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Trail: Learning the Java Language
Lesson: Language Basics
Section: Variables
Language Basics
Variables
Primitive Data Types
Arrays
Summary of Variables
Questions and Exercises
Operators
Assignment, Arithmetic, and Unary Operators
Equality, Relational, and Conditional Operators
Bitwise and Bit Shift Operators
Summary of Operators
Questions and Exercises
Expressions, Statements, and Blocks
Questions and Exercises
Control Flow Statements
The if-then and if-then-else Statements
The switch Statement
The while and do-while Statements
The for Statement
Branching Statements
Summary of Control Flow Statements
Questions and Exercises
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Arrays

> An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed. You've seen an example of arrays already, in the `main` method of the "Hello World!" application. This section discusses arrays in greater detail.

> An array of ten elements

> Each item in an array is called an element, and each element is accessed by its numerical index. As shown in the above illustration, numbering begins with 0. The 9th element, for example, would therefore be accessed at index 8.

> The following program, `ArrayDemo`, creates an array of integers, puts some values in it, and prints each value to standard output.

> >     class ArrayDemo {
> >          public static void main(String[] args) {
> >               int[] anArray;              // declares an array of integers

> >               anArray = new int[10];      // allocates memory for 10 integers

> >               anArray[0] = 100; // initialize first element
> >               anArray[1] = 200; // initialize second element
> >               anArray[2] = 300; // etc.
> >               anArray[3] = 400;
> >               anArray[4] = 500;
> >               anArray[5] = 600;
> >               anArray[6] = 700;
> >               anArray[7] = 800;
> >               anArray[8] = 900;
> >               anArray[9] = 1000;

> >               System.out.println("Element at index 0: " + anArray[0]);
> >               System.out.println("Element at index 1: " + anArray[1]);
> >               System.out.println("Element at index 2: " + anArray[2]);
> >               System.out.println("Element at index 3: " + anArray[3]);
> >               System.out.println("Element at index 4: " + anArray[4]);
> >               System.out.println("Element at index 5: " + anArray[5]);
> >               System.out.println("Element at index 6: " + anArray[6]);
> >               System.out.println("Element at index 7: " + anArray[7]);
> >               System.out.println("Element at index 8: " + anArray[8]);
> >               System.out.println("Element at index 9: " + anArray[9]);
> >          }
> >     }

> The output from this program is:

> >     Element at index 0: 100
> >     Element at index 1: 200
> >     Element at index 2: 300
> >     Element at index 3: 400
> >     Element at index 4: 500
> >     Element at index 5: 600
> >     Element at index 6: 700
> >     Element at index 7: 800
> >     Element at index 8: 900
> >     Element at index 9: 1000

> In a real-world programming situation, you'd probably use one of the supported looping constructs to iterate through each element of the array, rather than write each line individually as shown above. However, this example clearly illustrates the array syntax. You'll learn about the various looping constructs (`for`, `while`, and `do-while`) in the Control Flow section.

> ### Declaring a Variable to Refer to an Array

> The above program declares `anArray` with the following line of code:

> >     int[] anArray;          // declares an array of integers

> Like declarations for variables of other types, an array declaration has two components: the array's type and the array's name. An array's type is written as `type[]`, where `type` is the data type of the contained elements; the square brackets are special symbols indicating that this variable holds an array. The size of the array is not part of its type (which is why the brackets are empty). An array's name can be anything you want, provided that it follows the rules and conventions as previously discussed in the naming section. As with variables of other types, the declaration does not actually create an array — it simply tells the compiler that this variable will hold an array of the specified type.

> Similarly, you can declare arrays of other types:

> >     byte[] anArrayOfBytes;
> >     short[] anArrayOfShorts;
> >     long[] anArrayOfLongs;
> >     float[] anArrayOfFloats;
> >     double[] anArrayOfDoubles;
> >     boolean[] anArrayOfBooleans;
> >     char[] anArrayOfChars;
> >     String[] anArrayOfStrings;

> You can also place the square brackets after the array's name:

> >     float anArrayOfFloats[]; // this form is discouraged

> However, convention discourages this form; the brackets identify the array type and should appear with the type designation.

> ### Creating, Initializing, and Accessing an Array

> One way to create an array is with the `new` operator. The next statement in the `ArrayDemo` program allocates an array with enough memory for ten integer elements and assigns the array to the `anArray` variable.

> >     anArray = new int[10];  // create an array of integers

> If this statement were missing, the compiler would print an error like the following, and compilation would fail:

> >     ArrayDemo.java:4: Variable anArray may not have been initialized.

> The next few lines assign values to each element of the array:

> >     anArray[0] = 100; // initialize first element
> >     anArray[1] = 200; // initialize second element
> >     anArray[2] = 300; // etc.

> Each array element is accessed by its numerical index:

> >     System.out.println("Element 1 at index 0: " + anArray[0]);
> >     System.out.println("Element 2 at index 1: " + anArray[1]);
> >     System.out.println("Element 3 at index 2: " + anArray[2]);

> Alternatively, you can use the shortcut syntax to create and initialize an array:

> >     int[] anArray = {100, 200, 300, 400, 500, 600, 700, 800, 900, 1000};

> Here the length of the array is determined by the number of values provided between { and }.

> You can also declare an array of arrays (also known as a multidimensional array) by using two or more sets of square brackets, such as `String[][] names`. Each element, therefore, must be accessed by a corresponding number of index values.

> In the Java programming language, a multidimensional array is simply an array whose components are themselves arrays. This is unlike arrays in C or Fortran. A consequence of this is that the rows are allowed to vary in length, as shown in the following MultiDimArrayDemo program:

> >     class MultiDimArrayDemo {
> >         public static void main(String[] args) {
> >             String[][] names = {{"Mr. ", "Mrs. ", "Ms. "},
> >                                 {"Smith", "Jones"}};
> >             System.out.println(names[0][0] + names[1][0]); //Mr. Smith
> >             System.out.println(names[0][2] + names[1][1]); //Ms. Jones
> >         }
> >     }

> The output from this program is:

> >         Mr. Smith
> >         Ms. Jones

> Finally, you can use the built-in `length` property to determine the size of any array. The code

> >      System.out.println(anArray.length);

> will print the array's size to standard output.

> ### Copying Arrays

> The `System` class has an `arraycopy` method that you can use to efficiently copy data from one array into another:

> >     public static void arraycopy(Object src,
> >                                  int srcPos,
> >                                  Object dest,
> >                                  int destPos,
> >                                  int length)

> The two `Object` arguments specify the array to copy from and the array to copy to. The three `int` arguments specify the starting position in the source array, the starting position in the destination array, and the number of array elements to copy.

> The following program, `ArrayCopyDemo`, declares an array of `char` elements, spelling the word "decaffeinated". It uses `arraycopy` to copy a subsequence of array components into a second array:

> >     class ArrayCopyDemo {
> >         public static void main(String[] args) {
> >             char[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e',
> >     			    'i', 'n', 'a', 't', 'e', 'd' };
> >             char[] copyTo = new char[7];

> >             System.arraycopy(copyFrom, 2, copyTo, 0, 7);
> >             System.out.println(new String(copyTo));
> >         }
> >     }

> The output from this program is:

> >     caffein

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