CSC 245 Chapter 3, Spring 2015

Chapter 3 Collection Classes I Java Arrays I ADT for Bag of Integers • Specification • IntArrayBag ADT • Bag ADT • Invariants I Interactive Testing Applet I Sequence ADT Computer Programming II (v1.20) WK–2 Java Arrays I Contiguous locations of memory • Same name & type. I Elements accessed via subscripts • scores[2] • Index range is 0 to length − 1 • ArrayIndexOutOfBoundsException I Length of an array • Public data member, length. scores.length scores[0] scores[1] scores[2] scores[3] Computer Programming II (v1.20) WK–3 Java Arrays… I Declaration • int scores[] = new int[4]; • int scores[]; scores = new int[4]; • int scores[] = {7, 22, 19, 56}; I Assignment • scores[0] = 7; • scores[1] = 22; • scores[2] = 19; • scores[3] = 56; I Array as Object • int[] exams = scores; scores[0] scores[1] scores[2] scores[3] scores exams Computer Programming II (v1.20) WK–4 Java Arrays… I Clones • int exams[] = (int[]) scores.clone(); • exams[2] = 42; I Array Initialization • Numeric primitive types: zero. • Boolean primitive types: false. • Object references: null. [0] [1] [2] [3] 56 19 22 7 scores [0] [1] [2] [3] 56 42 22 7 exams Computer Programming II (v1.20) WK–5 Parameter Passing I Call-by-value • Caller passes a copy of a parameter to the called method. • Changes are not reflected in the original parameter. I Call-by-reference • Caller passes a reference to the parameter to the called method. • Changes made by the called method are seen by the caller. I Java Policy • Primitive types are passed by value—scalar quantities. • Object references, arrays, etc., are passed by reference. Computer Programming II (v1.20) WK–6 Java Arrays… Array Parameter Example I put42s Method public static void put42s (int[] data) • Put 42 in all elements of the array. I Parameters data—an array of integers. I Postcondition • All elements of array data have been set to 42. I Implementation public static void puts42 (int[ ] data) { int i; for (i = 0; i < data.length; i++) { data[i] = 42; } } I Using Parameters int[] example = new int[7]; put42s(example); Computer Programming II (v1.20) WK–7 Bag of Integers I Collections • Objects that can hold a group of items. • Implemented as a class. • Class defines methods to add, remove, & examine items. I Bag of Integers • Holds a collection of integers. • First implementation uses integer array. • Other implementations use linked lists… Computer Programming II (v1.20) WK–8 Bag of Integers… I Creation • Bag starts empty I Initial state. I Modifying Operations • Add number to a bag. I Allow multiple copies. • Remove number from a bag. I If number is present in bag! I Only one copy removed if more than one copy in bag. I Queries • How many occurrences of a given number? • How many numbers in bag? Computer Programming II (v1.20) WK–9 Bag ADT—Specification I Implementation • To be decided! I Class Definition • Class header. • Constructor(s). • Public methods. I Public Interface • Specification defines public interface. • Can use Bag ADT without knowing implementation details. Computer Programming II (v1.20) WK–10 Bag ADT—Specification… I Class Name • Implemented as class IntArrayBag. I Constructors • public IntArrayBag(int initialCapacity) I Create a bag with an initial capacity. I Capacity is automatically increased when capacity is reached. • public IntArrayBag() I Initial capacity of 10. Computer Programming II (v1.20) WK–11 Bag ADT—Specification… I Modification Methods • public void add(int element) I Add the integer, element, to the bag. I A new copy is added if already present. • public boolean remove(int target) I Remove the number target from the bag if present. I Return true if number present, false otherwise. • public void addAll(IntArrayBag addend) I Add contents of a bag to existing contents of this bag. Computer Programming II (v1.20) WK–12 Bag ADT—Specification… I Query Methods… • public int getCapacity() I Returns capacity of current bag. I Modification Methods… • public void ensureCapacity(int minimumCapacity) I Increases capacity of bag to a minimum. • public void trimToSize() I Reduces capacity of bag to number of items currently in bag. I Minimizes memory usage. Computer Programming II (v1.20) WK–13 Bag ADT—Specification… I Bag Producing Methods • public static IntArrayBag union (IntArrayBag b1, IntArrayBag b2) I Returns new bag containing numbers in both bags. • public Object clone() I Returns a new bag. I Contents same as contents of this bag. Computer Programming II (v1.20) WK–14 Bag ADT—Specification… I Query Methods • public int size() I Returns number of integers in bag. • public int countOccurrences(int target) I Determines number of times integer target appears in bag. • public int getCapacity() I Returns current capacity of bag. Computer Programming II (v1.20) WK–15 Bag ADT—Code Examples I Bag Creation & Adding Elements // New bag is empty. IntArrayBag firstBag = new IntArrayBag(); firstBag.add(8); // firstBag has 1 element: 8. firstBag.add(4); // firstBag has 2 elements: 8 & 4. firstBag.add(8); // firstBag has 3 elements: Two 8s & one 4. I At end, Bag firstBag contains 3 elements. • Two 8s and one 4. Computer Programming II (v1.20) WK–16 Bag ADT—Code Examples… I Query Methods System.out.println(firstBag.countOccurrences(1)); System.out.println(firstBag.countOccurrences(4)); System.out.println(firstBag.countOccurrences(8)); • Output I 0 I 1 I 2 Computer Programming II (v1.20) WK–17 Bag ADT—Code Examples… I Bag Creation & Adding Elements… IntArrayBag helter = new IntArrayBag(); IntArrayBag skelter = new IntArrayBag(); helter.add(8); skelter.add(4); skelter.add(8); // skelter has two elements: 4 & 8. // helter has one element: 8. helter.addAll(skelter); // helter has three elements: two 8s & one 4. Computer Programming II (v1.20) WK–18 Bag ADT—Code Examples… I Bag Creation… IntArrayBag part1 = new IntArrayBag(); IntArrayBag part2 = new IntArrayBag(); part1.add(8); part1.add(9); // Two elements in part1: 8 & 9. part2.add(4); part2.add(8); // Two elements in part2: 4 & 8. IntArrayBag total = IntArrayBag.union(part1, part2); // 4 elements in total: 4, 9, & two 8s. I Equivalent statements IntArrayBag total = new IntArrayBag(); total.addAll(part1); total.addAll(part2); Computer Programming II (v1.20) WK–19 Bag ADT—Code Examples… I Bag Creation… IntArrayBag b = new IntArrayBag(); b.add(42); IntArrayBag c = (IntArrayBag) b.clone(); • Bag b is different from bag c. • Bags b & c each contain one integer: 42. Computer Programming II (v1.20) WK–20 Bag ADT—Specification… I Out of Memory Limitation • Store integers in an array. • Allocate larger array when more capacity is needed. • Array allocated on heap. • Exception OutOfMemoryError indicates insufficient memory. I Other Limitations • Array indexed by integers. • Java integers limited to Integer.MAX_VALUE. • Machines normally run out of memory before exceeding integer limit. Computer Programming II (v1.20) WK–21 Class IntArrayBag—Specification… I Class IntArrayBag • An IntArrayBag is a collection of int numbers. The same number may appear multiple times in a bag. • Limitations 1. Bag capacity can change after creation, but maximum capacity is limited by machine memory. The constructor, addItem, clone, and union throw an OutOfMemoryError when memory is exhausted. 2. A bag's capacity cannot exceed the maximum integer 2,147,483,647 (Integer.MAX_VALUE). Any attempt to create a larger capacity results in a failure due to an arithmetic overflow. 3. Because of the slow linear algorithms of this class, large bags will have poor performance. Computer Programming II (v1.20) WK–22 Class IntArrayBag—Specification… I First Constructor public IntArrayBag() • Initialize an empty bag with an initial capacity of 10. Note that the add method works efficiently (without needing more memory) until this capacity is reached. • Parameters none . • Postcondition This bag is empty and has an initial capacity of 10. • Throws: OutOfMemoryError Indicates insufficient memory for: new int[10]. Computer Programming II (v1.20) WK–23 Class IntArrayBag—Specification… I Second Constructor public IntArrayBag(int initialCapacity) • Initialize empty bag with a specified initial capacity. The add method works efficiently (without needing more memory) until this capacity is reached. • Parameters initialCapacity—initial capacity of this bag. • Precondition initialCapacity is non-negative. • Postcondition This bag is empty and has a capacity of initialCapacity. • Throws: IllegalArgumentException—initialCapacity is negative. OutOfMemoryError—insufficient memory for: new int[initialCapacity]. Computer Programming II (v1.20) WK–24 Class IntArrayBag—Specification… I add Method public void add(int element) • Add a new element to this bag. Increase capacity of bag before adding element if need be. • Parameters element—new element that is being added. • Postcondition A new copy of the element has been added to this bag. • Throws: OutOfMemoryError—insufficient memory for increasing the capacity. • Note Capacity over Integer.MAX_VALUE generates arithmetic overflow. Computer Programming II (v1.20) WK–25 Class IntArrayBag—Specification… I addAll Method public void addAll(IntArrayBag addend) • Add the contents of another bag to this bag. • Parameters addend—a bag whose contents are being added to this bag. • Precondition The parameter, addend, is not null. • Postcondition The elements from addend have been added to this bag. • Throws: NullPointerException—parameter addend is null. OutOfMemoryError—insufficient memory for increasing the capacity. • Note Capacity over Integer.MAX_VALUE generates arithmetic overflow. Computer Programming II (v1.20) WK–26 Class IntArrayBag—Specification… I clone Method public Object clone() • Generate a copy of this bag. • Returns A copy of this bag. Changes to copy will not affect this bag or vice versa. Return value must be typecast to an IntArrayBag before used. • Throws OutOfMemoryError—insufficient memory for creating clone. Computer Programming II (v1.20) WK–27 Class IntArrayBag—Specification… I countOccurrences Method public int countOccurrences(int target) • Accessor method to count occurrences of a particular element in this bag. • Parameters target—the element that needs to be counted. • Returns The number of times that target occurs in this bag. Computer Programming II (v1.20) WK–28 Class IntArrayBag—Specification… I ensureCapacity Method public void ensureCapacity(int minumumCapacity) • Change the current capacity of this bag. • Parameters minumumCapacity—the new capacity for this bag. • Postcondition This bag's capacity has been changed to at least minumumCapacity. Capacity is unchanged if already greater than parameter. • Throws OutOfMemoryError—indicates insufficient memory for: new int[minimumCapacity]. Computer Programming II (v1.20) WK–29 Class IntArrayBag—Specification… I getCapacity Method public int getCapacity() • Accessor method that determines the current capacity of this bag. The add method works efficiently (w/o needing more memory) until this capacity is reached. • Returns the current capacity of this bag. Computer Programming II (v1.20) WK–30 Class IntArrayBag—Specification… I remove Method public boolean remove(int target) • Remove one copy of a specified element from this bag. • Parameters target—the element to remove from this bag. • Postcondition If target was found in this bag, then one copy has been removed and the method returns true. Otherwise, this bag remains unchanged and the method returns false. Computer Programming II (v1.20) WK–31 Class IntArrayBag—Specification… I size Method public int size() • Accessor method to determine the number of elements in this bag. • Returns the number of elements in this bag. Computer Programming II (v1.20) WK–32 Class IntArrayBag—Specification… I trimToSize Method public void trimToSize() • Reduce the current capacity of this bag to the number of elements in it. • Postcondition This bag's capacity has been changed to its actual size. • Throws OutOfMemoryError—indicates insufficient memory for altering capacity. Computer Programming II (v1.20) WK–33 Class IntArrayBag—Specification… I union Method public static IntArrayBag union (IntArrayBag b1, IntArrayBag b2) • Create a new bag that contains all the elements from two other bags. • Parameters b1—the first of two bags. b2—the second of two bags. • Precondition Neither b1 nor b2 is null. • Returns a new bag that is the union of b1 and b2. • Throws NullPointerException—indicates that of the arguments is null. OutOfMemoryError—indicates insufficient memory for the new bag. • Note Capacity over Integer.MAX_VALUE generates arithmetic overflow. Computer Programming II (v1.20) WK–34 Class IntArrayBag—Demo I Program Task • User types ages with a negative sentinel; program enters ages into a bag. • User types an age which the program removes from the bag. I Session % java BagDemonstration Type the ages of your family members. Type a negative number at the end and press return. 5 19 47 -1 Type those ages again. Press return after each age. Age: 19 Yes, I've got that age and will remove it. Age: 36 No, that age does not occur! Age: 5 Yes, I've got that age and will remove it. Age: 47 Yes, I've got that age and will remove it. May your family live long and prosper. % Computer Programming II (v1.20) WK–35 Class BagDemonstration // This small demonstration program shows how to use the // IntArrayBag class from the edu.colorado.collections package. import edu.colorado.collections.IntArrayBag; import edu.colorado.io.EasyReader; class BagDemonstration { private static EasyReader stdin = new EasyReader(System.in); public static void main(String[ ] args) { IntArrayBag ages = new IntArrayBag( ); getAges(ages); checkAges(ages); System.out.println("May your family live long and prosper."); } ... } Computer Programming II (v1.20) WK–36 Class BagDemonstration… // The getAges method prompts the user to type in the ages of family members. These ages // are read and placed in the ages bag, stopping when the user types a negative number. // This program does not worry about the possibility of running out of memory. public static void getAges(IntArrayBag ages) { int userInput; // An age from the user's family System.out.println("Type the ages of your family members."); System.out.println ("Type a negative number at the end and press return."); userInput = stdin.intInput( ); while (userInput >= 0) { ages.add(userInput); userInput = stdin.intInput( ); } } Computer Programming II (v1.20) WK–37 Class BagDemonstration… //The checkAges method prompts the user to type in the ages of family members once // again. Each age is removed from the ages bag when it is typed, stopping when the bag is // empty (or the user types a negative number). public static void checkAges(IntArrayBag ages) { int userInput; // An age from the user's family System.out.print("Type those ages again. "); System.out.println("Press return after each age."); while (ages.size( ) > 0) { userInput = stdin.intQuery("Age: "); if (ages.countOccurrences(userInput) == 0) System.out.println("No, that age does not occur!"); else { System.out.println("Yes, I've got that age and will remove it."); ages.remove(userInput); } } } Computer Programming II (v1.20) WK–38 Class BagDemonstration… I See BagDemonstration.java listing Computer Programming II (v1.20) WK–39 Class IntArrayBag—Design I Basic Structure • Store numbers in front part of array. • Order is irrelevant. I Example • Bag with three entries: two 4s & one 8. Array of integers Irrelevant Contents 8 [0] 4 [1] 8 [2] [3] [4] [5] ... Computer Programming II (v1.20) WK–40 Class IntArrayBag—Design… I Example… • Valid storage schemes. Array of integers Irrelevant Contents 8 [0] 8 [1] 4 [2] [3] [4] [5] ... 4 [0] 8 [1] 8 [2] [3] [4] [5] ... Computer Programming II (v1.20) WK–41 Class IntArrayBag—Design… I Basic Structure… • Store numbers in array. • Keep track of number of items in array. Array of integers Irrelevant Contents 8 [0] 8 [1] 4 [2] [3] [4] [5] ... 3 Number of items stored in array Computer Programming II (v1.20) WK–42 Class IntArrayBag—Design… I Basic Class Definition public class IntArrayBag implements Cloneable { private int[] data; // An array to store elements private int manyItems; // How much of the array is used? // public methods go here. } Computer Programming II (v1.20) WK–43 Invariant of an ADT I Definition • Rules governing the use of instance variables by methods. • Invariant is valid when a method is invoked. I Constructors are the exception. • Invariant is valid when a method call completes. • Document in class declaration. I Example—Class IntArrayBag • Number of elements is stored in instance variable manyItems. • Numbers stored in data[0] through data[manyItems – 1]. • For empty bag, do not care about items stored in array data. Computer Programming II (v1.20) WK–44 Class IntArrayBag—Design… I Constructor • Initializes instance variables to represent an empty bag. • Array size is initial capacity of bag. data [0] [1] [2] [3] [4] [5] ... Irrelevant Contents 0 manyItems Computer Programming II (v1.20) WK–45 Class IntArrayBag—Design… I Constructor—Implementation public IntArrayBag(int initialCapacity) { if (initialCapacity < 0) throw new IllegalArgumentException ("initialCapacity is –ve: " + initialCapacity"); manyItems = 0; data = new int[initialCapacity]; } Computer Programming II (v1.20) WK–46 Class IntArrayBag—Design… I add Method • Check capacity & double existing capacity if needed. • Store number in next free slot. • Increment element count. public void add(int element) b.add(42); I Before I After 4 8 2 b.manyItems b.data 4 8 7 3 b.manyItems b.data Computer Programming II (v1.20) WK–47 Class IntArrayBag—Design… I add Method—Implementation public void add(int element) { if (manyItems == data.length) { // Double the capacity and add 1: this works even if manyItems is 0. // However, in the case that manyItems*2+1 is beyond // Integer.MAX_VALUE, there will be an arithmetic overflow and // the bag will fail. ensureCapacity(manyItems * 2 + 1); } data[manyItems++] = element; } Computer Programming II (v1.20) WK–48 Class IntArrayBag—Design… I remove Method • Find index of target value in array. • Store item in last slot of array in target slot. • Decrement element count. public boolean remove(int element) b.remove(8); I Before I After 4 8 7 4 b.manyItems b.data 4 4 7 3 b.manyItems b.data 4 Computer Programming II (v1.20) WK–49 Class IntArrayBag—Design… I remove Method public boolean remove(int target) { int index; // Location of target in data array. // First, set index to location of target in data array; range is 0 to manyItems – 1; // If target is not in array, index will be set to manyItems. for (index = 0; (index < manyItems) && (target != data[index]); index++) ; // Do nothing if (index == manyItems) return false; // Target not found; nothing is removed. else { manyItems--; data[index] = data[manyItems]; return true; } } Computer Programming II (v1.20) WK–50 Class IntArrayBag—Design… I countOccurrences Method public int countOccurrences(int target) { int answer; int index; answer = 0; for (index = 0; index < manyItems; index++) { if (target == data[index]) answer++; } return answer; } Computer Programming II (v1.20) WK–51 Class IntArrayBag—Design… I addAll Method • Ensure capacity is big enough. • Copy items from addend.data to bag’s own data array. • Update number of items in bag. public void addAll(IntArrayBag addend) { // If addend is null, a NullPointerException is thrown. // If total number of items > Integer.MAX_VALUE, overflow failure. ensureCapacity(manyItems + addend.manyItems); System.arrayCopy(addend.data, 0, data, manyItems, addend.manyItems); manyItems += addend.manyItems; } Computer Programming II (v1.20) WK–52 Class IntArrayBag—Design… I union Method IntArrayBag b3 = IntArrayBag.union(b1, b2); public static IntArrayBag(IntArrayBag b1, IntArrayBag b2) { // Throw NullPointerException if b1 or b2 are null. // Failure by arithmetic overflow if total number of items is too big. IntArrayBag answer = new IntArrayBag(b1.getCapacity() + b2.getCapacity()); System.arrayCopy(b1.data, 0, answer.data, 0, b1.manyItems); System.arrayCopy(b2.data, 0, answer.data, b1.manyItems, b2.manyItems); answer.manyItems = b1.manyItems + b2.manyItems; return answer; } Computer Programming II (v1.20) WK–53 Class IntArrayBag—Design… I ensureCapacity Method public void ensureCapacity(int minimumCapacity) { int[] biggerArray; if (data.length < minimumCapacity) { biggerArray = new int[minimumCapacity]; System.arrayCopy(data, 0, biggerArray, 0, manyItems); data = biggerArray; } } Computer Programming II (v1.20) WK–54 Class IntArrayBag—Design… I getCapacity Method public int getCapacity() { return data.length; } I size Method public int size() { return manyItems; } Computer Programming II (v1.20) WK–55 Class IntArrayBag—Design… I trimToSize Method public void trimToSize() { int trimmedArray; if (data.length != manyItems) { trimmedArray = new int[manyItems]; System.arrayCopy(data, 0, trimmedArray, 0, manyItems); } } Computer Programming II (v1.20) WK–56 Class IntArrayBag—Design… I clone Method • Create copy of bag. • Create copy of objects referred by bag's instance variables. I super.clone(); I System.arrayCopy(…); null 0 answer.manyItems answer.data this.manyItems 3 this.data 4 8 7 3 answer.manyItems answer.data 4 8 7 Computer Programming II (v1.20) WK–57 Class IntArrayBag—Design… I clone Method—Implementation public Object clone() { // Clone an IntArrayBag object. IntArrayBag answer; try { answer = (IntArrayBag) super.clone(); } catch (CloneNotSupportedException e) { // Exception implies a programming error, most likely forgot "implements // Cloneable" in class header. throw new RuntimeException ("Class does not implement Cloneable"); } answer.data = (int[]) data.clone(); return answer; } Computer Programming II (v1.20) WK–58 Bag ADT—Time Analysis I Counting Operations • Expressed as a function of bag capacity (c) or size (n). I Loop for (index = 0; index < manyItems; index++) if (target == data[index]) answer++; • n × (number of statements in loop) ⇒ O(n) I Constructor • Allocates array of initialCapacity integers. • Integers initialized to zero. ⇒ O(c) Computer Programming II (v1.20) WK–59 Bag ADT—Time Analysis… c is the bag's capacity n1 & n2 are the sizes of the bags Linear in the size of the added bag Linear time Constant time c is the initial capacity O(c) clone O(n1+n2) b1.addAll(b2) with capacity increase O(n2) b1.addAll(b2) without capacity increase O(n) add with capacity increase O(1) add without capacity increase O(c) Constructor Time Analysis Operation Computer Programming II (v1.20) WK–60 Bag ADT—Time Analysis… c1 & c2 are the bags’ capacities O(c1+c2) union of b1 and b2 Linear time Constant time Linear time Constant time c is the specified minimum capacity Linear time O(n) trimToSize O(1) size O(n) remove O(1) getCapacity O(c) ensureCapacity O(n) countOccurrences Time Analysis Operation Computer Programming II (v1.20) WK–61 Applets for Testing Computer Programming II (v1.20) WK–62 Applets for Testing I Use public interface to exercise class methods. • Add elements. • Obtain size. • Count occurrences. Computer Programming II (v1.20) WK–63 Applets for Testing… I Template • Import statements • Class definition I IntArrayBag object I GUI components of applet. I init method. I Implementations of action listeners • Events: buttons or input fields. I Implementations of other methods. Computer Programming II (v1.20) WK–64 Applets for Testing… I Import Statements • Class being tested. I Not needed if class is in same directory. import edu.colorado.collections.IntArrayBag; • Java classes needed by applet. import java.applet.Applet; // Provides Applet class import java.awt.*; // Provides Button class… import java.awt.event.*; // ActionEvent, ActionListener. Computer Programming II (v1.20) WK–65 Applets for Testing… I Class Definition public class BagApplet extends Applet { // IntArrayBag object for applet to manipulate IntArrayBag b = new IntArrayBag(); // Declare applet’s components: buttons, text fields, & other GUI comps ... public void init() { ... } // Implementation of action listeners. ... // Implementation of other methods. ... } Computer Programming II (v1.20) WK–66 Applets for Testing… I Applet’s GUI Components • Buttons, labels, text fields, and a text area. // These are the interactive Components that will appear in the Applet. // We declare one Button for each IntArrayBag method that we want // to be able to test. If the method has an argument, then there is also a // TextField where the user can enter the value of the argument. At the // bottom, there is a TextArea to write messages. Button sizeButton = new Button("size( )"); Button addButton = new Button("add( )"); TextField elementText = new TextField(10); Button countOccurrencesButton = new Button("countOccurrences( )"); TextField targetText = new TextField(10); TextArea feedback = new TextArea(7, 60); Computer Programming II (v1.20) WK–67 Applets for Testing… I init Method • Called by applet viewer after creating applet object. I Equivalent to an application’s main method. • Adds GUI components. I Sets up layout of applet. • Activates GUI components. I Creates & attaches listeners to selected GUI components. Computer Programming II (v1.20) WK–68 Applets for Testing… I Adding GUI Components • Use add method. public void add(Component) • Adds a GUI component to applet object. I Buttons, labels,… • add(sizeButton); • add(new Label("Test program created a bag")); • Typical components: I Button—activates action when pressed. I Label—displays message on screen. I TextField—accepts or displays user input (one line). I TextArea—accepts or displays multiple lines of text. Computer Programming II (v1.20) WK–69 Applets for Testing… I Activating GUI Components • Create listeners. • Attach listeners to components. I Listeners • Describe actions to be taken in reaction to events. I Button presses, CR in a text field, etc. • SizeListener—sizeButton is clicked. • AddListener—addButton is clicked or CR entered in elementText field. • CountOccurrencesListener—countOccurrencesButton is clicked or CR entered in targetText field. Computer Programming II (v1.20) WK–70 Applets for Testing… I Implementing Action Listeners • Define inner class. I Visible only within another class definition. I Has access to all instance variables. • Attach instance of inner class as listener. I Pattern class implements ActionListener { void actionPerformed(ActionEvent event) { ... } } Computer Programming II (v1.20) WK–71 Applets for Testing… I Action Listeners—“size()” Button class SizeListener implements ActionListener { void actionPerformed(ActionEvent event) { feedback.append("The bag has size " + b.size + "\n"); } } I Attaching Listeners • sizeButton(addActionListener(new SizeListener())); Computer Programming II (v1.20) WK–72 Applets for Testing… I Action Listeners—“add()” Button & Text Field • Must handle empty or mistyped number. I NumberFormatException when interpreted as integer. • Use try-catch block. I Attaching Listener • AddListener addListener = new AddListener(); • addButton.addActionListener(addListener); • elementText.addActionListener(addListener); Computer Programming II (v1.20) WK–73 Applets for Testing… I Action Listeners—“add()” Button & Text Field class AddListener implements ActionListener { void actionPerformed(ActionEvent event) { try { String userInput = elementText.getText(); int element = Integer.parseInteger(userInput); b.add(element); feedback.append (element +" has been added to the bag"); } catch (NumberFormatException e) { feedback.append ("Type an integer before clicking button"); elementText.requestFocus(); elementText.selectAll(); } }} Computer Programming II (v1.20) WK–74 Applets for Testing I See BagApplet.java listing Computer Programming II (v1.20) WK–75 Applets for Testing… I HTML File • <APPLET CODE="BagApplet.class" WIDTH=480 HEIGHT=340> </APPLET> I Starting Applet • Browser • appletviewer Computer Programming II (v1.20) WK–76 Sequence ADT I Similar to a bag. • Collection of items. I Defines current item. I Has methods that step through elements. • Useable in a loop. I Contains doubles. Computer Programming II (v1.20) WK–77 Class DoubleArraySeq—Specification I Constructors • public void DoubleArraySeq() I Empty sequence with initial capacity of 10. • public void DoubleArraySeq(int capacity) I Empty sequence with specified initial capacity. I size Method • public int size() I Returns number of elements in the sequence. Computer Programming II (v1.20) WK–78 Class DoubleArraySeq—Specification… I Examining a Sequence • public void start() I Initialize current element to point to sequence’s first element. • public double getCurrent() I Return current element of sequence. • public void advance() I Update current to point to sequence’s next element. • public boolean isCurrent() I Does current point to an actual element of sequence? I Is current pointing past sequence’s edge? Computer Programming II (v1.20) WK–79 Class DoubleArraySeq—Specification… I Examining a Sequence—Example for (numbers.start(); numbers.isCurrent(); numbers.advance()) { System.out.println(numbers.getCurrent()); } • Output I 10.1 I 40.2 I 1.1 10.1 40.2 1.1 Computer Programming II (v1.20) WK–80 Class DoubleArraySeq—Specification… I addBefore & addAfter Methods • public void addBefore(double element) I Place new element before current. I Make new element current. I Sequence with no current adds at front. I Empty sequence adds first element. I May need to increase capacity. I seq.addBefore(10.0); • public void addAfter(double element) I Similar to addBefore method. I Place new element after current. 42.1 8.8 99.0 42.1 10.0 8.8 99.0 After Before Computer Programming II (v1.20) WK–81 Class DoubleArraySeq—Specification… I removeCurrent Method • public boolean removeCurrent() I Removes current element from sequence. I Curren