Arrays

Introduction

• Arrays store multiple variables of the same type.
• The array itself is always an object on the heap (even if it is storing primitive elements).
• Arrays can be multi-dimensional.
• Think of a multi-dimensional array as an array of arrays.

Declaring

• int[] scores; (Preferred) or int scores[]; (Legal, but bad)
• String[] names;
• int[][] matrix;

Constructing

• int[] scores = new int[10];
• This will create a new array object on the heap. All the int values will be assigned their default values, Object references will be assigned null.
• The array size must be present, when there is no initializer.
• double[] rates = new double[]; //illegal
• Thread[] pool = new Thread[10]; //No thread objects are created here, only the array object to hold ten thread references is created.
• int[][] matrix = new int[5][];
• Only the size of the array needs to be specified - in this case 5 says that matrix 2-D array can store 5 1-D array objects. So only the 5 is sufficient to be specified.
• Of course we can also specify both dimensions like int[][] matrix = new int[3][2]; Which menas that matrix can store 3 1-D arrays each of length 2.

Initializing

• Initializing means assigning values to the elements of the array.
• Initializing can be done after the array is declared and constructed.
• Initializing can also be combined along with the array declaration and construction.

String[] names; //Declare
names = new String[3]; //Construct
//Initialize
names[0] = "A";
names[1] = "B";
names[2] = "C";

//All in one
String[] names = new String[] {"A", "B", "C"};

//Multi-dimensional - initing a 3x3 int array
int[][] matrix = { {1,2,3}, {4,5,6}, {7,8,9} };

Illegal	Legal
int[5] scores; //No size in LHS. int[] scores = new int[]; //No size specified; int[3] scores = new int[3] {1, 2, 3}; //Cant specify both size and initializer int[] scores; scores = {1,2,3}; //Intializer being used, without array being constructed.	int[] scores; scores = new int[] {1,2,3}; //OK int[] scores = {1, 2, 3 }; //OK - concise. int[] scores = new int[] {1, 2 , 3}; /OK