Arrays in Java

ARRAYS

What is an Array?

• An array is a group of variables of the same type that is referred to by a common name
• Arrays are objects
• Indexing is 0-based, meaning that the variables contained in arrays are ordered with indexes 0 to n-1, where n is the number of elements or size of the array
• The size of an array must be specified by an int value
• They may contain primitive data types as well as objects of a class
• There are two types of arrays: single dimensional and multidimensional
• Arrays: Groups of Same-Type Variables

Pros and Cons of Arrays

• The Good
• An array optimizes the code, making it easy to sort or retrieve data
• Any data located at any index position can be accessed
• Arrays provide fast access to data
• The Bad
• An array can store only a fixed number of elements
• Arrays can hold only a collection of similar items; the items cannot be of different types
• Insertion and deletion in the middle of arrays are tedious (causes shifts)
• Requires a large enough block of memory; non-ideal if the size of the data grows exceedingly large
• Arrays are easily corrupted

Declaring a 1-D Array

• Declaring and instantiating a one-dimensional array can be combined as shown in the format below:
• type name[] = new type[size];
• type is the data type the array will store
• name is the reference of the array
• size is the number of elements the array will contain
• Examples:
• double numbers[] = new double[12];
• String names[] = {"Pam", "Jim", "Dwight", "Michael"}
• int heights[] = new int[6];
• Understanding Declarations

Some Methods and Examples

• public static void sort(Object[] a)
• Sorts the specified array in ascending order
• public static void fill(int[] a, int val)
• Assigns every index of the array to the specified value; can be utilized with other types (boolean, byte, char, double, etc.)
• public static List asList(type[] a)
• Returns a fixed-size list backed by the specified array
• Sorting an Array
• Fill Method
• asList Method

Multidimensional Arrays

• The easiest multidimensional array to understand is a two-dimensional array, which is an array of arrays
• Think of the number of rows in a 2-D array as the number of arrays found within the array
• The picture to the right provides a visual representation of a 2-D array; this one array contains 3 (rows) arrays, with each additional array containing 4 elements
• A three-dimensional array is an array of arrays of arrays
• This pattern continues with every additional dimension you wish to add
• 

Declaring a 2-D Array

• 2-D arrays are declared and instantiated using the format below:
• type[][] name = new type[rows][columns];
• type is the data type the array will contain
• name is the name of the array which will reference it
• rows is the number of rows in the array
• columns is the number of columns in the arry
• Examples:
• int[][] calories = new int[5][7];
• double[][] height = {{46.2, 48.3, 52.1}, {51.4, 49.9, 54.6}, {61.3, 64.2, 49.8}};
• Visualizing a 2-D Array
• Examples of 2-D Declarations