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Chapter 10 – Arrays and ArrayList s

Chapter 10 – Arrays and ArrayList s

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Chapter 10 – Arrays and ArrayList s

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  1. Chapter 10 – Arrays and ArrayLists 1 • Array Basics • Array Declaration • Array Creation • Array Element Initialization • Array Default Values • Array length Property • Partially Filled Arrays • Copying an Array • Histograms • Searching an Array • Sorting an Array

  2. Chapter 10 – Arrays and ArrayLists 2 • Selection Sort • Two-Dimensional Arrays • Arrays of Objects • The ArrayList Class • How to Create an ArrayList Object • Adding Elements to an ArrayList Object • How to Access an Element Within an ArrayList • How to Update an ArrayList Object • Additional ArrayList Methods • Printing or Concatenating an ArrayList • Storing Primitives in an ArrayList • ArrayList Example Using Anonymous Objects and the For-Each Loop • ArrayList Objects Versus Standard Arrays

  3. Array Basics 3 • A class stores a group of related data, and it stores the methods that operate on that data. • An array is a limited version of a class. • Like a class, an array also stores a group of related data, but an array does not store methods. • Another difference between an array and a class is that an array's data must all be of the same type. • Here's a picture of an array that holds a list of speed-dial phone numbers. Each of the five boxes is called an array element and each box stores one phone number. first speed-dial phone number last speed-dial phone number

  4. Array Basics 4 • A class uses dot notation to access one of its members. • On the other hand, an array uses square brackets around an index to access one of its elements. • The rightmost column shows how to access each of the 5 elements in the phoneList array. • Note that the index values start at 0 instead of 1 and the last index value is one less than the number of elements in the array. 5 elements

  5. Array Basics 5 • Here's how you can change the first phone number to 2013434: phoneList[0] = 2013434; • And here's how you can print the second phone number: System.out.println(phoneList[1]);

  6. Array Basics 6 /************************************************************* * SpeedDialList.java * Dean & Dean * * This program creates a cell phone speed-dial phone number * list and prints the created list. *************************************************************/ import java.util.Scanner; public class SpeedDialList { public static void main(String[] args) { Scanner stdIn = new Scanner(System.in); long[] phoneList; // list of phone numbers int sizeOfList; // number of phone numbers long phoneNum; // an entered phone number System.out.print( "How many speed-dial numbers would you like to enter? "); sizeOfList = stdIn.nextInt(); phoneList = new long[sizeOfList];

  7. Array Basics 7 for (int i=0; i<sizeOfList; i++) { System.out.print("Enter phone number: "); phoneNum = stdIn.nextLong(); phoneList[i] = phoneNum; } // end for System.out.println("\nSpeed Dial List:"); for (int i=0; i<sizeOfList; i++) { System.out.println((i + 1) + ". " + phoneList[i]); } // end for } // end main } // end class SpeedDialList

  8. Array Declaration 9 • An array is a variable and, as such, it must be declared before you can use it. • Array declaration syntax: <element-type>[] <array-variable>; • Array declaration examples: int[] ids; double[] workHours; String[] names;

  9. Array Creation 10 • In Java, arrays are objects. • As with all objects, arrays are created/instantiated with the new operator. • Syntax for creating and assigning an array object: <array-variable> = new <element-type>[<array-size>]; • Example: long[] phoneList; phoneList = new long[10]; <element-type> indicates the type of each element in the array <array-size> indicates the number of elements in the array array creation

  10. Array Creation 11 • It's legal to combine an array's declaration, creation, and assignment operations. Here's an example: long[] phoneList = new long[10]; • Provide a single statement that declares, creates, and assigns a 100-element array that stores book titles.

  11. Array Element Initialization 12 • An array initializer is a single statement made up of an array declaration, creation, and {} assignment. • Array element initialization syntax: <element-type>[] <array-name> = {<element-values-list>}; • Array element initialization example: String[] students = {"Christopher", "TJ", "Ellie"}; • When an array initializer is used, the size of the array equals the number of elements in the initialization list. • Note that with an array initializer, you creating an array object without using the new operator.

  12. Array Default Values 13 • An array is an object and an array's elements are the instance variables for an array object. As such, an array's elements get default values when the array is instantiated, the same as any other instance variables get default values. • Here are the default values for array elements (they're also the default values for all instance variables):

  13. Array length Property 14 • Suppose you have a five-element colors array that's been initialized like this: String[] colors = {"blue", "gray", "lime", "teal", "yellow"}; • Here's how to print such an array: for (int i=0; i<colors.length; i++) { System.out.println(colors[i]); } • To obtain an array's length, specify array name, dot, and then length. • Note that length is used in two different ways: • length is a String method • length is an array property • Mnemonic acronym to help you remember when to use parentheses with length: • ANSY (arrays no, strings yes) Note how an array object's length property gets the array's size.

  14. Array length Property and Partially Filled Arrays 15 import java.util.Scanner; public class SpeedDialList2 { public static void main(String[] args) { Scanner stdIn = new Scanner(System.in); String[] phoneList = new String[100]; // phone numbers int filledElements = 0; // number of phone numbers String phoneNum; // an entered phone number System.out.print("Enter phone number (or q to quit): "); phoneNum = stdIn.nextLine(); while (!phoneNum.equalsIgnoreCase("q") && filledElements < phoneList.length) { if (phoneNum.length() < 1 || phoneNum.length() > 16) { System.out.println("Invalid entry." + " Must enter between 1 and 16 characters."); } Array length property does not use ( )'s. String length method uses ( )'s.

  15. Array length Property and Partially Filled Arrays 16 else { phoneList[filledElements] = phoneNum; filledElements++; } System.out.print("Enter phone number (or q to quit): "); phoneNum = stdIn.nextLine(); } // end while System.out.println("\nSpeed Dial List:"); for (int i=0; i<filledElements; i++) { System.out.println((i + 1) + ". " + phoneList[i]); } // end for } // end main } // end class SpeedDialList2

  16. Partially Filled Arrays 17 • A partially filled array is when you use some of an array's elements, but not all. • If you have a partially filled array, you have to keep track of the number of filled elements in the array so you can process the filled elements differently from the non-filled elements. • In the SpeedDialList2 program, note how the filledElements variable keeps track of the number of phone numbers in the array.

  17. Copying an Array 18 • As with all objects and their associated reference variables, if you assign one array reference variable to another array reference variable, both array reference variables then point to the same single array object. • What's the problem with that scenario? More specifically, what's wrong with the following code if the goal is to make a copy of arr1? arr2 = arr1;

  18. Copying an Array 19 • Usually when you make a copy of an array, you'll want the copy and the original to point to different array objects. To do that, assign array elements one at a time. • For example, suppose you use arrays to hold a store's prices, one array for each month's prices. And you'd like to copy January's price array into February's price array and make a change in February's second price. The program on the next slide does that by assigning array elements one at a time. Here's the program's output: Jan Feb 1.29 1.29 9.99 10.99 22.00 22.00 4.55 4.55 7.35 7.35 6.49 6.49

  19. Copying an Array 20 public class ArrayCopy { public static void main(String[] args) { double[] pricesJanuary = {1.29, 9.99, 22.50, 4.55, 7.35, 6.49}; double[] pricesFebruary = new double[pricesJanuary.length]; for (int i=0; i<pricesJanuary.length; i++) { pricesFebruary[i] = pricesJanuary[i]; } pricesFebruary[1] = 10.99; System.out.printf("%7s%7s\n", "Jan", "Feb"); for (int i=0; i<pricesJanuary.length; i++) { System.out.printf("%7.2f%7.2f\n", pricesJanuary[i], pricesFebruary[i]); } } // end main } // end class ArrayCopy

  20. Histograms 21 • A histogram is a graph that displays quantities for a set of categories. It indicates category quantities with bars - shorter bars equate to smaller quantities, longer bars equate to larger quantities. • For example, the histogram on the next slide shows quantities of frozen desserts produced in the United States in 2003.

  21. Histograms 22

  22. Histograms 23 • Problem Description: • Suppose you have three coins. When you flip all three, you're curious how likely it is you'll get zero heads, how likely it is you'll get one head, how likely it is you'll get two heads, and how likely it is you'll get three heads. In other words, you're curious about the frequency distribution for the number of heads. • Solution: • Write a main method that simulates throwing the three coins a million times. • Print the simulation results in the form of a histogram: • For each of the four cases (zero heads, one head, etc.), print a bar that represents the number of times the case occurred. • To mimic a bar, print a certain number of *'s where each * represents 1% of the total number of simulation iterations.

  23. Histograms 24 • Output for the coin-flip program: Number of times each head count occurred: 0 124685 ************ 1 374759 ************************************* 2 375420 ************************************** 3 125136 ************* • Program implementation tips: • Use a four-element frequency array to keep track of the number of times each head-count value occurs. The frequency[0] element holds the number of times none of the three coins lands heads up. The frequency[1] element holds the number of times one of the three coins lands heads up. • Each element in the frequency array is called a "bin." • After each simulation iteration, add 1 to the appropriate bin. For example, increment the frequency[3] bin if all three of the coins land heads up.

  24. Histograms 25 public class CoinFlips { public static void main(String[] args) { final int NUM_OF_COINS = 3; // number of coins that are flipped final int NUM_OF_REPS = 1000000; // total number of repetitions // The frequency array holds the number of times a particular number // of heads occurred. int[] frequency; int heads; // number of heads in the current group of flips float fractionOfReps; // Number of times a particular head count occurred // divided by total number of repetitions. int numOfAsterisks; // Number of asterisks in histogram for a // particular head count. frequency = new int[NUM_OF_COINS + 1]; for (int rep=0; rep<NUM_OF_REPS; rep++) { // perform a group of flips heads = 0; for (int i=0; i<NUM_OF_COINS; i++) { heads += (int) (Math.random() * 2); } frequency[heads]++; // update appropriate bin } // end for

  25. Histograms 26

  26. Histograms 27 System.out.println( "Number of times each head count occurred:"); for (heads=0; heads<=NUM_OF_COINS; heads++) { System.out.print( " " + heads + " " + frequency[heads] + " "); fractionOfReps = (float) frequency[heads] / NUM_OF_REPS; numOfAsterisks = Math.round(fractionOfReps * 100); for (int i=0; i<numOfAsterisks; i++) { System.out.print("*"); } System.out.println(); } // end for } // end main } // end class CoinFlips

  27. Searching an array 28 • Often, you'll need to determine whether an array contains a particular value. Here's the pseudocode algorithm for conducting a sequential search for a particular value within a list array: i ← 0 while i < number of filled elements if list[i] equals the searched-for value <do something and stop the loop> increment i

  28. Searching an array 29 • Problem description: • Write a helper method named findStudent that searches for an id value within an array of student id's. • The findStudent method should receive an id parameter and return the index value of id's location within a studentIds array instance variable. • If id's value is not found, then return -1. • As always, use appropriate access modifiers (public vs. private, class method vs. instance method).

  29. Sorting an Array 31 • Sorting is a very common task in programming. • Examples: • Sort e-mails in an inbox – by date, by sender • Sort songs – by title, by author • Sort student records – by student ID

  30. Selection Sort 32 • There are many different sorting algorithms with varying degrees of complexity and efficiency. Since this is your first exposure to sorting, we'll cover a simple algorithm - the selection sort algorithm. Here it is: for (i0; i<list's_length; i++) find the smallest value in list from list[i] down to the bottom of the list swap the found value with list[i] list (original) list (sorted)

  31. Sorting an Array 33 • You could include a sorting method in every class that needs sorting functionality. • For example, you could include: • A studentSort method in a Students class that sorts students by student id. • A bookSort method in a Books class that sorts books by ISBN number. • A productSort method in a Products class that sorts products by product id. • But suppose that you want to make a generic sorting method that receives an array as a parameter (e.g., studentIds, bookISBNs, or productIds) and sorts it.

  32. Sorting an Array 34 • To make the sorting method generic so that it can be used by multiple classes, put the sort method in a utility class. • A utility class is a class with general-purpose methods that other classes can easily use. To make the methods easy to use, use class methods (as opposed to instance methods). • Why would it be easy for other classes to use the sort method if the sort method is implemented as a class method?

  33. Sorting an Array 35 • If you make the sort method an instance method, then you'll be required to instantiate the sort method's enclosing class prior to calling the sort method. For example, assuming the sort method's enclosing class is named Sort: Sort s = new Sort(); s.sort(studentIds); • On the other hand, if you make the sort method a class method, then you are not required to instantiate the sort method's enclosing class prior to calling the sort method. Instead, you simply need to prefix the sort method call with the class name and then a dot. For example: Sort.sort(studentIds); • Thus, in the interest of simplicity and elegance, let's make the sort method a class method.

  34. Selection Sort 36 public class Sort { public static void sort(int[] list) { int j; for (int i=0; i<list.length-1; i++) { j = indexOfNextSmallest(list, i); swap(list, i, j); } } // end sort private static int indexOfNextSmallest(int[] list, int startIndex) { int min = list[startIndex]; int minIndex = startIndex; for (int i=startIndex+1; i<list.length; i++) { if (list[i] < min) { min = list[i]; minIndex = i; } } // end for return minIndex; } // end indexOfNextSmallest

  35. Selection Sort 37 private static void swap(int[] list, int i, int j) { int temp; temp = list[i]; list[i] = list[j]; list[j] = temp; } // end swap } // end Sort public class SortDriver { public static void main(String[] args) { int[] studentIds = {3333, 1234, 2222, 1000}; Sort.sort(studentIds); for (int i=0; i<studentIds.length; i++) { System.out.print(studentIds[i] + " "); } } // end main } // end SortDriver

  36. Two-Dimensional Arrays 38 • If you have a group of related data that's organized in a table format, consider using a two-dimensional array. • Two-dimensional arrays use the same basic syntax as one-dimensional arrays except for a second pair of []'s. • the first index identifies the row and the second index identifies the column position within a row. • For example, here's a two-row by three-column array named x: column indexes row indexes

  37. Two-Dimensional Arrays 39 • As with one-dimensional arrays, there are two ways to assign values into a two-dimensional array’s elements: 1) an array initializer, 2) assignment statements. • Here’s how you can declare the previous slide's x array and assign values into its elements, using an array initializer: int[][] x = {{8,-2,4}, {1,0,5}}; • You can use the array initializer technique only if you know the assigned values when you first declare the array. Otherwise, you need to provide array element assignment statements that are separate from the array's declaration and creation. initializer for a 2-row by 3-column array

  38. Two-Dimensional Arrays 40 • The following code fragment declares and creates the x array in one statement, and assigns values to x's elements in a separate statement. int[][] x = new int[2][3]; for (int i=0; i<x.length; i++) { for (int j=0; j<x[0].length; j++) { System.out.print( "Enter value for row " + i + ", col " + j); x[i][j] = stdIn.nextInt(); } // end for j } // end for i Declare and create a 2-row by 3-column array. Assign a value to the element at row i column j.

  39. Two-Dimensional Arrays 41 • Bottom line: To loop through the rows in a two-dimensional array, use <array-name>.length . And to loop through the elements within a particular row, use <array-name>[0].length. For example: for (int i=0; i<x.length; i++) { for (int j=0; j<x[0].length; j++) { ...

  40. Two-Dimensional Arrays 42 • The upcoming FlightTimes program: • Uses a two-dimensional array to store this table of flight times between cities: Wch Top KC Col StL Wch 0 22 30 42 55 Top 23 0 14 25 37 KC 31 9 0 11 28 Col 44 27 12 0 12 StL 59 41 30 14 0 • Contains a promptForFlightTime method that prompts the user for a departure city and a destination city and prints the flight time for that flight. • Contains a displayFlightTimesTable method that prints the table. It takes 25 minutes to fly from Topeka, KS to Columbia, MO.

  41. Two-Dimensional Arrays 43 public class FlightTimesDriver { public static void main(String[] args) { int[][] flightTimes = { {0, 22, 30, 42, 55}, {23, 0, 14, 25, 37}, {31, 9, 0, 11, 28}, {44, 27, 12, 0, 12}, {59, 41, 30, 14, 0} }; // Define terminals in the Kansas-Missouri region. String[] cities = {"Wch", "Top", "KC", "Col", "StL"}; FlightTimes ft = new FlightTimes(flightTimes, cities); System.out.println("\nFlight times for KansMo Airlines:\n"); ft.displayFlightTimesTable(); System.out.println(); ft.promptForFlightTime(); } // end main } // end class FlightTimesDriver

  42. Two-Dimensional Arrays 44 import java.util.Scanner; public class FlightTimes { private int[][] flightTimes; // table of flight times private String[] cities; // names of cities in flightTimes table public FlightTimes(int[][] ft, String[] c) { flightTimes = ft; cities = c; } //******************************************** // This method prompts the user for departure and destination cities // and prints the associated flight time. public void promptForFlightTime() { Scanner stdIn = new Scanner(System.in); int departure; // index for departure city int destination; // index for destination city

  43. Two-Dimensional Arrays 45 for (int i=0; i<cities.length; i++) { System.out.println(i+1 + " = " + cities[i]); } System.out.print("Enter departure city's number: "); departure = stdIn.nextInt() - 1; System.out.print("Enter destination city's number: "); destination = stdIn.nextInt() - 1; System.out.println("Flight time = " + flightTimes[departure][destination] + " minutes."); } // end promptForFlightTime //******************************************** // This method prints a table of all flight times. <Insert displayFlightTimesTable method here.> } // end class FlightTimes

  44. Arrays of Objects 47 • Suppose you need to keep track of total sales for each sales clerk in a department store. • In the following clerks array, each array element holds a reference for a SalesClerk object. • Each SalesClerk object holds a sales clerk's name and a total-sales value for the sales clerk. • If sales clerk Amanda sells two items for $55.45 and $22.01, then you'd like to store 77.46 for her total-sales value. Daniel, 6.25 Josh, 58.12 Amanda, 77.46

  45. Arrays of Objects 48 • Show how the clerks array gets filled with the input shown below.

  46. Arrays of Objects 49 import java.util.Scanner; public class SalesClerksDriver { public static void main(String[] args) { Scanner stdIn = new Scanner(System.in); SalesClerks clerks = new SalesClerks(2); String name; System.out.print("Enter clerk's name (q to quit): "); name = stdIn.nextLine(); while (!name.equals("q")) { System.out.print("Enter sale amount: "); clerks.addSale(name, stdIn.nextDouble()); stdIn.nextLine(); // flush newline System.out.print("Enter clerk's name (q to quit): "); name = stdIn.nextLine(); } // end while clerks.dumpData(); } // end main } // end SalesClerksDriver

  47. Arrays of Objects 50 class SalesClerks { private SalesClerk[] clerks; // contains names and sales private int filledElements = 0; // number of elements filled //*********************************************************** public SalesClerks(int initialSize) { clerks = new SalesClerk[initialSize]; } // end SalesClerks constructor //*********************************************************** // Process a sale for the clerk whose name is passed in. // If the name is not already in the clerks array, // create a new object and insert a reference to it in the // next array element, doubling array length if necessary. public void addSale(String name, double amount) { int clerkIndex = findClerk(name); if (clerkIndex == -1) // add a new clerk { if (filledElements == clerks.length) { doubleLength(); }

  48. Arrays of Objects 51 clerkIndex = filledElements; clerks[clerkIndex] = new SalesClerk(name); filledElements++; } // end if clerks[clerkIndex].adjustSales(amount); } // end addSale //********************************************************** // Print all the data - sales clerk names and sales. public void dumpData() { for (int i=0; i<filledElements; i++) { System.out.printf("%s: %6.2f\n", clerks[i].getName(), clerks[i].getSales()); } } // end dumpData //**********************************************************

  49. Arrays of Objects 52 // Search for the given name. If found, return the index. // Otherwise, return -1. private int findClerk(String name) { for (int i=0; i<filledElements; i++) { if (clerks[i].getName().equals(name)) { return i; } } // end for return -1; } // end findClerk //********************************************************** // Double the length of the array. private void doubleLength() { SalesClerk[] clerks2 = new SalesClerk[2 * clerks.length]; System.arraycopy(clerks, 0, clerks2, 0, clerks.length); clerks = clerks2; } // end doubleLength } // end class SalesClerks The arraycopy method copies the first argument's array (starting at the second argument's position) to the third argument's array (starting at the fourth argument's position). The fifth argument specifies the number of elements that are to be copied.

  50. Arrays of Objects 53 public class SalesClerk { private String name; // sales clerk's name private double sales = 0.0; // total sales for clerk //******************************************************* public SalesClerk(String name) { this.name = name; } //******************************************************* public String getName() { return name; } public double getSales() { return sales; } //******************************************************* // Adjust clerk's total sales by adding the passed-in sale. public void adjustSales(double amount) { sales += amount; } } // end class SalesClerk