50];

We can even declare, allocate, and initialize an array at the same time providing a list of the initial values inside brackets like so:int[] k = {1, 2, 3};
float[] yt = {0.0f, 1.2f, 3.4f, -9.87f, 65.4f, 0.0f, 567.9f};

Two Dimensional Arrays
Declaring, Allocating and Initializing Two Dimensional Arrays
Two dimensional arrays are declared, allocated and initialized much like one dimensional arrays. However we have to specify two dimensions rather than one, and we typically use two nested for loops to fill the array. forThe array examples above are filled with the sum of their row and column indices. Here’s some code that would create and fill such an array:
class FillArray {
public static void main (String args[]) {
int[][] M;
M = new int[4][5];
for (int row=0; row
In two-dimensional arrays ArrayIndexOutOfBounds errors occur whenever you exceed the maximum column index or row index. Unlike two-dimensional C arrays, two-dimensional Java arrays are not just one-dimensional arrays indexed in a funny way.Multidimensional Arrays
You don’t have to stop with two dimensional arrays. Java lets you have arrays of three, four or more dimensions. However chances are pretty good that if you need more than three dimensions in an array, you’re probably using the wrong data structure. Even three dimensional arrays are exceptionally rare outside of scientific and engineering applications.The syntax for three dimensional arrays is a direct extension of that for two-dimensional arrays. Here’s a program that declares, allocates and initializes a three-dimensional array:
class Fill3DArray {
public static void main (String args[]) {
int[][][] M;
M = new int[4][5][3];
for (int row=0; row
Example 1 : declaring and initializing 1-dimensional arrays
An array groups elements of the same type. It makes it easy to manipulate the information contained in them.
class Arrays1{
public static void main(String args[]){
// this declares an array named x with the type “array of int” and of
// size 10, meaning 10 elements, x[0], x[1] , … , x[9] ; the first term
// is x[0] and the last term x[9] NOT x[10].
int x[] = new int[10];
// print out the values of x[i] and they are all equal to 0.
for(int i=0; i
Example 2 : Find the sum of the numbers 2.5, 4.5, 8.9, 5.0 and 8.9
class Arrays2{
public static void main(String args[]){
// this declares an array named fl with the type “array of int” and
// initialize its elements
float fl[] = {2.5f, 4.5f, 8.9f, 5.0f, 8.9f};
// find the sum by adding all elements of the array fl
float sum = 0.0f;
for(int i=0; i
Check that the sum displayed is 29.8.
Example 3 : declaring and initializing 2-dimensional arrays
class Arrays3{
public static void main(String args[]){
// this declares a 2-dimensional array named x[i][j] of size 4 (4 elements)
// its elements are x[0][0], x[0][1], x[1][0] and x[1][1].
// the first index i indicates the row and the second index indicates the
// column if you think of this array as a matrix.
int x[][] = new int[2][2];
// print out the values of x[i][j] and they are all equal to 0.0.
for(int i=0; i

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