Arrays

“An array is a fixed-length sequence of values of the same type, which can be an object type or a primitive type” (Horstmann, 2002, p. 530). Each value is stored in an element of the array, and each element is identified by its index: an integer which is the element’s sequence number.

Array elements are numbered from 0. For example: an array of 10 elements has indexes which run from 0 through 9. In general: the last element of an array is at length - 1.

An array is an object, and every array object knows its length: the number of elements which can be stored within it. The length of an array is stored in a public field called length and can be accessed with the dot operator.

To summarize: arrays

1. are objects
2. know their length via a public field: length
3. contain elements numbered from 0
4. contain elements all of the same type
5. may contain elements whose type is also an array

Initialization

Each element of an array is automatically initialized upon its creation to

• 0, for numeric elements
• false, for boolean elements
• '\u0000', for char elements
• null, for object elements

/*
 * Example 1:
 *
 * Declare object reference and create array
 * with explicit size, then initialize.
 */

int[] validYear = new int[3];
…
validYear[0] = 2000;
validYear[1] = 2001;
validYear[2] = 2002;

/*
 * Example 2:
 *
 * Declare object reference then create array and
 * initialize simultaneously. Size of array is
 * calculated by compiler.
 */

String[] vitamin = {"chocolate", "sugar", "caffeine"};

// Every array knows its length.
System.out.println(vitamin.length);

/*
 * Example 3:
 *
 * Declare object reference first;
 * create array and initialize later.
 */

int[] validYear;
…
validYear = new int[3];
validYear[0] = 2000;
validYear[1] = 2001;
validYear[2] = 2002;

/* Example 4:
 *
 * Declare object reference first;
 * create array and initialize later.
 */

char[] grade;
…
grade = new char[] {'A', 'B', 'C', 'D', 'F'};

Reference

Horstmann, C.S. (2002). Big Java. John Wiley & Sons, Inc. ISBN 0-471-40248-6.

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