Arrays

1
Arrays

• Chapter 6

2
Objectives

• learn about arrays and how to use them in Java
  programs
• learn how to use array parameters and how to
  define methods that return an array
• learn the proper use of an array as an instance
  variable
• learn about multidimensional arrays

3
Objectives, cont.

• (optional) learn about text fields and text areas
  in applets
• (optional) learn to draw arbitrary polygons and
  polylines in applets

4
Outline

• Array Basics
• Arrays in Classes and Methods
• Programming with Arrays and Classes
• Sorting Arrays
• Multidimensional Arrays
• (optional) Graphics Supplement

5
Introduction to Arrays

• An array is an object used to store a (possibly
  large) collection of data.
• All the data stored in the array must be of the
  same type.
• An array object has a small number of predefined
  methods.

6
Introduction to Arrays, cont.

• Sometimes you want to know several things about a
  collection of data
• the average
• the number of items below the average
• the number of items above the average
• etc.
• This requires all the items to be stored as
  variables of one kind or another.

7
Introduction to Arrays, cont.

• Arrays satisfy this need.
• Even though an array is an object in Java, it can
  be convenient to think of it as a collection of
  variables of the same type.

8
Array Basics Outline

• Creating and Accessing Arrays
• Array Details
• The length Instance Variable
• Initializing Arrays

9
Creating and Accessing Arrays

• example
• double temperature new double7
• is like declaring seven variables of type double,
  named
• temperature0, temperature1, temperature2,
  temperature3, temperature4, temperature5,
  temperature6.

10
Creating and Accessing Arrays, cont.

• These variables can be used just like any other
  variables of type double.
• examples
• temperature3 32.0
• temperature6 temperature3 5
• System.out.println(temperature6)
• temperatureindex 66.5
• These variables are called indexed variables,
  elements, or subscripted variables.

11
Creating and Accessing Arrays, cont.

• The integer expression within the square brackets
  is called the index (or subscript).

12
Creating and Accessing Arrays, cont.

• class ArrayOfTemperatures

13
Creating and Accessing Arrays, cont.
14
Array Details

• syntax for creating an array
• Base_Type Array_Name
• new Base_TypeLength
• example
• int pressure new int100
• or
• int pressure
• pressure new int100

15
Array Details, cont.

• The type of the elements is called the base type.
• The base type of an array can be any type
  including a class type.
• for example,
• Species entry new Species3
• The number of elements is the length or size of
  the array.

16
Brackets

• Brackets serve three purposes
• creating the type name
• example int pressure
• creating the new array
• pressure new int100
• naming an indexed variable of the array
• pressure3 keyboard.nextInt()

17
Array Terminology
18
Singular Array Names

• Whether an array holds singular primitive types
  or singular class types, singular names make the
  code more self-documenting.
• for example,
• entry2
• makes more sense than
• entries2.

19
The length Instance Variable

• An array has only one public instance variable,
  length.
• The length variable stores the number of elements
  the array can hold.
• Using Array_Name.length typically produces
  clearer code than using an integer literal.

20
The length Instance Variable, cont

• class ArrayOfTemperatures2

21
Indices and length

• The indices of an array start with 0 and end with
  Array_Name.length-1.
• When a for loop is used to step through an array,
  the loop control variable should start at 0 and
  end at length-1.
• example
• for (lcv 0 lcv lt temperature.length lcv)

22
Array Index Out of Bounds

• Every index must evaluate to an integer which is
  not less than 0 and not greater than
  Array_Name.length-1.
• Otherwise, the index is said to be out of bounds
  or invalid.
• An out-of-bounds index will produce a run-time
  error.

23
Incomplete Array Processing

• Loops fail to process an entire array correctly
  when they
• begin with an index other than 0
• end with an index other than length-1.
• Examples
• for (i 1 i lt oops.length-1 index)
• for (i 1 i lt oops.length index)
• for (i 0 i lt oops.length index)
• for (i 0 i lt oops.length-1 index)

24
Initializing Arrays

• An array can be initialized at the time it is
  declared.
• example
• double reading 3, 3, 15.8, 9.7
• The size of the array is determined by the number
  of values in the initializer list.

25
Initializing Arrays, cont.

• Uninitialized array elements are set to the
  default value of the base type.
• However, its better to use either an initializer
  list or a for loop.
• int count new int100
• for (int i 0, i lt count.length, i)
• counti 0

26
Arrays in Classes and Methods

• Arrays can be used as instance variables in
  classes.
• Both an indexed variable of an array and an
  entire array can be a argument of a method.
• Methods can return an indexed variable of an
  array or an entire array.

27
Case Study Using an Array as an Instance Variable

• Prepare a sales report showing
• which sales associate (or associates) has the
  highest sales
• how each associates sales compare to the
  average.
• needed for each associate
• name
• sales figures

28
Case Study Using an Array as an Instance
Variable, cont.

• tasks
• obtain the data (getFigures)
• update the instance variables (update)
• display the results (displayResults)

29
Case Study Using an Array as an Instance
Variable, cont.

• class SalesAssociate

30
Case Study Using an Array as an Instance
Variable, cont.

• The program uses an array to keep track of the
  data for all sales associates.

31
Case Study Using an Array as an Instance
Variable, cont.

• class SalesReporter

32
Case Study Using an Array as an Instance
Variable, cont.

• class SalesReporter, contd.

33
Case Study Using an Array as an Instance
Variable, cont.
34
Indexed Variables as Method Arguments

• An indexed variable can be used anywhere that any
  other variable of the base type of the array can
  be used.
• Hence, an indexed variable can be an argument to
  a method.

35
Indexed Variables as Method Arguments, cont.

• class ArgumentDemo

36
Indexed Variables as Method Arguments, cont.

• Note that the results would be the same if the
  arguments provided to method average were
  interchanged.

37
Indexed Variables as Method Arguments, cont.
38
Array Subtleties

• If the base type of an array is a primitive type,
  then a method to which an array element is passed
  creates a copy of the array element, and cannot
  change the original array element.
• If the base type of an array is a class type,
  then a method to which an array element is passed
  creates an alias, and the referenced object can
  be changed.

39
Entire Arrays as Method Arguments

• An entire array can be used as a single argument
  passed to a method.
• example
• double a new double10
• SampleClass.change(a)
• ...
• public static void change(double d)
• No brackets accompany the argument.
• Method change accepts an array of any size.

40
Arguments for the Method main

• Recall the heading for method main
• public static void main(String args)
• Method main takes an array of String values as
  its argument.

41
Arguments for the Method main, cont.

• An array of String values can be provided in the
  command line.
• example
• java TestProgram Mary Lou
• args0 is set to Mary
• args1 is set to Lou
• System.out.println(Hello args0
• args1)
• prints Hello Mary Lou.

42
Use of and with Arrays

• The assignment operator and the equality
  operator , when used with arrays, behave the
  same as when used with other objects.
• The assignment operator creates an alias, not a
  copy of the array.
• The equality operator determines if two
  references contain the same memory address, not
  if two arrays contain the same values.

43
Making a Copy of an Array

• example
• int a new int50
• int b new int50
• ...
• for (int j 0 j lt a.length j)
• bj aj

44
Determining the Equality of Two Arrays

• To determine if two arrays at different memory
  locations contain the same elements in the same
  order, define an equals method which determines
  if
• both arrays have the same number of elements
• each element in the first array is the same as
  the corresponding element in the second array.

45
Determining the Equality of Two Arrays, cont.

• A while loop can be used.
• A boolean variable match is set to true.
• Each element in the first array is compared to
  the corresponding element in the second array.
• If two elements are different, match is set to
  false and the while loop is exited.
• Otherwise, the loop terminates with match still
  set to true.

46
Determining the Equality of Two Arrays, cont.

• class TestEquals

47
Determining the Equality of Two Arrays, cont.
48
Methods that Return Arrays

• A method can return an array.
• The mechanism is basically the same as for any
  other returned type.
• example
• public static Base_Type Method_Name
  (Parameter_List)
• Base_Type Array_Name
• return Array_Name
• The method need not be public or static.

49
Methods that Return Arrays, cont.

• class ReturnArrayDemo

50
Programming with Arrays and Classes Outline

• Programming Example
• Partially Filled Arrays
• Searching an Array

51
Programming Example A Specialized List Class

• An array can be an instance variable of a class
  that is accessible only through the class
  methods.
• We will define a class whose objects store lists
  of items, such as a grocery list or a list of
  things to do.
• Well name the class OneWayNoRepeatsList.

52
Programming Example A Specialized List Class,
cont.

• some features of the class
• a method for adding an item to the list unless it
  is on the list already
• an array of strings to hold the items
• numbering starting with 1 rather than 0

53
Programming Example A Specialized List Class,
cont.

• a maximum list size
• accessor and mutator methods, but no methods for
  changing or deleting items
• a method to erase the entire list

54
Using Class OneWayNowRepeatsList

• Lets discover how OneWayNoRepeatsList works
  before considering the definition of the class.

55
Using Class OneWayNowRepeatsList, cont.

• class ListDemo

56
Using Class OneWayNowRepeatsList, cont.
57
More Features of the Class

• We need a means of determining that the end of
  the list has been reached.
• Let toDoList.getEntryAt(position) return the
  value null.

58
More Features of the Class, cont.

• class OneWayNoRepeatsList

59
More Features of the Class, cont.

• class OneWayNoRepeatsList, contd.

60
More Features of the Class, cont.

• three ways to detect the end of a list
• when the array is at capacity
• when position is at the last entry
• when position advances beyond the last entry and
  null is returned

61
Partially Filled Arrays

• Sometimes you need some, but not all of the
  indexed variables in an array.
• In such situations, it is important to keep track
  of how much of the array has been used and/or the
  index of the last entry so that only meaningful
  values are accessed.

62
Partially Filled Arrays, cont.
63
Searching an Array

• Method onList in class OneWayNoRepeatsList
  searches the array entry to see if parameter item
  is in the array entry.
• This is an example of a sequential search of an
  array.
• The array elements are examined from first to
  last to see in an item is in the array already.

64
Returning an Array Instance Variable

• If a array reference variable can be copied, the
  elements of the array can be accessed, perhaps
  improperly, and changed using the alias, even if
  the array has a private designation.
• Even when a copy of the array is provided, if its
  base type is not a primitive type or type String,
  its elements can be changed unless they, too, are
  copied.

65
Sorting Arrays Outline

• Selection Sort
• Other Sorting Algorithms

66
Sorting Arrays

• Sometime we want numbers in an array sorted from
  smallest to largest, or from largest to smallest.
• Sometimes we want the strings referenced by an
  array to be in alphabetical order.
• Sorting techniques typically are easy to adapt to
  sort any type that can be ordered.

67
Selection Sort

• The selection sort arranges the values in an an
  array so that
• a0 lt a1 lt a2 lt aa.length-1
• The selection sort places the smallest item in
  a0, the next smallest item in a1, and so on
  for all but the last item.
• for(i 0 i lta.length-1 i)
• place the ith smallest item in ai

68
Selection Sort, cont.

• Selection sort begins by finding the smallest
  item in the array and swapping it with the item
  in a0.
• Selection sort continues by finding the smallest
  item in the remainder of the array and swapping
  it with the next item in array a.
• Selection sort terminates when only one item
  remains.

69
Selection Sort, cont.
70
Selection Sort, cont.

• class SelectionSort

71
Selection Sort, cont.
class SelectionSortDemo
72
Selection Sort, cont.
73
Swapping Elements

• To swap two elements ai and aj, one of them
  must be saved temporarily.

74
Swapping Elements, cont.

• class interchange

75
Other Sorting Algorithms

• The selection sort is not particularly efficient,
  but it is easy to code.
• More efficient algorithms, such as quicksort, are
  more difficult to code.

76
Multidimensional Arrays

• Introduction to Multidimensional Arrays
• Multidimensional-Array Basics
• Multidimensional-Array Parameters and Returned
  Values
• Implementation of Multidimensional Arrays
• (optional) Ragged Arrays
• Programming Example

77
Introduction to Multidimensional Arrays

• An array with more than one index sometimes is
  useful.
• example savings account balances at various
  interest rates for various numbers of years
• columns for interest rates
• rows for years
• This two-dimensional table calls for a
  two-dimensional array.

78
Introduction to Multidimensional Arrays, cont.
79
Introduction to Multidimensional Arrays, cont.
80
Introduction to Multidimensional Arrays, cont.

• An array with n indices is called an
  n-dimensional array.
• The arrays we considered previously, which had
  one index, were one-dimensional arrays.

81
Multidimensional-Array Basics

• example declaration
• int table new int 106
• or
• int table
• table new int106
• The number of bracket pairs in the declaration is
  the same as the number of indices.

82
Multidimensional-Array Basics, cont.

• syntax
• Base_Type Array_Name
• new Base_TypeLength_1Length_n
• examples
• char page new char 10080
• double threeDPicture
• new double102030
• SomeClass entry
• new SomeClass10080

83
Multidimensional-Array Basics, cont.

• Nested for loops can be used to change the values
  of the elements of the array and to display the
  values of the elements of the array.

84
Multidimensional-Array Basics, cont.

• class InterestTable

85
Multidimensional-Array Basics, cont.
86
Multidimensional-Array Parameters

• Methods may have multidimensional-array
  parameters.

87
Multidimensional-Array Parameters, cont.

• class InterestTable2

88
Multidimensional-Array Returned Values

• A method may return a multidimensional array.
• example
• public static double corner(double
  sArray, int i)
• double temp new doubleii
• return temp

89
Implementation of Multidimensional Arrays

• Multidimensional arrays are implemented in Java
  using one-dimensional arrays.
• Consider
• int table new int106
• The array table is a one-dimensional array of
  length 10.
• Its base type is int.
• Therefore, it is an array of arrays.

90
Implementation of Multidimensional Arrays, cont.

• This permits us to use the length instance
  variable, instead of an integer literal, to
  control a for loop used to initialize or print
  the values of the elements of an array.
• example
• for (r 0 r lt table.length r)
• for (c 0 c lt tabler.length c)

91
Implementation of Multidimensional Arrays, cont.

• redefined method showTable

92
Ragged Arrays

• Since a two-dimensional array in Java is an array
  of arrays, each row can have a different number
  of elements (columns).
• Arrays in which rows have different numbers of
  elements are called ragged arrays.

93
Ragged Arrays, cont.

• Example
• int b new int3
• b0 new int5
• b1 new int7
• b2 new int4

94
Programming Example Employee Time Records

• Two-dimensional array hours stores hours worked
  by each employee for each of the five workdays.
• Each row represents a day of the work week,
  beginning with Monday as day 0.
• Each column represents an employee, beginning
  with employee 1.

95
Programming Example Employee Time Records, cont.

• class TimeBook
• includes a stub for method setHours.

96
Programming Example Employee Time Records, cont.

• class TimeBook, contd.

97
Programming Example Employee Time Records, cont.
98
Programming Example Employee Time Records, cont.
99
(optional) Graphics Supplement Outline

• Part 1 Text Areas and Text Fields
• Part 2 Drawing Polygons and Polylines

100
Text Areas and Text Fields

• A text area is a window that can be used for text
  input and text output.
• any number of lines
• any number of characters per line
• A text field allows only one line of characters.
• Text area and text fields provide areas for
  changeable text in a GUI.

101
Programming Example A Question and Answer Applet

• The applet accepts a question, requests some
  advice, and provides an answer.
• The applet contains some advice for the first
  user, but then takes the advice provided by one
  user, and uses it to answer the next user.

102
Programming Example, cont.

• class Oracle

103
Programming Example, cont.
104
JTextArea Objects

• A text area is an object of the class JTextArea.
• A JTextArea can be created with
• private JTextArea theText
• theText
• new JTextArea(LINES, CHAR_PER_LINE)
• Typically, this occurs inside the init method.

105
JTextArea Objects, cont.

• The text in the text area can be read using
• String question theText.getText()
• The text in the text area can be set using
• theText.setText(Thats difficult.\n
• I need some advice.\n
• Give me some advice and click.)
• Text can be entered in excess of the specified
  size, but may not be entirely visible.

106
JTextField Objects

• A text field is an object of the class
  JTextField.
• A JTextField can be created with
• private JTextField theText
• theText
• new JTextField(NUMBER_OF_CHARS)
• Typically, this occurs inside the init method.

107
JTextField Objects, cont.

• The text in the text field can be read using
• String question theText.getText()
• The text in the text area can be set using
• theText.setText(Thats all, folks.)
• Text can be entered in excess of the specified
  size, but may not be entirely visible.

108
Drawing Polygons and Polylines

• The methods drawPolygon and fillPolygon allow you
  to draw polygons which are closed figures made of
  of line segments that meet at vertices but do not
  cross.
• A polyline is similar to a polygon, but uses the
  method drawPolyline and need not be closed.

109
Drawing Polygons and Polylines, cont.
110
Drawing Polygons and Polylines, cont.

• class House

111
Drawing Polygons and Polylines, cont.
112
Method drawPolygon

• syntax
• canvas.drawPolygon(Array_of_xs, Array_of_ys,
  Number_of_Points)
• example
• private int xCoord 150, 150, 200, 250,
  250
• private int yCoord 100, 40, 20, 40, 100
• canvas.drawPolygon(xCoord, yCoord,
  xCoord.length)

113
Method fillPolygon

• syntax
• canvas.fillPolygon(Array_of_xs, Array_of_ys,
  Number_of_Points)
• example
• private int xCoord 150, 150, 200, 250,
  250
• private int yCoord 100, 40, 20, 40, 100
• canvas.fillPolygon(xCoord, yCoord,
  xCoord.length)

114
Method drawPolyline

• syntax
• canvas.drawPolyline(Array_of_xs, Array_of_ys,
  Number_of_Points)
• example
• private int xCoord 150, 150, 200, 250,
  250
• private int yCoord 100, 40, 20, 40, 100
• canvas.drawPolyline(xCoord, yCoord,
  xCoord.length)

115
Method drawPolyline, cont.

• There is no automatic line segment from the last
  point to the first point.
• As a consequence, the figure typically is not
  closed.
• A polyline cannot be filled.

116
Summary

• You have learned about arrays and how to use them
  in Java programs.
• You have learned how to use array parameters and
  how to define methods that return an array.
• You have learned the proper use of an array as an
  instance variable.
• You have learned about multidimensional arrays.

117
Summary, cont.

• (optional) You have learned about text fields and
  text areas in applets.
• (optional) You have learned to draw arbitrary
  polygons and polylines in applets.