# Chapter 10 – Arrays and ArrayLists

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Chapter 10 – Arrays and ArrayLists
Array Basics Array Declaration Array Creation Array Element Initialization Array Default Values Array length Property Partially Filled Arrays Copying an Array Searching an Array Sorting an Array Selection Sort

Chapter 10 – Arrays and ArrayLists
2 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

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 phone numbers. Each of the five boxes is called an array element and each box stores one phone number. phoneList first phone number last phone number

how to access each element
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. index phoneList how to access each element phoneList[0] 1 phoneList[1] 2 phoneList[2] 3 phoneList[3] 4 phoneList[4] 5 elements

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

Array Basics 6 /************************************************************* * ContactList.java * Dean & Dean * * This program creates a cell phone contacts phone number * list and prints the created list. *************************************************************/ import java.util.Scanner; public class ContactList { 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 contact numbers would you like to enter? "); sizeOfList = stdIn.nextInt(); phoneList = new long[sizeOfList];

Array Basics for (int i=0; i<sizeOfList; i++) {
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("\nContacts List:"); System.out.println((i + 1) + ". " + phoneList[i]); } // end main } // end class ContactList

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;

Array Creation In Java, arrays are objects.
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

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.

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 = {"Hamoud", "Lee", "Brandon"}; 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 create an array object without using the new operator.

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 instance variables and class variables): Array element's type Default value integer floating point 0.0 boolean false reference null For example, what are the default values below? float[] gpas = new float[1000]; String[] states = new String[50];

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.

Array length Property and Partially Filled Arrays
15 import java.util.Scanner; public class ContactList2 { 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.

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("\nContact List:"); for (int i=0; i<filledElements; i++) System.out.println((i + 1) + ". " + phoneList[i]); } // end for } // end main } // end class ContactList2

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 ContactList2 program, note how the filledElements variable keeps track of the number of phone numbers in the array.

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;

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

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"); System.out.printf("%7.2f%7.2f\n", pricesJanuary[i], pricesFebruary[i]); } // end main } // end class ArrayCopy

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.

Histograms 22

Histograms Problem Description: Solution:
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.

Histograms Output for the coin-flip program:
24 Output for the coin-flip program: Number of times each head count occurred: ************ ************************************* ************************************** ************* 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.

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 = ; // 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

Histograms heads frequency 1st group of flips 1 0 1 2
26 heads frequency 1st group of flips 1 0 1 2 2nd group of flips 2 0 1 3 3rd group of flips ···

Histograms System.out.println(
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

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

Searching an array Problem description:
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).

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

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) 1 2 3 4 5 10 -3 20 2 -3 10 5 20 2 -3 2 5 20 10 -3 2 5 20 10 -3 2 5 10 20

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.

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?

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.

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

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

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: x how to access each element 1 2 8 -2 4 x[0][0] x[0][1] x[0][2] 5 x[1][0] x[1][1] x[1][2] column indexes row indexes

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

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.

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++) ...

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 Top KC Col StL 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.

Two-Dimensional Arrays
43 public class FlightTimesDriver { public static void main(String[] args) int[][] flightTimes = {0, 22, 30, 42, 55}, {23, 0, 9, 25, 37}, {31, 10, 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

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

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

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 Derrick sells two items for \$55.45 and \$22.01, then you'd like to store for his total-sales value. clerks clerks[0] clerks[1] clerks[2] clerks[3] null Oybek, 6.25 Chris, 58.12 Derrick, 77.46

Arrays of Objects 48 Using the input shown below, how would the clerks array get filled? input filledElements Oybek 6.25 Chris 58.12 Derrick 40 -6.25 12.88 clerks null

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 } // end while clerks.dumpData(); } // end main } // end SalesClerksDriver

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(); }

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

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.

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

Arrays of Objects Beware:
54 Beware: To create an array of objects, you must instantiate the array with the new operator, and you must also instantiate each object that's stored in the array with individual new operators. For example, the SalesClerks constructor instantiates an array of SalesClerk objects like this: clerks = new SalesClerk[maxClerks]; You might think that since the above instantiation specifies maxClerks number of SalesClerk objects, the JVM instantiates all the SalesClerk objects. On the contrary, the JVM only instantiates an array object and each array element holds null. To fill up the clerks array with SalesClerk objects, you need to instantiate the SalesClerk objects individually.

The ArrayList Class 55 The ArrayList class provides the basic functionality that comes with a standard array, plus it provides additional functionality. The basic functionality: An ArrayList stores an ordered collection of values and allows access to the values via an index. The added functionality: An ArrayList grows and shrinks dynamically by inserting and deleting elements at any specified location.

How to Create an ArrayList Object
56 The ArrayList class is defined in the Java API's java.util package, so for files that use the ArrayList class, import it like this: import java.util.ArrayList; To initialize an ArrayList reference variable, use this syntax: ArrayList<element-type> reference-variable = new ArrayList<element-type>(); For example, here's how to initialize an ArrayList reference variable named students: ArrayList<Student> students = new ArrayList<Student>(); Compare the above ArrayList example to this corresponding standard-array example: Student[] students = new Student[100];

How to Create an ArrayList Object
57 What are the syntax differences between the ArrayList example and the standard-array example? With the ArrayList example: Use angled brackets to specify the type for the elements. <element-type> must be a class name (not a primitive). Do not specify the number of elements (because ArrayList objects start out with no elements).

Adding Elements to an ArrayList Object
58 To add an element to the end of an ArrayList object, use this syntax: ArrayList-reference-variable.add(item); The item that's added must be the same type as the type specified in the ArrayList's declaration. Write a code fragment that creates this ArrayList object: computerScientists "Ada Lovelace" 1 "Grace Hopper" 2 "Carol Bartz"

Java API API stands for application programming interface.
59 API stands for application programming interface. The Java API is the interface to the huge library of pre-built Java classes. As a programmer, you don't need to know the internals of those classes; you just need to know how to use them. Or said another way, you just need to know how to interface with them. To interface with them, you need to use their public methods. To use a method, you need to know what type of argument(s) to pass to it and what type of value it returns. A method's API shows the method's parameters and its return type. The standard way to show that information is to show the method's heading. For example, here's the API heading for the Math class's pow method: public static double pow(double num, double power)

How to Access an Element Within an ArrayList
60 With standard arrays, you use square brackets to access and update an element. ArrayList objects don't use square brackets. Instead, they use a get method to access an element and a set method to update an element. Here's the API heading for the ArrayList's get method: public E get(int index) Semantics: The index parameter specifies the position of the desired element within the ArrayList calling object. As with standard arrays, the first element is at position 0, the second element is at position 1, etc. If index refers to a nonexistent element, then a runtime error occurs. If index is valid, then get returns the element at the specified position.

How to Access an Element Within an ArrayList
61 Note the E return type for the ArrayList's get method: public E get(int index) The E stands for element. It represents the data type of the ArrayList's elements. It's the same as the element-type specified in the ArrayList's initialization: ArrayList<element-type> reference-variable = new ArrayList<element-type>();

How to Update an ArrayList Object
62 The set method allows you to assign a value to an existing ArrayList element. Here's its API heading: public E set(int index, E elem) Semantics: The index parameter specifies the position of the element you're interested in. If index refers to a nonexistent element, then a runtime error occurs. If index is valid, then set assigns the elem parameter to the specified element, overlaying whatever was there originally. E represents the data type of the ArrayList's elements.

How to Update an ArrayList Object
63 Draw a picture of the colors ArrayList after this code fragment executes: String mixedColor; ArrayList<String> colors = new ArrayList<String>(); colors.add("red"); colors.add("green"); colors.add("blue"); mixedColor = colors.get(0) + colors.get(1); colors.set(2, mixedColor);

64 public void add(int index, E elem) Starting with the specified index position, shift the original elements to higher-indexed positions. Then insert the elem parameter at the specified index position. public void clear() Remove all elements from the list. public int indexOf(Object elem) Search for the first occurrence of the elem parameter within the list. If it's found, return its index position. If it's not found, return -1. public boolean isEmpty() Return true if the list contains no elements. public E remove(int index) Remove the element at the specified index position, shift all higher-indexed elements to lower-indexed positions, and return the removed element. public int size() Return the number of elements in the list. Object is a generic class that can be used as a class type for any object.

Example ArrayList Program
66 import java.util.ArrayList; public class Survivor { public static void main(String[] args) int loserIndex; // index of person who gets kicked out String loser; // person who gets kicked out ArrayList<String> tribe = new ArrayList<String>(); tribe.add("Richard"); tribe.add("Jerri"); tribe.add("Colby"); tribe.add("Amber"); tribe.add("Rupert"); loserIndex = (int) (Math.random() * 5); loser = tribe.remove(loserIndex); System.out.println("Sorry, " + loser + ". The tribe has spoken. You must leave immediately."); System.out.println("Remaining: " + tribe); } // end main } // end Survivor

Printing or Concatenating an ArrayList
67 If you attempt to print or concatenate an ArrayList, the ArrayList returns a comma-separated list of ArrayList elements surrounded by square brackets, []. For example, in the Survivor program, if Colby is removed, the last line prints this: Remaining: [Richard, Jerri, Amber, Rupert]

Storing Primitives in an ArrayList
68 As mentioned previously, ArrayLists store references. For example, in the Survivor program, tribe is an ArrayList of strings, and strings are reference types. If you need to store primitives in an ArrayList, you can't do it directly, but you can do it if the primitives are wrapped up in wrapper classes. Ever since Java 5.0, the "wrapping up" process has been done behind the scenes. For ArrayLists, it's done automatically if a wrapper class is used in an ArrayList declaration. The StockAverage program on the next slide reads int stock values and stores them in an ArrayList. After all stock values are entered, the program calculates the average stock value. Why is an ArrayList appropriate for calculating a stock average?

Storing Primitives in an ArrayList
69 import java.util.Scanner; import java.util.ArrayList; public class StockAverage { public static void main(String[] args) Scanner stdIn = new Scanner(System.in); ArrayList<Double> stocks = new ArrayList<Double>(); double stock; // a stock value double stockSum = 0; // sum of stock values System.out.print("Enter a stock value (-1 to quit): "); stock = stdIn.nextDouble(); while (stock >= 0) stocks.add(stock); } // end while Must be a wrapper class, not a primitive. Automatic boxing (“autoboxing”) takes place here.

Storing Primitives in an ArrayList
70 for (int i=0; i<stocks.size(); i++) { stock = stocks.get(i); stockSum += stock; } if (stocks.size() != 0) System.out.printf("\nAverage stock value = \$%.2f\n", stockSum / stocks.size()); } // end main } // end class StockAverage Where does automatic unboxing take place?

ArrayList Example Using Anonymous Objects and the For-Each Loop
71 When storing objects in an ArrayList, it's common to create an object and add it to the ArrayList all in the same statement. For example, the upcoming BearStore program stores Bear objects in an ArrayList. In storing a Bear object, the program creates a Bear object and adds it to the bears ArrayList, all in the same statement: bears.add(new Bear("Acme", "brown teddy")); An anonymous object is an object that's instantiated, but it's not stored in a variable (and with no variable, there's no name for it; thus, we say it's "anonymous").

ArrayList Example Using Anonymous Objects and the For-Each Loop
72 import java.util.Scanner; import java.util.ArrayList; public class BearStore { ArrayList<Bear> bears = new ArrayList<Bear>(); //********************************************************** // Fill store with specified number of standard teddy bears. public void addStdBears(int num) for (int i=0; i<num; i++) bears.add(new Bear("Acme", "brown teddy")); } } // end addStdBears anonymous object

ArrayList Example Using Anonymous Objects and the For-Each Loop
73 //********************************************************** // Fill store with specified number of customized bears. public void addUserSpecifiedBears(int num) { for (int i=0; i<num; i++) bears.add(getUserSpecifiedBear()); } } // end addUserSpecifiedBears // Prompt user for a customized bear name and return bear. private Bear getUserSpecifiedBear() Scanner stdIn = new Scanner(System.in); String maker, type; System.out.print("Enter bear's maker: "); maker = stdIn.nextLine(); System.out.print("Enter bear's type: "); type = stdIn.nextLine(); return new Bear(maker, type); } // end getUserSpecifiedBear anonymous object

ArrayList Example Using Anonymous Objects and the For-Each Loop
74 //********************************************************** // Print all the bears in the store. public void displayInventory() { for (Bear bear : bears) bear.display(); } } // end displayInventory public static void main(String[] args) BearStore store = new BearStore(); store.addStdBears(3); store.addUserSpecifiedBears(2); store.displayInventory(); } // end main } // end BearStore class

ArrayList Example Using Anonymous Objects and the For-Each Loop
75 public class Bear { private final String MAKER; // bear's manufacturer private final String TYPE; // type of bear //********************************************************** public Bear(String maker, String type) MAKER = maker; TYPE = type; } public void display() System.out.println(MAKER + " " + TYPE); } // end Bear class

Anonymous Objects 76 The bear store program contains several specific examples of using anonymous objects. In general, you'll see anonymous objects being used in two circumstances: Passing a newly created object into a method or constructor. For example: bears.add(new Bear("Acme", "brown teddy")); Returning a newly created object from a method. For example: return new Bear(maker, type);

For-Each Loop 77 Note the for-each loop in the BearStore's displayInventory method: public void displayInventory() { for (Bear bear : bears) bear.display(); } } // end displayInventory For-each loop syntax for an ArrayList: for (<element-type> <element-name> : <ArrayList-reference-variable>) Read this as "for each bear in bears, …" For each iteration through the loop, bear accesses the next element in the bears ArrayList.

For-Each Loop 78 Note that using the for-each loop is an option, not a requirement. Here's an alternative displayInventory implementation that uses a standard for loop: public void displayInventory() { for (int i=0; i<bears.size(); i++) bears.get(i).display(); } } // end displayInventory The for-each loop implementation is preferred because it is simpler.

For-Each Loop 79 Be aware that you can use for-each loops for more than just ArrayLists. You can also use them with standard arrays. Here's a for-each loop example that prints the numbers in a primes array: int[] primes = {2, 3, 5, 7, 11, 13}; for (int p : primes) { System.out.println(p); }

For-Each Loop 80 The for-each loop can be used only with things that have elements - Java API collections and arrays. (The ArrayList is a Java API collection. To learn about Java’s other collections, go to

For-Each Loop Be aware of several issues when using a for-each loop:
81 Be aware of several issues when using a for-each loop: It's new with Java 5.0, so it won't work with older compilers. The for-each loop doesn't use an index variable to loop through its elements. That can be a benefit in that it leads to less cluttered code. But it's a drawback if there's a need for an index within the loop. For example, given the primes array in the earlier slide, which type of loop (standard or for-each) should you use to print the following? primes[0] = 2 primes[1] = 3 ... primes[5] = 13 As a practice exercise, provide a standard for loop that prints the above and also provide a for-each loop that prints the above.

ArrayList Objects Versus Standard Arrays
82 Benefits of an ArrayList Over a Standard Array Benefits of a Standard Array Over an ArrayList It's easy to increase the size of an ArrayList – just call add. A standard array uses []'s to access array elements (which is easier than using get and set methods). It's easy for a programmer to insert or remove an element to or from the interior of an ArrayList – just call add or remove and specify the element's index position. A standard array is more efficient with storing primitive values.

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