Arrays A problem with simple variables One variable holds one value The value may change over time but at any given time a variable holds a single value If you want to keep track of many values you need many variables All of these variables need to have names What if you need to keep track of hundreds or thousands of values Multiple values An array lets you associate one name with a fixed but possibly large number of values All values must have the same type The values are distinguished by a numerical index between 0 and array size minus 1 0 9 1 myArra 12 43 y 2 6 3 4 5 6 83 14 57 109 12 7 0 8 6 Indexing into arrays To reference a single array element use array name index Indexed elements can be used just like simple variables You can access their values You can modify their values An array index is sometimes called a subscript Using array elements 0 9 1 myArra 12 43 y 2 6 3 4 5 6 83 14 57 109 12 7 0 8 6 Examples x myArray 1 sets x to 43 myArray 4 99 replaces 14 with 99 m 5 y myArray m sets y to 57 z myArray myArray 9 sets z to 109 Array values An array may hold any type of value All values in an array must be the same type For example you can have an array of integers an array of Strings an array of Person an array of arrays of String an array of Object Strings and arrays Strings and arrays both have special syntax Strings are objects and can be used as objects Arrays are objects but Arrays are created using special syntax new type size instead of new Person If an array holds elements of type T then the array s type is array of T Declaration versus definition Arrays are objects Creating arrays is like creating other objects the declaration only provides type information the new definition actually allocates space declaration and definition may be separate or combined Example for ordinary objects Person p declaration p new Person John definition Person p new Person John combined Declaring and defining arrays Example for array objects int myArray declaration This declares myArray to be an array of integers Notice that the size is not part of the type myArray new int 10 definition new int 10 creates the array the rest is an ordinary assignment statement int myArray new int 10 both Array assignment Array assignment is object assignment Object assignment does not copy values Person p1 Person p2 p1 new Person John p2 p1 p1 and p2 refer to the same person Array assignment does not copy values int a1 int a2 a1 new int 10 a2 a1 a1 and a2 refer to the same array An array s size is not part of its type When you declare an array you declare its type you must not specify its size Example String names When you define the array you allocate space you must specify its size Example names new String 50 This is true even when the two are combined Example String names new String 50 Array assignment When you assign an array value to an array variable the types must be compatible The following is not legal double dub new int 10 illegal The following is legal int myArray new int 10 and later in the program myArray new int 500 legal Legal because array size is not part of its type Example of array use I Suppose you want to find the largest value in an array scores of 10 integers int largestScore 0 for int i 0 i 10 i if scores i largestScore largestScore scores i By the way do you see an error in the above program What if all values in the array are negative Example of array use II To find the largest value in an array scores of 10 possibly negative integers int largestScore scores 0 for int i 1 i 10 i if scores i largestScore largestScore scores i Example of array use III Suppose you want to find the largest value in an array scores and the location in which you found it int largestScore scores 0 int index 0 for int i 1 i 10 i if scores i largestScore largestScore scores i index i Array names I The rules for variables apply to arrays Rule 25 Use lowercase for the first word and capitalize only the first letter of each subsequent word that appears in a variable name Rule 26 Use nouns to name variables Rule 27 Pluralize the names of collection references Array names II Here s what the naming rules mean Capitalization is just like any other variable Array names should be plural nouns Example array names scores phoneNumbers preferredCustomers Length of an array Arrays are objects Every array has an instance constant length that tells how large the array is Example for int i 0 i scores length i System out println scores i Use of length is always preferred over using a constant such as 10 Strings have a length method Magic numbers Use names instead of numbers in your code Names help document the code numbers don t It may be hard to tell why a particular number is used we call it a magic number This is a pejorative term You might change your name about the value of a constant say more than ten scores You can change the value of a name in one place An array s length is always correct NullPointerException Suppose you declare a Person p but you don t define it Until you define p it has the special value null null is a legal value for any kind of object null can be assigned tested and printed But if you try to use a field or method of null such as p name or p birthday the error you get is a nullPointerException Arrays of objects Suppose you declare and define an array of objects Person people new Person 20 There is nothing wrong with this array but it has 20 references to Persons in it all of these references are initially null you have not yet defined 20 Persons For example people 12 name will give you a nullPointerException Initializing arrays I Here s one way to initialize an array of objects Person people new Person 20 for int i 0 i people length i people i new Person John This approach has a slight drawback all the array elements are alike Initializing arrays II There is a special syntax for giving initial values to the elements of arrays This syntax can be used in place of new type size It can only be used in an array declaration The syntax is value value value Examples int primes 2 3 5 7 11 13 19 String languages Java C C Array literals You can create an array literal with the following syntax type value1 value2 valueN Examples myPrintArray new int 2 3 5 7 11 int foo foo …
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