Arrays

Arrays  When a value is to be used in a program, a variable is declared to hold that value  What about storing the results of exams for a large class…200 results = 200 variable declarations?  Solution is to use Arrays… An array is a structure that holds multiple values of the same type. The length of an array is established when the array is created (at runtime). After creation, an array is a fixed-length structure.

 Arrays are Objects and must be declared as such (use the new key word) int arrayExample[] = new int[10];  Creates an array of 10 Integers (all elements are of the same datatype e.g. int, float, Student etc.)  Two parts to declaration… int arrayExample[] tells compiler there will be an array of integers referenced by the name arrayExample new int[10]assigns the size of the Array (sets it aside in memory – space for 10 integers needed) Arrays

Where confusion can arise… Arrays can be declared and initialised in either of the following ways… int[] arrayExample = new int[10]; int arrayExample[] = new int[10];  Both are correct and are down to preference, many different text books and programmers use both styles Declare & Create in one statement

 Each place holder in an array is known as an element (we have 10 elements of type integer)  Each element is uniquely identified by an index  To refer to an element… arrayExample[index]  NB Index values start at 0!!!!!  Each element in an array has the same name as the array but a different value (index) in the square brackets Arrays

Creating an Array  rainfall = new double[12];  This creates an array which can hold 12 elements.  The first element will be stored at –rainfall[0]  The last element will be stored at –rainfall[11]  NOTE: the index of the first position in an array is 0.

 If you try to reference an element using a higher index, this will result in an Exception (error) - ArrayIndexOutOfBoundsException  When Arrays are first created the elements of the Arrays are initialised to 0 (false for Boolean and null for objects) Arrays

Dynamic Initialisation dataType nameOfArray[] = { };  There is no need for a ‘new’ statement, it will be done automatically  Example int values[] = {4,2,7,1};

See Example 1

Arrays of Objects  Arrays can hold objects in the same way they held fundamental data types  Declaration for the array is the same: className arrayName[]; // Array creation step 1 arrayName = new className[3]; // Array step 2

Accessing objects in an array Use the array index as if it were an object variable arrayName[0].methodName();arrayName[1].variableName;

Example class Student { String name; String ID; int mark; public void setName (String n) {.....} public void setid (String Id) { } Student bis[] = new Student[3]; bis[0] = new Student(); bis[0].setName ("Paul"); bis[0].setId ("987654");

See Example 2