Arrays in Java Programming Language slides

Chapter 6: Arrays
Chapter 6: Arrays
Presentation slides for
Java Software Solutions
Foundations of Program Design
Third Edition
by John Lewis and William Loftus
Java Software Solutions is published by Addison-Wesley
Presentation slides are copyright 2002 by John Lewis and William Loftus. All rights reserved.
Instructors using the textbook may use and modify these slides for pedagogical purposes.

2
Arrays
Arrays
 Arrays are objects that help us organize large amounts of
information
 Chapter 6 focuses on:
• array declaration and use
• passing arrays and array elements as parameters
• arrays of objects
• sorting elements in an array
• multidimensional arrays
• the ArrayList class
• polygons and polylines
• more button components

3
Arrays
Arrays
 An array is an ordered list of values
0 1 2 3 4 5 6 7 8 9
79 87 94 82 67 98 87 81 74 91
An array of size N is indexed from zero to N-1
scores
The entire array
has a single name
Each value has a numeric index
This array holds 10 values that are indexed from 0 to 9

4
Arrays
Arrays
 A particular value in an array is referenced using the
array name followed by the index in brackets
 For example, the expression
scores[2]
refers to the value 94 (the 3rd value in the array)
 That expression represents a place to store a single
integer and can be used wherever an integer variable can
be used

5
Arrays
Arrays
 For example, an array element can be assigned a value,
printed, or used in a calculation:
scores[2] = 89;
scores[first] = scores[first] + 2;
mean = (scores[0] + scores[1])/2;
System.out.println ("Top = " + scores[5]);

6
Arrays
Arrays
 The values held in an array are called array elements
 An array stores multiple values of the same type (the
element type)
 The element type can be a primitive type or an object
reference
 Therefore, we can create an array of integers, or an array
of characters, or an array of String objects, etc.
 In Java, the array itself is an object
 Therefore the name of the array is a object reference
variable, and the array itself must be instantiated

7
Declaring Arrays
Declaring Arrays
 The scores array could be declared as follows:
int[] scores = new int[10];
 The type of the variable scores is int[] (an array of
integers)
 Note that the type of the array does not specify its size,
but each object of that type has a specific size
 The reference variable scores is set to a new array
object that can hold 10 integers
 See BasicArray.java (page 322)

8
Declaring Arrays
Declaring Arrays
 Some examples of array declarations:
float[] prices = new float[500];
boolean[] flags;
flags = new boolean[20];
char[] codes = new char[1750];

9
Bounds Checking
Bounds Checking
 Once an array is created, it has a fixed size
 An index used in an array reference must specify a valid
element
 That is, the index value must be in bounds (0 to N-1)
 The Java interpreter throws an
ArrayIndexOutOfBoundsException if an array
index is out of bounds
 This is called automatic bounds checking

10
Bounds Checking
Bounds Checking
 For example, if the array codes can hold 100 values, it
can be indexed using only the numbers 0 to 99
 If count has the value 100, then the following reference
will cause an exception to be thrown:
System.out.println (codes[count]);
 It’s common to introduce off-by-one errors when using
arrays
for (int index=0; index <= 100; index++)
codes[index] = index*50 + epsilon;
problem

11
Bounds Checking
Bounds Checking
 Each array object has a public constant called length
that stores the size of the array
 It is referenced using the array name:
scores.length
 Note that length holds the number of elements, not the
largest index
 See ReverseOrder.java (page 324)
 See LetterCount.java (page 326)

12
Alternate Array Syntax
Alternate Array Syntax
 The brackets of the array type can be associated with the
element type or with the name of the array
 Therefore the following declarations are equivalent:
float[] prices;
float prices[];
 The first format generally is more readable

13
Initializer Lists
Initializer Lists
 An initializer list can be used to instantiate and initialize
an array in one step
 The values are delimited by braces and separated by
commas
 Examples:
int[] units = {147, 323, 89, 933, 540,
269, 97, 114, 298, 476};
char[] letterGrades = {'A', 'B', 'C', 'D', ’F'};

14
Initializer Lists
Initializer Lists
 Note that when an initializer list is used:
• the new operator is not used
• no size value is specified
 The size of the array is determined by the number of
items in the initializer list
 An initializer list can only be used only in the array
declaration
 See Primes.java (page 330)

15
Arrays as Parameters
Arrays as Parameters
 An entire array can be passed as a parameter to a
method
 Like any other object, the reference to the array is
passed, making the formal and actual parameters aliases
of each other
 Changing an array element within the method changes
the original
 An array element can be passed to a method as well, and
follows the parameter passing rules of that element's type

16
Arrays of Objects
Arrays of Objects
 The elements of an array can be object references
 The following declaration reserves space to store 25
references to String objects
String[] words = new String[25];
 It does NOT create the String objects themselves
 Each object stored in an array must be instantiated
separately
 See GradeRange.java (page 332)

17
Command-Line Arguments
Command-Line Arguments
 The signature of the main method indicates that it takes
an array of String objects as a parameter
 These values come from command-line arguments that
are provided when the interpreter is invoked
 For example, the following invocation of the interpreter
passes an array of three String objects into main:
> java StateEval pennsylvania texas arizona
 These strings are stored at indexes 0-2 of the parameter
 See NameTag.java (page 334)

18
Arrays of Objects
Arrays of Objects
 Objects can have arrays as instance variables
 Many useful structures can be created with arrays and
objects
 The software designer must determine carefully an
organization of data and objects that makes sense for the
situation
 See Tunes.java (page 335)
 See CDCollection.java (page 337)
 See CD.java (page 340)

19
Sorting
Sorting
 Sorting is the process of arranging a list of items in a
particular order
 The sorting process is based on specific value(s)
• sorting a list of test scores in ascending numeric order
• sorting a list of people alphabetically by last name
 There are many algorithms for sorting a list of items
 These algorithms vary in efficiency
 We will examine two specific algorithms:
• Selection Sort
• Insertion Sort

20
Selection Sort
Selection Sort
 The approach of Selection Sort:
• select a value and put it in its final place into the list
• repeat for all other values
 In more detail:
• find the smallest value in the list
• switch it with the value in the first position
• find the next smallest value in the list
• switch it with the value in the second position
• repeat until all values are in their proper places

21
Selection Sort
Selection Sort
 An example:
original: 3 9 6 1 2
smallest is 1: 1 9 6 3 2
 See SortGrades.java (page 342)
 See Sorts.java (page 343) -- the selectionSort
method

22
Swapping
Swapping
 Swapping is the process of exchanging two values
 Swapping requires three assignment statements
temp = first;
first = second;
second = temp;

23
Insertion Sort
Insertion Sort
 The approach of Insertion Sort:
• pick any item and insert it into its proper place in a sorted sublist
• repeat until all items have been inserted
 In more detail:
• consider the first item to be a sorted sublist (of one item)
• insert the second item into the sorted sublist, shifting the first item
as needed to make room to insert the new addition
• insert the third item into the sorted sublist (of two items), shifting
items as necessary
• repeat until all values are inserted into their proper positions

24
Insertion Sort
Insertion Sort
 An example:
original: 3 9 6 1 2
insert 9: 3 9 6 1 2
insert 6: 3 6 9 1 2
insert 1: 1 3 6 9 2
insert 2: 1 2 3 6 9
 See Sorts.java (page 343) -- the insertionSort
method

25
Sorting Objects
Sorting Objects
 Integers have an inherent order, but the ordering criteria
of a collection of objects must be defined
 Recall that a Java interface can be used as a type name
and guarantees that a particular class implements
particular methods
 We can use the Comparable interface and the
compareTo method to develop a generic sort for a set of
objects
 See SortPhoneList.java (page 347)
 See Contact.java (page 348)
 See Sorts.java (page 343) – the second
insertionSort method

26
Comparing Sorts
Comparing Sorts
 Both Selection and Insertion sorts are similar in efficiency
 They both have outer loops that scan all elements, and
inner loops that compare the value of the outer loop with
almost all values in the list
 Approximately n2
number of comparisons are made to
sort a list of size n
 We therefore say that these sorts are of order n2
 Other sorts are more efficient: order n log2 n

27
Two-Dimensional Arrays
 A one-dimensional array stores a list of elements
 A two-dimensional array can be thought of as a table of
elements, with rows and columns
one
dimension
two
dimensions

28
 To be precise, a two-dimensional array in Java is an
array of arrays
 A two-dimensional array is declared by specifying the
size of each dimension separately:
int[][] scores = new int[12][50];
 A two-dimensional array element is referenced using two
index values
value = scores[3][6]
 The array stored in one row or column can be specified
using one index

29
Expression Type Description
scores int[][] 2D array of integers, or
array of integer arrays
scores[5] int[] array of integers
scores[5][12] int integer
 See TwoDArray.java (page 351)
 See SodaSurvey.java (page 352)

30
Multidimensional Arrays
Multidimensional Arrays
 An array can have many dimensions
 If it has more than one dimension, it is called a
multidimensional array
 Each dimension subdivides the previous one into the
specified number of elements
 Each array dimension has its own length constant
 Because each dimension is an array of array references,
the arrays within one dimension can be of different
lengths
• these are sometimes called ragged arrays

31
The ArrayList Class
The ArrayList Class
 The ArrayList class is part of the java.util package
 Like an array, it can store a list of values and reference
them with an index
 Unlike an array, an ArrayList object grows and shrinks
as needed
 Items can be inserted or removed with a single method
invocation
 It stores references to the Object class, which allows it
to store any kind of object
 See Beatles.java (page 357)

32
ArrayList
ArrayList Efficiency
Efficiency
 The ArrayList class is implemented using an array
 The code of the ArrayList class automatically expands
the array's capacity to accommodate additional elements
 The array is manipulated so that indexes remain
continuous as elements are added or removed
 If elements are added to and removed from the end of
the list, this processing is fairly efficient
 If elements are inserted and removed from the middle of
the list, the elements are constantly being shifted around

33
Polygons and Polylines
Polygons and Polylines
 Arrays often are helpful in graphics processing
 Polygons and polylines are shapes that can be defined
by values stored in arrays
 A polyline is similar to a polygon except that its endpoints
do not meet, and it cannot be filled
 See Rocket.java (page 360)

34
The Rocket Program
The Rocket Program

35
The Polygon
The Polygon Class
Class
 The Polygon class, defined in the java.awt package
can be used to define and draw a polygon
 Two versions of the overloaded drawPolygon and
fillPolygon methods each take a single Polygon
object as a parameter
 A Polygon object encapsulates the coordinates of the
polygon

36
Check Boxes
Check Boxes
 A check box is a button that can be toggled on or off
 A check box is represented by the JCheckBox class
 A change of state generates an item event
 The ItemListener interface corresponds to item
events
 The itemStateChanged method of the listener
responds when a check box changes state

37
The StyleOptions Program
 A frame is a container that can be used to create stand-
alone GUI applications
 A frame is represented by the JFrame class
 A Font object represents by the font's:
• family name (such as Times or Courier)
• style (bold, italic, or both)
• font size
 See StyleOptions.java (page 364)
 See StyleGUI.java (page 365)

38

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Radio Buttons
Radio Buttons
 A set of radio buttons represents a set of mutually
exclusive options
 When a radio button from a group is selected, the other
button currently "on" in the group is toggled off
 A radio button generates an action event
 See QuoteOptions.java (page 368)
 See QuoteGUI.java (page 369)

40
The QuoteOptions Program
The QuoteOptions Program

41
Summary
Summary
 Chapter 6 has focused on:
• array declaration and use
• passing arrays and array elements as parameters
• arrays of objects
• sorting elements in an array
• multidimensional arrays
• the ArrayList class
• polygons and polylines
• more button components

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