Object Oriented Programming. Java-Lecture 6 - Arrays

Transcription

1 Object Oriented Programming Java-Lecture 6 - Arrays

2 Arrays Arrays are data structures consisting of related data items of the same type In Java arrays are objects -> they are considered reference types We need array name and index or subscript of the array element to access the array element Every array variable knows its own length and stores it in a length (which is a public final variable) instance variable

3 Declaring and Creating Arrays Array objects too occupy space in memory, so array objects are also created using the new key word To declare an Array of 12 integer variables we have to use the following line of java code int[] c = new int[12]; (or) int[] c; c = new int[12]; Type[] arrayobjectname = new Type[arraySize]; Or Type arrayobjectname[] = new Type[arraySize]; Arrays elements will have default value zero for numeric primitive type elements, false for Boolean elements and null for references Int[] a, b ; // Declares two array references a and b Int a[], b; // Declares a as array reference object and b as integer variable.

4 Array Example public class InitArrays public static void main(string[] args) int[] array = new int[10]; System.out.printf("%8s%8s\n", "Index", "Value"); for(int i = 0; i < array.length; i++) System.out.printf("%8d%8d\n", i, array[i]); Output: Index Value

5 Array Initialization Example public class ArraysInit public static void main(string[] args) int[] array = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10; // new operation behind the scene System.out.printf("%8s%8s\n", "Index", "Value"); for(int i = 0; i < array.length; i++) System.out.printf("%8d%8d\n", i, array[i]); Output: Index Value

6 Assigning array values at run time import java.util.scanner; public class ArraysInit public static void main(string[] args) Scanner input = new Scanner(System.in); System.out.print("Enter Array size:"); int size = input.nextint(); int[] array = new int[size]; for(int i = 0; i < array.length; i++) array[i] = 2 * i;; System.out.printf("%8s%8s\n", "Index", "Value"); for(int i = 0; i < array.length; i++) System.out.printf("%8d%8d\n", i, array[i]); Output: Enter Array size:10 Index Value

7 Enhanced for loop The enhanced for statement iterates through the elements of an array without using a counter and avoids the possibility of out of array access. Syntax: for( parameter : arrayname) Statements Example: Normal for loop Vs Enhanced for loop for(int i = 0; i < array.length; i++) sum = sum + array[i]; for(int element : array) sum = sum + element; Can be used only to obtain the array elements. It cannot be used to modify the array elements.

8 Exercise Write and program which counts and displays how many times each face of the dice has appeared if the dice is rolled for times. Use Random class simulate the dice rolling (Home exercise) Case study: Card Shuffling and Dealing simulation

9 Passing Arrays to methods To pass an array argument to a method specify the name of the array without any brackets Example: Array Declaration: int[] array = new int[10]; Method declaration: modifyarray(int[] a) Method body statements Method call: modifyarray(array); Every array object knows its length, we no need to pass length of the array as an additional argument When array is passed it is passed as a reference (changes in the array elements inside method will effect the actual array : Pass by reference) When an individual primitive type array element is passed its passed as a separate copy of the value (changes to the value of the element inside method will not affect the array : Pass by Value)

10 Exercise Write class GradeBook and GradeBookTest to process course marks using array to store the course marks.

11 Multidimensional Arrays Java does not support multidimensional arrays directly We can specify one-dimensional arrays whose elements are also one-dimensional arrays Declaration: int[][] a = new int[3][4]; //Fixed rows and columns Declaration: with different row lengths int[][] b = new int[2][]; b[0] = new int[2]; b[1] = new int[3]; Initialization: int[][] b = 1, 2, 3, 4, 5; //variable row lengths

12 Home Exercise Use two dimensional array to store continuous evaluation of the student (i.e. minors, assignments and major examination) Total of all marks of the student to get the final marks and grade of the student Write functions like, min, max, average and computegrade

13 Variable-Length Argument Lists We can create methods that receives an unspecified number of arguments A type followed by an ellipsis ( ) in a method s parameter list indicates that the method receives a variable number of arguments of that particular type The use of ellipsis can occur only once in a parameter list, should be the last parameter of the method Java treats variable length-length argument list as an array whose elements are all of same type

14 Examples Variable length argument list example Processing command line arguments in java example