Notes

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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.

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

Arrays An array is an ordered list of values

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 (which is the 3rd value in the array) That expression represents a place to store a single integer which can be used wherever an integer variable can be used

Arrays For example, value in an array can be assigned a value, printed, or used in a calculation scores[2] = 89; scores[first] = scores[first] + 2; mean = (scores[0] + scores[9])/2; System.out.println (“Top = “ + scores[5]);

Arrays An array stores multiple values of the same type That type can be primitive types or object references 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

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

BasicArray.java

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

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

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

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 (just like any other object):         scores.length Note that length holds the number of elements, not the largest index See ReverseOrder.java (page xxx) See LetterCount.java (page xxx)

ReverseOrder.java

LetterCount.java

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

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', ’E'};

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 declaration of an array See Primes.java (page xxx)

Primes.java

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

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 xxx)

GradeRange.java

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 DoIt pennsylvania texas california These strings are stored at indexes 0-2 of the parameter See NameTag.java (page xxx)

NameTag.java

Arrays of Objects Objects can have arrays as instance variables Therefore, many useful structures can be created simply 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 xxx) See CDCollection.java (page xxx) See CD.java (page xxx)

Tunes.java (Figure 6.3 here)

CDCollection.java

CD.java

Sorting Sorting is the process of arranging a list of items in a particular order There must be some value on which the order is based 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

Selection Sort The approach of Selection Sort: select a value and put it in its final place into the sort 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

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

SortGrade.java

Sorts.java

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

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

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 xxx) -- the insertionSort method

Sorts.java

Sorting Objects Integers have an inherent order, but the order of a collection of objects must be defined by the person defining the class Recall that a Java interface can be used as a type name and guarantees that a particular class implementes 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 xxx) See Contact.java (page xxx) See Sorts.java (page xxx)

SortPhoneList.java

Contact.java

Sorts.java

Comparing Sorts Both Selection and Insertion sorts are similar in efficiency The 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 Therefore 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

Two-Dimensional Arrays A one-dimensional array stores a simple list of values A two-dimensional array can be thought of as a table of values, with rows and columns Because each dimension is an array of array references, the arrays within one dimension can be of different lengths Sometimes these are called ragged arrays

Two-Dimensional Arrays A two-dimensional array element is referenced using two index values value = scores [3][6] To be precise, a two-dimensional array in Java is an array of arrays

Two-Dimensional Arrays (Figure 6.6 here) See TwoDArray.java (page xxx) See SodaSurvey.java (page xxx)

TwoDArray.java

SodaSurvey.java

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

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

Some Methods of the ArrayList Class (Figure 6.8 here) See Beatles.java (page xxx)

ArrayList Efficiency The ArrayList class is implemented using an array. The array expands beyond its initial capacity to accommodate additional elements Methods manipulate the array so that indexes remain continuous as elements are added or removed (Figure 6.9 here)

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 xxx)

Rocket.java

The Polygon 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 take a single Polygon object as a parameter A Polygon object encapsulated the coordinates of the polygon

Some Methods of the Polygon Class (Figure 6.9 here)

Other Button Components 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 See StyleOptions.java (page xxx) See StylePanel.java (page xxx)

StyleOptions.java

StylePanel.java

The Font Class A Font object is defined by the font name, the font style, and the font size The style of a font can be plain, bold, italic, or bold and italic together The itemStateChanged method of the listener responds when a check box changes state

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 xxx) See QuotePanel.java (page xxx)

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