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Trail: Learning the Java Language
Lesson: Data Structures

## Arrays and Strings

> Like other programming languages, Java allows you to collect and manage multiple values through an array object. You manage data comprised of multiple characters through a `String` object.

### Arrays

> The `countChars` method doesn't use an array, but the `Count` class's `main` method declares an array as a parameter to `main`. This section shows you how to create and use arrays in your Java programs.

> As for other variables, before you can use an array you must first declare it. Again, like other variables, a declaration of an array has two primary components: the array's type and the array's name. An array's type includes the data type of the elements contained within the array. For example, the data type for an array that contained all integer elements is array of integers. You cannot have a generic array--the data type of its elements must be identified when the array is declared. Here's a declaration for an array of integers:

> >     int[] arrayOfInts;

> The `int[]` part of the declaration indicates that `arrayOfInts` is an array of integers. The declaration does not allocate any memory to contain the array elements.

> If your program attempted to assign or access any values to any elements of `arrayOfInts` before memory for it has been allocated the compiler will print an error like this one and refuse to compile your program.

> >     testing.java:64: Variable arrayOfInts may not
> >         have been initialized.

> To allocate memory for the elements of the array, you must instantiate the array. You do this using Java's `new` operator. (Actually, the steps you take to create an array are similar to the steps you take to create an object from a class: declaration, instantiation, and initialization. You can learn more about creating objects in the Creating Objects section of the next lesson.

> The following statement allocates enough memory for `arrayOfInts` to contain ten integer elements.

> >     int[] arrayOfInts = new int[10]

> In general, when creating an array, you use the `new` operator, plus the data type of the array elements, plus the number of elements desired enclosed within square brackets ('[' and ']').

> >     elementType[] arrayName = new elementType[arraySize]

> Now that some memory has been allocated for your array, you can assign values to its elements and retrieve those values:

> >     for (int j = 0; j < arrayOfInts.length; j ++) {
> >         arrayOfInts[j] = j;
> >         System.out.println("[j] = " + arrayOfInts[j]);
> >     }

> This example shows that to reference an array element, you append square brackets to the array name. Between the square brackets indicate (either with a variable or some other expression) the index of the element you want to access. Note that in Java, array indices begin at 0 and end at the array length minus 1.

> There's another interesting element (so to speak) in the small code sample above. The `for` loop iterates over each element of `arrayOfInts`, assigning values to its elements and printing out those values. Notice the use of `arrayOfInts.length` to retrieve the current length of the array. `length` is a property provided for all Java arrays.

> Let's look again at the `main` method that calls `countChars`. In particular note the use of the `args` array:

> >     import java.io.*;
> >     public class Count {
> >         // ... countChars method omitted ...
> >         public static void main(String[] args) throws Exception {
> >             if (args.length >= 1)
> >                 countChars(new FileReader(args[0]));
> >             else
> >                 System.err.println("Usage: Count filename");
> >         }
> >     }

> The Java runtime allocates the space for the args array, so `main` doesn't have to bother with it. The `main` method ensures that there's at least one element in the `args` array, and if there is, it uses the first element in the array (presumably the name of a file) to open a `FileReader`.

> Arrays can contain any legal Java data type including reference types such as objects or other arrays. For example, the following declares an array that can contain ten `String` objects.

> >     String[] arrayOfStrings = new String[10];

> The elements in this array are reference types, that is, each element contains a reference to a `String` object. At this point, enough memory has been allocated to contain the `String` references, but no memory has been allocated for the `String`s themselves. If you attempted to access one of `arrayOfStrings` elements at this point, you would get a `NullPointerException` because the array is empty and contains no `String`s and no `String` objects. This is often a source of some confusion for programmers new to the Java language. You have to allocate the actual `String` objects separately:

> >     for (int i = 0; i < arrayOfStrings.length; i ++) {
> >         arrayOfStrings[i] = new String("Hello " + i);
> >     }

### Strings

> A sequence of character data is called a string and is implemented in the Java environment by the `String` class (a member of the `java.lang` package). `Count`'s `main` method uses `String`s in its the declaration of the `args` array:

> >     String[] args

> This code explicitly declares an array named `args` that contains `String` objects. The empty brackets indicate that the length of the array is unknown at compilation time because the array is passed in at runtime.

> The `countChars` method also uses two `String`s both in the form of a literal string (a string of characters between double quotation marks):

> >     "Counted "
> >         . . .
> >     " chars."

> The program implicitly allocates two `String` objects, one for each of the two literal strings shown previously.

> `String` objects are immutable--that is, they cannot be changed once they've been created. The `java.lang` package provides a different class, `StringBuffer`, which you can use to create and manipulate character data on the fly. Using String and StringBuffer covers thoroughly the use of both the `String` and `StringBuffer` classes.

> ### String Concatenation

> > Java lets you concatenate strings together easily using the `+` operator. The `countChars` method uses this feature of the Java language to print its output. The following code snippet concatenates three strings together to produce its output:

> > >     "Counted " + count + " chars."

> > Two of the strings concatenated together are literal strings: "`Counted` " and "`chars.`" The third string--the one in the middle--is actually an integer that first gets converted to a string and then is concatenated to the others.

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