1. IT259 Foundation of Programming Using Java Unit 9 Seminar : (Chapter 8 ) Instructor : Vladimir Gubanov, PhD Email : vgubanov@kaplan.eduvgubanov@kaplan.edu

2. A reminder: 1. Our Seminars : they will be each Thursday, from 9 PM to 10 PM EST 2. My Office Hours : Mondays, 7PM to 8PM Saturdays, 9AM to 10 AM 3. My email : vgubanov@kaplan.edu

3. Java Programming, Fifth Edition Chapter Eight: Arrays

4. Java Programming, Fifth Edition 4 Objectives Declare and initialize an array Declare an array of objects Search an array for an exact match and for a range match Manipulate arrays of Strings Sort array elements Use two-dimensional and multidimensional arrays Use the Arrays class Use the ArrayList class

5. 5 Array Basics List or series of values all referenced by the same name – single identifier given to entire list Data structure that may contain any number of variables, which must be of the same data type Individual variables in the data structure are called elements Elements accessed through an index – each element has it own index value –Index also called subscript –Elements are sometimes referred to as indexed or subscripted variables

6. Java Programming, Fifth Edition 6 Declaring and Initializing an Array Array –Named list of data items –All have same type Declare array variable –Same way as declaring any simple variable –Insert pair of square brackets after type : double[] salesFigure; int[] idNum; Have the arrays been created in the statements above ?

7. Java Programming, Fifth Edition 7 Declaring and Initializing an Array Reserve memory space – create the array : salesFigure = new double[20]; double[] salesFigure = new double[20]; Java arrays can be of any data type Use subscript –Integer contained within square brackets –Indicates one of array’s variables –Can use any subscript, e.g., from 0 through 19 when working with array that has 20 elements –Example: salesFigure[0] = 2100.00;

8. Declaring and Initializing an Array Java Programming, Fifth Edition 8

9. 9 Initializing an Array Variable with reference type : holds memory address where value stored Array names - represent computer memory addresses Use keyword new to define array –Array name acquires actual memory address value int[] someNums = new int[10]; –Each element of someNums has value of 0 char array elements –Assigned ‘\u0000’ boolean array elements –Automatically assigned value false

10. The Declaration Statement for Arrays Declare arrays as public or private –Defaults to private Location of declaration determines scope and lifetime of array variables Declare an array by placing opening and closing square brackets after the data type Declare the number of elements or the initial values of array elements (which determine the number of elements) All array elements must be the same data type Can an array in Java be resized as in VB ?

11. Java Programming, Fifth Edition 11 Use array initializers to assign values to array elements Assign nondefault values to array elements upon creation int[] tenMult = {10, 20, 30, 40, 50, 60}; Power of arrays –Use subscripts that are variables, not only constant subscripts –Use a loop to perform array operations for (sub = 0; sub < 5; ++sub) scoreArray[sub] += 3;

12. Java Programming, Fifth Edition 12 Using Subscripts with an Array Subscripts can be constants, variables or numeric expressions Convenient to declare symbolic constant equal to size of array final int NUMBER_OF_SCORES = 5; Length property –Contains number of elements in array Use for loops when need to iterate over array elements for(sub = 0; sub < scoreArray.length; ++sub) scoreArray[sub] += 3;

13. Java Programming, Fifth Edition 13 Enhanced for loop –Cycle through array –No need to specify starting and ending points for loop control variable for(int val : scoreArray) System.out.println(val); –What will this code do ? –Is it an analog of for each loop in C# or VB ?

14. Java Programming, Fifth Edition 14 Array of Objects Array element can be object of any class Create array of Employee objects Employee[] emp = new Employee[7]; To feel in array elements one need to create seven objects each time calling an Employee class constructor final double PAYRATE = 6.35; for(int x = 0; x < NUM_EMPLOYEES; ++x) emp[x] = new Employee(101 + x, PAYRATE); What will be the content of each Employee array element when an array is created ?

15. Java Programming, Fifth Edition 15 Searching For an Exact Match Determine whether variable holds one of many valid values Searching array : –Compare variable to list of values in array for(int x = 0; x < validValues.length; ++x) { if(itemOrdered == validValues[x]) –validItem = true; }

16. Java Programming, Fifth Edition 16 Searching an Array For an Exact Match Alternative for searching –Use while loop Parallel arrays –One with same number of elements as another –Values in corresponding elements related Next slide : search using two parallel arrays

17. Java Programming, Fifth Edition 17 What is going on here ? How does the code work ?

18. Searching an Array For a Range Match Range match –Compare value to endpoints of numerical ranges –Find category in which value belongs Java Programming, Fifth Edition 18

19. 19 Arrays as Return Types Methods can have arrays as their return type ( a good way to return a set of values from a method !) Example method heading public static int [ ] GetScores(ref int gameCnt) Example call to the method int [ ] points = new int [1000]; points = GetScores(ref gameCnt); –Method would include a return statement with an array

20. Java Programming, Fifth Edition 20 Passing Arrays to and Returning Arrays from Methods Pass single array element to method –Same as passing variable –Passed by value Copy of value made and used in receiving method All primitive types passed this way Pass whole array to method – by reference –Object holds memory address where values stored –Receiving method gets copy of array’s actual memory address –Receiving method has ability to alter original values in array elements

21. Java Programming, Fifth Edition 21

22. Java Programming, Fifth Edition 22 Manipulating Arrays of Strings Create and initialize array of Strings String[] deptName = {"Accounting", "Human Resources", "Sales"}; for(int a = 0; a < deptName.length; ++a) System.out.println(deptName[a]); What will be printed to the display ?

23. Manipulating Arrays of Strings : Java Programming, Fifth Edition 23 What does this code do and how does it work ?

24. Java Programming, Fifth Edition 24 Sorting Array Elements Sorting –Process of arranging series of objects in some logical order Ascending order –Begin with object that has lowest value Descending order –Start with object that has largest value

25. Java Programming, Fifth Edition 25 Sorting Array Elements Simplest possible sort –Involves two values that are out of order –Swap two values temp = valA; // 16 goes to temp valA = valB; // 2 goes to valA valB = temp; // 16 goes to valB

26. Java Programming, Fifth Edition 26 Sorting Array Elements Bubble sort Continue to compare pairs of items –Swapping them if they are out of order Smallest items “bubble” to top of list Eventually creating sorted list

27. Sorting Array Elements : more efficient bubble sort Java Programming, Fifth Edition 27 Why is this code more eefficient ?

28. Java Programming, Fifth Edition 28 Sorting Arrays of Objects –Same way that you sort arrays of primitive types –Major difference Sort based on particular object field

29. Java Programming, Fifth Edition 29 Using Two-Dimensional and Multidimensional Arrays One-dimensional or single-dimensional array –Picture as column of values –Elements accessed using single subscript Two-dimensional arrays –Two or more columns of values –Rows and columns –Use two subscripts –Matrix or table int[][] someNumbers = new int[3][4]; How to reference the element in second row and third column?

30. 30 Initializing Two-Dimensional Array int[][] rents = { {400, 450, 510}, {500, 560, 630}, {625, 676, 740}, {1000, 1250, 1600} }; public static void displayScores(int[][]scoresArray) length field holds the number of rows in the array rents.length Each row has a length field that holds the number of columns in the row rents[1].length

31. Understanding Ragged Arrays Two-dimensional array with rows of different length Two steps when creating a ragged array : –Define the number of rows for a two- dimensional array But not defining the number of columns in the rows –Then declare the individual rows Java Programming, Fifth Edition 31

32. 32 Jagged Arrays Rectangular arrays always have a rectangular shape, like a table —jagged arrays do not Also called ‘arrays of arrays’ Example int[ ] [ ] anArray = new int[4] [ ]; anArray [0] = new int[] {100, 200}; anArray [1] = new int[] {11, 22, 37}; anArray [2] = new int[] {16, 72, 83, 99, 106}; anArray [3] = new int[] {1, 2, 3, 4};

33. Java Programming, Fifth Edition 33 Using Multidimensional Arrays Multidimensional arrays –More than two dimensions Create arrays of any size, with any number of indices –Keep track of order of variables needed as subscripts –No limits in array’s dimension –Don’t exhaust computer’s memory

34. Multi-Dimensional Arrays Limited only by your imagination as far as the number of dimensions Format for creating three-dimensional array type [,, ] identifier = new type [integral value, integral value, integral value]; Example (rectangular) int [,, ] calories = new int [4,7,3]; (4 week; 7 days; 3 meals) Allocates storage for 84 element s Type an example of declaration for 4 dimensional array

35. Java Programming, Fifth Edition 35 Using the Arrays Class Arrays class –Contains many useful methods for manipulating arrays –static methods Use with class name Without instantiating Arrays object –binarySearch() method Convenient way to search through sorted lists of values of various data types List must be in order

36. Using the Arrays Class : some available methods Java Programming, Fifth Edition 36

37. Using the ArrayList Class Advantages over the Arrays class –Dynamically resizable –Can add an item at any point in an ArrayList container –Can remove an item at any point in an ArrayList container Capacity –Number of items ArrayList can hold without having to increase its size Java Programming, Fifth Edition 37

38. Some methods of ArrayList Class Java Programming, Fifth Edition 38

39. Limitations of ArrayList Class Cannot use an ArrayList to store primitive types Must cast elements to the appropriate reference type –Or you must declare a reference type in the ArrayList declaration Java Programming, Fifth Edition 39

40. Java Programming, Fifth Edition 40 Summary Array –Named list of data items –All have same type Array names –Represent computer memory addresses Shorten many array-based tasks –Use variable as subscript length field –Contains number of elements in array

41. Java Programming, Fifth Edition 41 Summary (continued) Search array to find match to value Perform range match Pass single array element to method Sorting –Process of arranging series of objects in some logical order Two-dimensional arrays –Both rows and columns Arrays class ArrayList class

42. Quiz True or False: The first element of an array is referenced by index 0. Elements in arrays must be of the same ________ ____________. True or False: You can initialize the memory location of the array at the same time that you declare the array. Arrays are pass by ________ by default. True or False: One way to make a duplicate of all the elements in the array is to use the assignment operator. True or False: You can return an array of 1000 elements back from a value returning method. 42

43. Quiz(cont) What are the two types of two-dimensional arrays that can be created in Java ? True or False: With a jagged array, one row may have 3 columns while another row may have 10 columns. Two-dimensional arrays are referenced much like you reference a _____________using ________ indexes. In order to define a four dimensional array, how many commas would appear in the declaration inside the square bracket? True or False: An array in Java can increase in sizes once it is dimensioned. True or False: You can reference individual elements in an ArrayList using an index. 43

44. Quiz True or False: You declare an array variable in the same way you declare any simple variable, but you insert a pair of curly brackets after the type. Answer: False A(n) ____ is an integer contained within square brackets that indicates one of an array’s variables, or elements. Answer: subscript How would you set the value of the first element in the someNums array to 46? Answer: someNums[0] = 46; True or False: To use a method that belongs to an object that is part of an array, you insert the appropriate subscript notation after the array name and before the dot that precedes the method name. Answer: True 44

45. Quiz (cont.) A(n) ____ array is one with the same number of elements as another and for which the values in corresponding elements are related. Answer: parallel Arrays, like all nonprimitive objects, are ____ types; this means that the object actually holds a memory address where the values are stored and the receiving method gets a copy of the array’s actual memory address. Answer: reference When you place objects in order, beginning with the object that has the lowest value, you are sorting in ____ order. Answer: ascending 45

46. Quiz (cont.) True or False: In a binary sort, you continue to compare pairs of items, swapping them if they are out of order, so that the smallest items “bubble” to the top of the list, eventually creating a sorted list. Answer: False When mathematicians use a(n) ____ array, they often call it a matrix or a table. Answer: two-dimensional True or False: It is important that the list be in order before you use it in a call to binarySearch() Answer: True Is it possible to store elements with different data types in the same array? Explain why or why not. True or False: In Java, if a class is public (that is, if you use the public access modifier before the class name), you must save the class in a file with exactly the same name and a.class extension. Answer: False and *= To compile a file named First.java, you type ____ First.java and then press Enter. Answer: javac To run the First application from the command line, you type ____ First. Answer: java 46

47. This is the end of our Unit 9 Seminar Any questions ? 47