1. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Starting Out with Java: Early Objects Third Edition by Tony Gaddis as modified for CSCI 1250/1260 Chapter 7: Arrays and the Array List Class

2. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-2 Chapter Topics Chapter 7 discusses the following main topics:  Introduction to Arrays  Processing Array Contents  Passing Arrays as Arguments to Methods  Some Useful Array Algorithms and Operations  Returning Arrays from Methods  String Arrays  Arrays of Objects

3. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-3 Chapter Topics Chapter 7 discusses the following main topics:  The Sequential Search Algorithm  Two-Dimensional Arrays  Arrays with Three or More Dimensions  The Selection Sort and the Binary Search  Command-Line Arguments  The ArrayList Class

4. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-4 Introduction to Arrays one value at a timePrimitive variables are designed to hold only one value at a time. collection of values singletypeindexedArrays allow us to create a collection of values of a single type that is indexed. integersdoubles StringsAn array can store any type of data but only one type of data per array – in other words, all items in the collection must be of the same type (all integers, all doubles, all Strings, etc.). arrayAn array is a list of data elements.

5. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-5 Creating Arrays An array is an object so it needs an object reference. // Declare a reference to an array that will hold integers. int[] numbers; numbersThe next step creates the array and assigns its address to the numbers variable // Create a new array that will hold 6 integers. numbers = new int[6]; Array element values are initialized to 0. Array indexes always start at 0. The last subscript is always the one less than the size. 0 index 0 0 index 1 0 index 2 0 index 3 0 index 4 0 index 5

6. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-6 Creating Arrays It is possible to declare an array reference and create it in the same statement. int[] numbers = new int[6]; Arrays may be of any type. Arrays of objects are discussed later. float[] temperatures = new float[100]; char[] letters = new char[41]; long[] units = new long[50]; double[] sizes = new double[1200];

7. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-7 Creating Arrays The array size must be a non-negative number. It may be a literal value, a constant, or variable. final int ARRAY_SIZE = 6; int[] numbers = new int[ARRAY_SIZE]; Using a named constant is preferred because the array size may be used in many places in the code and this way provides for easy changes, if and when they are needed. an array size is fixedcannot be changedOnce created, an array size is fixed and cannot be changed.

8. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-8 Accessing the Elements of an Array An array is accessed by:  the reference name  a subscript that identifies which element in the array to access. numbers[0] = 20; //pronounced "numbers sub zero" numbers[0] 0 numbers[1] 0 numbers[2] 0 numbers[3] 0 numbers[4] 0 numbers[5]20

9. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-9 Inputting and Outputting Array Elements Array elements can be treated as any other variables. accessedarrayname subscriptThey are simply accessed by the array name and a subscript. See example: ArrayDemo1.javaArrayDemo1.java subscriptsvariablesArray subscripts can be variables (such as for loop counters). See example: ArrayDemo2.javaArrayDemo2.java

10. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-10 I/O with Array Elements

11. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-11 I/O with Array Elements

12. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-12 Bounds Checking zero array length - 1Array indexes always start at zero and continue to (array length - 1). int[ ] values = new int[10]; array 09This array would have indexes 0 through 9. See example: InvalidSubscript.javaInvalidSubscript.java

13. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-13 Bounds Checking Exception thrown when index is 3

14. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-14 Off-by-One Errors off-by-oneIt is very easy to be off-by-one when accessing arrays. // This code has an off-by-one error. int[] numbers = new int[100]; for (int i = 1; i <= 100; i++) numbers[i] = 99; ArrayIndexOutOfBoundsExceptionWhen i reaches 100, this code would throw an ArrayIndexOutOfBoundsException because there is no position number 100 – the last one is position 99. Here, the equal sign allows the loop to continue on to index 100, where 99 is the last index in the array.

15. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-15 Array Initialization When relatively few items need to be initialized, an initialization list can be used to initialize the array. int[]days = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; The numbers in the list are stored in the array in order:  days[0]  days[0] is assigned 31,  days[1]  days[1] is assigned 28,  days[2]  days[2] is assigned 31,  days[3]  days[3] is assigned 30,  etc. See example: ArrayInitialization.javaArrayInitialization.java

16. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-16 Array Initialization

17. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-17 Alternate Array Declaration Previously we showed arrays being declared: int[ ] numbers;  However, the brackets can also go here: int numbers[ ];  These are equivalent but the first style is more common. Multiple arrays can be declared on the same line with the original notation. int[ ] numbers, codes, scores; With the alternate notation each variable must have its own set of brackets. int numbers[ ], codes[ ], scores; scores int  The scores variable in this instance is simply an ordinary int variable.

18. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-18 Processing Array Contents Processing a data item in an array is done in the same ways as any other variable. grossPay = hours[3] * payRate; Pre and post increment/decrement also works the same: int[] score = {7, 8, 9, 10, 11}; ++score[2]; // Pre-increment operation score[4]++; // Post-increment operation See example: PayArray.javaPayArray.java

19. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-19 Processing Array Contents

20. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-20 Processing Array Contents relational operationsArray elements can be used in relational operations: if(cost[20] < cost[0]) {//statements} loopconditionsThey can be used in loop conditions: while(value[count] != 0) {//statements}

21. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-21 Array Length Arrays are objects public fieldlengthconstantArrays are objects and provide a public field named length that is a constant that can be tested, displayed, compared, etc. double[] temperatures = new double[25]; length 25  The length of this array is 25. lengthThe length of an array can be obtained via its length constant. int size = temperatures.length; size  The variable size will contain 25.

22. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-22 The Enhanced for Loop read onlySimplified array processing (read only) allfor each item in the array, do …Always goes through all elements. Can be thought of as “for each item in the array, do … ” General: for(datatype elementVariableName : arrayName) { statement; statement;} Example: int[] numbers = {3, 6, 9}; for(int val : numbers) { System.out.println("The next value is " + val); } for each val numbers Read this as “for each integer, val, in the array named numbers” … canuse values we cannot change them This means we can use values in the array with this technique, but we cannot change them.

23. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-23 Array Size lengthThe length constant can be used in a loop to provide automatic bounding. for(int i = 0; i < temperatures.length; i++) { System.out.println("Temperature " + i ": " System.out.println("Temperature " + i ": " + temperatures[i]); + temperatures[i]);} start0endoneless thanarray length Index subscripts start at 0 and end at one less than the array length.

24. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-24 Array Size You can let the user specify the size of an array: int numTests; int[] tests; Scanner keyboard = new Scanner(System.in); System.out.print("How many tests do you have? "); numTests = keyboard.nextInt(); tests = new int[numTests]; See example: DisplayTestScores.javaDisplayTestScores.java

25. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-25 Array Size

26. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-26 Reassigning Array References An array reference can be assigned to another array of the same type. // Create an array referenced by the numbers variable. int[] numbers = new int[10]; // Reassign numbers to a new array. numbers = new int[5]; After the first (10-element) array no longer has a reference to it, it will be garbage collected.

27. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-27 Reassigning Array References Address numbers The numbers variable holds the address of an int int array. int[] numbers = new int[10];

28. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-28 Reassigning Array References Address numbers The numbers variable holds the address of an int int array. numbers = new int[5]; This array gets marked for garbage collection

29. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-29 Copying Arrays notThis is not the way to copy an array. int[] array1 = { 2, 4, 6, 8, 10 }; int[] array2 = array1; // This does not copy array1. 2 Address array1 array1 holds an address to the array Address array2 array2 holds an address to the array 46810 Example: SameArray.java SameArray.java Remember: shallow copy vs. deep copy

30. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-30 Copying Arrays cannotYou cannot copy an array by merely assigning one reference variable to another. copy the individual elementsonearrayotherone at a time in a loopTo copy an array, you must copy the individual elements of one array to the other, one at a time in a loop int[] firstArray = {5, 10, 15, 20, 25 }; int[] secondArray = new int[5]; for (int i = 0; i < firstArray.length; i++) secondArray[i] = firstArray[i]; secondArray[i] = firstArray[i]; firstArray secondArrayThis code copies each element of the firstArray to the corresponding element of the secondArray.

31. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-31 Passing Array Elements to a Method single element of an arrayWhen a single element of an array is passed to a method it is handled like any other variable. See example: PassElements.javaPassElements.java

32. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-32 Passing a single element of an array

33. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-33 Passing Arrays as Arguments objectsArrays are objects. Their references can be passed to methods like any other object reference variable. 510152025 Address showArray(numbers); 303540 public static void showArray(int[] array) { for (int i = 0; i < array.length; i++) for (int i = 0; i < array.length; i++) System.out.print(array[i] + " "); System.out.print(array[i] + " ");} Example: PassArray.javaPassArray.java

34. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-34 Comparing Arrays array references same array objectThe == operator determines only whether array references point to the same array object. To compare the contents of an array: int[] firstArray = { 2, 4, 6, 8, 10 }; int[] secondArray = { 2, 4, 6, 8, 10 }; // Assume they are equal until we have evidence they are not boolean arraysEqual = true; int i = 0; if (firstArray.length != secondArray.length) arraysEqual = false; arraysEqual = false; while (arraysEqual && i < firstArray.length) { if (firstArray[i] != secondArray[i]) if (firstArray[i] != secondArray[i]) arraysEqual = false; arraysEqual = false; i++; i++;} if (arraysEqual) System.out.println("The arrays are equal."); System.out.println("The arrays are equal.");else System.out.println("The arrays are not equal."); System.out.println("The arrays are not equal.");

35. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-35 Array Algorithms There are many algorithms for doing common things with arrays.  Fill (or populate) an array  Display array  Find largest/smallest value in an array  Determine if an array contains a particular value  Sort the data in the array Some of these algorithms are given in examples in subsequent slides

36. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-36 Finding the largest For small sets of values, we can almost “see” the answer without giving much thought. For larger sets of values, we need a mental “algorithm”. The computer cannot just see the answer. It needs an algorithm. –We scan the items one at a time. –The first is the largest we have seen when it is the only one we have seen. –Scan the rest until we find a larger one. –Remember it. –Repeat previous two steps until finished. –What we are remembering at the end is the largest.

37. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-37 Useful Array Operations Finding the Highest Value int [] numbers = new int[50]; // Assume the array has now been filled with integers int highest = numbers[0]; // First is largest so far for (int i = 1; i < numbers.length; i++) { if (numbers[i] > highest) highest = numbers[i]; } Finding the Lowest Value int lowest = numbers[0]; for (int i = 1; i < numbers.length; i++) { if (numbers[i] < lowest) if (numbers[i] < lowest) lowest = numbers[i]; } Starting with the second item – the one in position 1 – scan the rest, comparing each to the largest seen so far

38. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-38 Useful Array Operations Summing Array Elements: int total = 0; // Initialize accumulator for (int i = 0; i < units.length; i++) total += units[i]; total += units[i]; Averaging Array Elements: double total = 0; // Initialize accumulator double average; // Will hold the average for (int i = 0; i < scores.length; i++) total += scores[i]; average = total / scores.length; Example: SalesData.java, Sales.javaSalesData.javaSales.java

39. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-39 Example

40. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-40 Example Continued

41. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-41 Partially Filled Arrays Typically, if the amount of data that an array must hold is unknown:  Set the size of the array to the largest number of elements you MIGHT need length  use a counting variable to keep track of how much valid data is in the array (remember the length of the array is the number of values it can hold – not the number it does hold, which could be fewer).… int[] array = new int[100]; int count = 0; … System.out.print("Enter a number or -1 to quit: "); System.out.print("Enter a number or -1 to quit: "); number = keyboard.nextInt(); number = keyboard.nextInt(); while (number != -1 && count <= 99) while (number != -1 && count <= 99) { array[count] = number; array[count] = number; count++; count++; System.out.print("Enter a number or -1 to quit: "); System.out.print("Enter a number or -1 to quit: "); number = keyboard.nextInt(); number = keyboard.nextInt(); } … Assume number, and keyboard were previously declared and keyboard references a Scanner object

42. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-42 Arrays and Files Saving the contents of an array to a file: int[] numbers = {10, 20, 30, 40, 50}; PrintWriter outputFile = new PrintWriter ("Values.txt"); new PrintWriter ("Values.txt"); for (int i = 0; i < numbers.length; i++) outputFile.println(numbers[i]); outputFile.println(numbers[i]);outputFile.close();

43. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-43 Arrays and Files Reading the contents of a file into an array: final int SIZE = 5; final int SIZE = 5; // Assuming we know the size int[] numbers = new int[SIZE]; int i = 0; File file = new File ("Values.txt"); Scanner inputFile = new Scanner(file); while (inputFile.hasNext() && i < numbers.length) { numbers[i] = inputFile.nextInt(); numbers[i] = inputFile.nextInt(); i++; i++;}inputFile.close();

44. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-44 Returning an Array Reference A method can return a reference to an array. The return type of the method must be declared as an array of the proper type. public static double[] getArray( ) { double[] array = { 1.2, 2.3, 4.5, 6.7, 8.9 }; return array; } getArray public static returnsarray of doublesThe getArray method is a public static method that returns an array of doubles. See example: ReturnArray.javaReturnArray.java

45. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-45 Returning an Array Reference

46. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-46 String Arrays Arrays are not limited to primitive data. StringAn array of String objects can be created: String[] names = { "Bill", "Susan", "Steven", "Jean" }; names The names variable holds the address to the array. String A String array is an array String of references to String objects. Address “Bill” “Susan” “Steven” “Jean” address names[1] names[0] names[3] names[2] Example: MonthDays.java MonthDays.java

47. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-47 String Array Example

48. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-48 String Arrays newIf an initialization list is not provided, the new keyword must be used to create the array: String[] names = new String[4]; names The names variable holds the address to the array. Address null names[1] names[0] names[3] names[2]

49. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-49 String Arrays When an array is created in this manner, each element of the array must be initialized individually. names The names variable holds the address to the array. Address null names[0] = "Bill"; names[1] = "Susan"; names[2] = "Steven"; names[3] = "Jean"; “Bill” “Susan” “Steven” “Jean” names[1] names[0] names[3] names[2]

50. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-50 Calling String Methods On Array Elements StringString objects have several methods, including:  toUpperCase  compareTo  equals  charAt StringStringEach element of a String array is a String object. arrayname indexMethods can be used by using the array name and index as before.System.out.println(names[0].toUpperCase()); char letter = names[3].charAt(0);

51. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-51 lengthlength The length Field & The length Method Arraysfieldattribute lengthArrays have a final field (attribute) named length. Stringmethod lengthString objects have a method named length. StringTo display the length of each string held in a String array: length for (int i = 0; i < names.length; i++) length() System.out.println(names[i].length()); array length attributeAn array’s length is an attribute (field)  You do not use a set of parentheses after its name. StringlengthA String ’s length is a method String length  You do place the parentheses after the name of the String class’s length method.

52. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-52 Arrays of Objects Strings are objects. Arrays can contain objects – Strings or any other types of objects. InventoryItem[] inventory = new InventoryItem[5]; inventory InventoryItem The inventory variable holds the address of an array of InventoryItem objects. Address null inventory[1] inventory[0] inventory[3] inventory[2] null inventory[4]

53. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-53 Arrays of Objects Each element needs to be initialized. for (int i = 0; i < inventory.length; i++) inventory[i] = new InventoryItem( ); inventory[i] = new InventoryItem( ); Example: ObjectArray.javaObjectArray.java inventory The inventory variable holds the address InventoryItem of an InventoryItem array. Address inventory[1] inventory[0] inventory[3] inventory[2] description: units: “” 0 description: units: “” 0 description: units: “” 0 description: units: “” 0 description: units: “” 0 Address inventory[4] Constructor for InventoryItem initializes each of these items

54. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-54 Object Array

55. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-55 The Sequential Search Algorithm search algorithmA search algorithm is a method of locating a specific item in a larger collection of data. personPeople  Locating a person in an array of 10,000 People bookBooks  Locating a book in an array of 100,000 Books partParts  Locating a part in an array of 20,000 Parts sequentialsearchalgorithmThe sequential search algorithm uses a loop to:  sequentially step through an array,  compare each element with the search key (the item for which we are searching), and  stop when the value is found or the end of the array is encountered. See example: SearchArray.javaSearchArray.java

56. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-56 Sequential Search

57. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-57 Selection Sort selection sortIn a selection sort algorithm:  The largest value in the array is located and exchanged with the last element.  Then the next largest value is located and exchanged with the next to last element.  This process continues until all of the elements have been placed in their proper order.  See example: SelectionSortDemo.javaSelectionSortDemo.java

58. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-58 Selection Sort

59. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-59 Selection Sort

60. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-60 Binary Search A binary search:  requires an array sorted in ascending order.  starts with the element in the middle of the array.  If that element is the desired value, the search is over. Otherwise, the value in the middle element is either greater or less than the desired value If it is greater than the desired value, search in the first half of the array. Otherwise, search the last half of the array.  Repeat as needed while adjusting start and end points of the search. See example: BinarySearchDemo.javaBinarySearchDemo.java

61. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-61 Binary Search Demo

62. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-62 Two-Dimensional Arrays A two-dimensional array is an array of arrays. It can be thought of as having rows and columns. row 0 column 1column 2column 3column 0 row 1 row 2 row 3

63. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-63 Declaring a two-dimensional array requires two sets of brackets and two size declarators  The first one is for the number of rows  The second one is for the number of columns. double[][] scores = new double[3][4]; The two sets of brackets in the data type indicate that the scores variable will reference a two-dimensional array. Notice that each size declarator is enclosed in its own set of brackets. Two-Dimensional Arrays two dimensional arrayrowscolumns

64. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-64 Accessing Two-Dimensional Array Elements When processing the data in a two-dimensional array, each element has two subscripts:  one for its row and  another for its column.

65. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-65 Accessing Two-Dimensional Array Elements scores[0][3]scores[0][2]scores[0][1]scores[0][0] row 0 column 1column 2column 3column 0 row 1 row 2 scores The scores variable holds the address of a doubles 2D array of doubles. Address scores[1][3]scores[1][2]scores[1][1]scores[1][0] scores[2][3]scores[2][2]scores[2][1]scores[2][0]

66. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-66 Accessing Two-Dimensional Array Elements Accessing one of the elements in a two- dimensional array requires the use of both subscripts. scores[2][1] = 95; 0000 row 0 column 1column 2column 3 column 0 row 1 row 2 Address 0000 00950 scores The scores variable holds the address of a doubles 2D array of doubles.

67. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-67 Accessing Two-Dimensional Array Elements Programs that process two-dimensional arrays can do so with nested loops. To fill the scores array: rowrowrow for (int row = 0; row < 3; row++) { colcolcol for (int col = 0; col < 4; col++) { System.out.print("Enter a score: "); rowcol scores[row][col] = keyboard.nextDouble(); } Number of rows, not the largest subscript Number of columns, not the largest subscript keyboard references a Scanner object

68. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-68 Accessing Two-Dimensional Array Elements scoresTo display the scores array: rowrowrow for (int row = 0; row < 3; row++) { colcolcol for (int col = 0; col < 4; col++) { rowcol System.out.println(scores[row][col]); } See example: CorpSales.javaCorpSales.java

69. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-69 Corporate Sales Example

70. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-70 Initializing a Two-Dimensional Array Initializing a two-dimensional array requires enclosing each row’s initialization list in its own set of braces. int[][] numbers = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; Java automatically creates the array and fills its elements with the initialization values.  row 0 {1, 2, 3}  row 1 {4, 5, 6}  row 2 {7, 8, 9} Declares an array with three rows and three columns.

71. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-71 Initializing a Two-Dimensional Array 321 row 0 column 1column 2column 0 row 1 row 2 Address 654 987 numbers The numbers variable holds the address of a int 2D array of int values. int[][] numbers = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}; produces:

72. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-72 The length Field Two-dimensional arrays are arrays of one- dimensional arrays. lengthnumber of rowsThe length field of the array gives the number of rows in the array. rowlength columnsEach row has its own length constant that tells how many columns are in that row. rowdifferentnumber columnsEach row can have a different number of columns.

73. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-73 The length Field lengthTo access the length fields of the array: int[][] numbers = { { 1, 2, 3, 4 }, { 5, 6, 7 }, { 9, 10, 11, 12 } }; for (int row = 0; row < numbers.length; row++) { for (int col = 0; col < numbers[row].length; col++) System.out.println(numbers[row][col]); } See example: Lengths.javaLengths.java Rows may have varying lengths Number of rows Length of this row

74. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-74 Array Example

75. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-75 Summing The Elements of a Two-Dimensional Array int[][] numbers = { { 1, 2, 3, 4 }, {5, 6, 7, 8}, {9, 10, 11, 12} }; int total; total = 0; rowrowrow for (int row = 0; row < numbers.length; row++) { colcolrowcol for (int col = 0; col < numbers[row].length; col++) rowcol total += numbers[row][col]; } System.out.println("The total is " + total);

76. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-76 Summing The Rows of a Two-Dimensional Array int[][] numbers = {{ 1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}}; int total; for (int row = 0; row < numbers.length; row++) { total = 0; colcolcol for (int col = 0; col < numbers[row].length; col++) col total += numbers[row][col]; System.out.println("Total of row " + row + " is " + total); }

77. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-77 Summing The Columns of a Two-Dimensional Array int[][] numbers = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}}; int total; colcolcol for (int col = 0; col < numbers[0].length; col++) { total = 0; for (int row = 0; row < numbers.length; row++) col total += numbers[row][col]; System.out.println("Total of column " col + col + " is " + total); }

78. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-78 Passing and Returning Two- Dimensional Array References There is no difference between passing a single or two-dimensional array as an argument to a method. The method must accept a two-dimensional array as a parameter. See example: Pass2Darray.javaPass2Darray.java

79. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-79 Passing 2D Array

80. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-80 Ragged Arrays ragged arrayWhen the rows of a two-dimensional array are of different lengths, the array is known as a ragged array. You can create a ragged array by creating a two- dimensional array with a specific number of rows, but no specific number of columns. int [][] ragged = new int [4][]; Then create the individual rows, perhaps with different lengths. ragged[0] = new int [3]; ragged[1] = new int [4]; ragged[2] = new int [5]; ragged[3] = new int [6];

81. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-81 More Than Two Dimensions Java does not limit the number of dimensions that an array may have. More than three dimensions is hard to visualize, but can be useful in some programming problems.

82. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-82 More Than Two Dimensions - Example String[ ][ ][ ][ ][ ] courses = new String [numUniv] [numColleges] [numDept] [numFac] [numCourses]; courses[3][0][1][5][2] is course number 2 taught by instructor number 5 in department number 1 in college number 0 of university number 3 in the state

83. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-83 Command-Line Arguments A Java program can receive arguments from the operating system command-line. mainThe main method has a header that looks like this: public static void main(String[] args) StringThe main method receives a String array as a parameter. argsThe array that is passed into the args parameter comes from the operating system command-line.

84. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-84 Command-Line Arguments To run the example: java CommandLine How does this work? args[0] is assigned "How" args[0] is assigned "does" args[0] is assigned "this" args[0] is assigned "work?" Example: CommandLine.javaCommandLine.java It is not required that the name of main ’s parameter array be args.

85. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-85 Command Line Example

86. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-86 Variable-Length Argument Lists vararg…Special type parameter – vararg…  Vararg  Vararg parameters are actually arrays ellipsis  The ellipsis is required.  Examples: VarArgsDemo1.java, VarargsDemo2.javaVarArgsDemo1.javaVarargsDemo2.java public static int sum(int... numbers) { int total = 0; // Accumulator // Add all the values in the numbers array. for (int val : numbers) total += val; total += val; // Return the total. return total; }

87. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-87 The ArrayList Class ArrayListSimilar to an array, an ArrayList allows object storage ArrayListUnlike an array, an ArrayList object:  Automatically expands  Automatically expands when a new item is added  Automatically shrinks  Automatically shrinks when items are removed Requires: import java.util.ArrayList;import java.util.ArrayList;

88. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-88 Creating and Using a Generic ArrayList ArrayListCreate an ArrayList object with no-args constructor ArrayList nameList = new ArrayList (); ArrayListaddTo populate the ArrayList, use the add method:nameList.add("James");nameList.add("Catherine"); sizeTo get the current size, call the size method nameList.size(); // returns 2 Type of Data stored in ArrayList

89. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-89 Creating and Using an ArrayList ArrayListgetTo access items in an ArrayList, use the get methodnameList.get(3); get 0 In this statement 3 is the index of the item to get. The index starts with 0 as with ordinary arrays. Example: ArrayListDemo1.javaArrayListDemo1.java

90. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-90 Generic ArrayList Demo1

91. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-91 Using an ArrayList ArrayListtoString ArrayListThe ArrayList class's toString method r eturns a string representing all items in the ArrayListSystem.out.println(nameList); This statement yields : [ James, Catherine ] ArrayListremove ArrayListThe ArrayList class's remove method removes designated item from the ArrayListnameList.remove(1); This statement removes the second item. See example: ArrayListDemo3.javaArrayListDemo3.java

92. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-92 ArrayList Demo3

93. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-93 Using an ArrayList ArrayListadd ArrayListThe ArrayList class's add method with one argument adds new items to the end of the ArrayList at a location of choice addTo insert items at a location of choice, use the add method with two arguments: nameList.add(1, "Mary"); String This statement inserts the String "Mary" at index 1 setTo replace an existing item, use the set method: nameList.set(1, "Becky"); This statement replaces “Mary” with “Becky” See example: ArrayListDemo4.javaArrayListDemo4.java

94. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-94 ArrayList Demo4 Note size gives how many items are currently in the ArrayList

95. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-95 Using an ArrayList ArrayListcapacityAn ArrayList has a capacity, which is the number of items it can hold without increasing its size. defaultcapacityArrayList 10 itemsThe default capacity of an ArrayList is 10 items. differentcapacity parameterizedconstructorTo designate a different initial capacity, use a parameterized constructor : ArrayList list = new ArrayList (100);

96. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-96 Generic ArrayList Demo2

97. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-97 Generic ArrayList Demo3 Enhanced for loop works with ArrayLists as well

98. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-98 instanceof Student GradStudentPartTimeStudentHonorsStudentSuppose Student is a class from which other classes such as GradStudent, PartTimeStudent, HonorsStudent and others have been derived. ArrayListstudentListGiven an ArrayList such as studentList below ArrayList studentList = new ArrayList ( ); Studentany of the classes derived from StudentArrayList  Objects of the class Student as well as objects of any of the classes derived from Student may be added to this ArrayList. studentListStudent GradStudent  Sometimes, however, we may need to treat one of the objects in the studentList as the type of Student it actually is (because we need functionality that only a GradStudent has, for example).  In this case, we need to be able to ask a question about what type one of the objects actually is. instanceof  We use instanceof in this case.

99. Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-99 instanceof For example, Student stdt = studentList.get (6); if (stdt instanceof GradStudent) { GradStudent grad = (GradStudent) stdt; // use the grad object now with all // GradStudent functionality...} Cast operator – says please treat this as a GradStudent – OK because we just verified it is a GradStudent