Sams Teach Yourself Java in 24 Hours, 7th Edition (2014)

The previous examples create arrays, but they do not store any values in them. To do this, you can use the new keyword along with the variable type or store values in the array within { and } marks. When using new, you must specify how many different items are stored in the array. Each item in an array is called an element. The following statement creates an array and sets aside space for the values that it holds:

int[] elfSeniority = new int[250];

This example creates an array of integers called elfSeniority. The array has 250 elements that can store the months that each of Santa’s elves has been employed at the Pole. (If Santa runs a union shop, this information is extremely important to track.)

When you create an array with the new statement, you must specify the number of elements. Each element of the array is given an initial value that depends on the type of the array. All numeric arrays have the initial value 0, char arrays equal '\0', and boolean arrays have the valuefalse. A String array and all other objects are created with the initial value of null.

For arrays that are not extremely large, you can set up their initial values at the same time that you create them. The following example creates an array of strings and gives them initial values:

String[] reindeerNames = { "Dasher", "Dancer", "Prancer", "Vixen",
"Comet", "Cupid", "Donder", "Blitzen" };

The information that should be stored in elements of the array is placed between { and } brackets with commas separating each element. The number of elements in the array is set to the number of elements in the comma-separated list.

Array elements are numbered, beginning with 0 for the first element. A specific element can be accessed by referring to this number within [ and ] brackets. The preceding statement accomplishes the same thing as the following code:

String[] reindeerNames = new String[8];
reindeerNames[0] = "Dasher";
reindeerNames[1] = "Dancer";
reindeerNames[2] = "Prancer";
reindeerNames[3] = "Vixen";
reindeerNames[4] = "Comet";
reindeerNames[5] = "Cupid";
reindeerNames[6] = "Donder";
reindeerNames[7] = "Blitzen";

Each element of the array must be of the same type. Here, a string is used to hold each of the reindeer names.

After the array is created, you cannot make room for more elements. Even if you recall the most famous reindeer of all, you can’t add “Rudolph” as the ninth element of the reindeerNames array. The Java compiler won’t let poor Rudolph join in any reindeerNames.

Using Arrays

You use arrays in a program as you would any variable, with one difference: The element number of the array item must be provided between the square brackets next to the array’s name. You can use an array element anywhere a variable could be used. The following statements all use arrays that have already been defined in this hour’s examples:

elfSeniority[193] += 1;
niceChild[9428] = "Eli";
currentNation = 413;
if (hostileAirTravelNations[currentNation] == true) {
sendGiftByMail();
}

Because the first element of an array is numbered 0 instead of 1, this means that the highest number is one less than you might expect. Consider the following statement:

String[] topGifts = new String[10];

This statement creates an array of string variables numbered from 0 to 9. If you referred to topGifts[10] somewhere in the program, you would get an error message referring to an ArrayIndexOutOfBoundsException.

Exception is another word for error in Java programs. This exception is an “array index out of bounds” error, which means that a program tried to use an array element that doesn’t exist within its defined boundaries. You learn more about exceptions during Hour 18, “Handling Errors in a Program.”

If you want to check the upper limit of an array so you can avoid going beyond that limit, a variable called length is associated with each array that is created. The length variable is an integer that contains the number of elements an array holds. The following example creates an array and then reports its length:

String[] reindeerNames = { "Dasher", "Dancer", "Prancer", "Vixen",
"Comet", "Cupid", "Donder", "Blitzen", "Rudolph" };
System.out.println("There are " + reindeerNames.length + " reindeer.");

In this example, the value of reindeerNames.length is 9, which means that the highest element number you can specify is 8.

You can work with text in Java as a string or an array of characters. When you’re working with strings, one useful technique is to put each character in a string into its own element of a character array. To do this, call the string’s toCharArray() method, which produces a char array with the same number of elements as the length of the string.

This hour’s first project uses both of the techniques introduced in this section. The SpaceRemover program displays a string with all space characters replaced with periods (.).

To get started, open the Java24 project in NetBeans, choose File, New File and create a new Empty Java File called SpaceRemover. Enter Listing 9.1 in the source editor and save it when you’re done.

LISTING 9.1 The Full Text of SpaceRemover.java