1. Array Lists and Arrays Sections 13.1, 13.3 Common Errors 13.1, 13.2
Chapter 13
Array Lists and Arrays
Sections 13.1, 13.3
Common Errors 13.1, 13.2
Fran Trees
Drew University

2. Dr. Seuss again: "Too Many Daves"
Did I ever tell you that Mrs. McCave Had twenty-three sons, and she named them all Dave?
Well, she did. And that wasn't a smart thing to do. You see, when she wants one, and calls out "Yoo-Hoo!
Come into the house, Dave!" she doesn't get one. All twenty-three Daves of hers come on the run!
Fran Trees
Drew University

3. "Too Many Daves"
This makes things quite difficult at the McCaves' As you can imagine, with so many Daves. And often she wishes that, when they were born, She had named one of them Bodkin Van Horn. And one of them Hoos-Foos. And one of them Snimm.
And one of them Hot-Shot. And one Sunny Jim. Another one Putt-Putt. Another one Moon Face. Another one Marvin O'Gravel Balloon Face. And one of them Zanzibar Buck-Buck McFate...
But she didn't do it. And now it's too late.
Fran Trees
Drew University

4. This is not all that bad…..
Come into the house, Dave!" she doesn't get one. All twenty-three Daves of hers come on the run!
Fran Trees
Drew University

5. ArrayList
It is very common for applications to require us to store a large amount of data.
Array Lists store large amounts of data in a single collection that can be referred to with a single variable.
Fran Trees
Drew University

6. ArrayList
An ArrayList is a sequence of objects that grows and shrinks as needed.
The ArrayList class is part of the java.util package of the Java standard class library.
Fran Trees
Drew University

7. ArrayList Each element in the sequence can be accessed separately.
We can explicitly overwrite an object at a specified position in the sequence, thus changing its value.
We can inspect the object at a specified location in the sequence.
We can add an object into a specified position of the sequence.
We can add an object to the end of the sequence.
We can remove an object from a specified location in the sequence.
Fran Trees
Drew University

8. interface java.util.List
boolean add(Object x) // appends x to the end of list; returns true
int size()
// returns the number of elements in this list
Object get(int index) // returns the element at the specified position in this list.
Object set(int index, Object x) // replaces the element at index with x // returns the element formerly at the specified position
Iterator iterator()
ListIterator listIterator()
Fran Trees
Drew University

9. class java.util.ArrayList implements java.util.List
Methods in addition to the List methods:
void add(int index, Object x) // inserts x at position index, sliding elements // at position index and higher to the right // (adds 1 to their indices) and adjusts size
Object remove(int index) // removes element from position index, sliding
// subsequent elements to the left (subtracts 1 from their
//indices) and adjusts size // returns the element at the specified position in this list.
Fran Trees
Drew University

10. An example: import java.util.ArrayList; import java.util.Iterator;
public class ArrayListTest {
public static void main (String [] arg)
System.out.println("\nArrayListTest \n");
ArrayList aList = new ArrayList();
aList.add("Dan");
aList.add("George");
aList.add("Mary");
System.out.println("aList contains:");
for(int x = 0; x < aList.size(); x++)
System.out.println(aList.get(x)); }
Fran Trees
Drew University

11. Using an Iterator import java.util.ArrayList;
import java.util.Iterator;
public class ArrayListTest {
public static void main (String [] arg)
System.out.println("\nArrayListTest \n");
ArrayList aList = new ArrayList();
aList.add("Dan");
aList.add("George");
aList.add("Mary");
System.out.println("aList contains:");
Iterator it = aList.iterator();
while (it.hasNext())
System.out.println(it.next()); }
Fran Trees
Drew University

12. What happens? ArrayList students = new ArrayList();
students.add("Mary");
students.add("James");
students.add("Kevin");
students.add(1, "Tanya");
String temp = (String)students.get(3);
System.out.println(temp);
students.remove(2);
students.set(1, "John");
System.out.println(students.size());
Fran Trees
Drew University

13. What happens? Mary James Kevin Mary Tanya James Kevin Mary Tanya Kevin
ArrayList students = new ArrayList();
students.add("Mary");
students.add("James");
students.add("Kevin");
students.add(1, "Tanya");
String temp = (String)students.get(3);
System.out.println(temp);
students.remove(2);
students.set(1, "John");
Mary
James
Kevin
Mary
Tanya
James
Kevin
temp
Mary
Tanya
Kevin
Mary
John
Kevin
Fran Trees
Drew University

14. An ArrayList is a sequence of objects.
Array lists can hold any kind of object.
ArrayList athletes = new ArrayList();
ArrayList csci6 = new ArrayList();
ArrayList accounts = new ArrayList();
The ArrayList method add adds an Object.
The ArrayList method get returns an Object.
Fran Trees
Drew University

15. An ArrayList is a sequence of objects.
Array lists can hold any kind of object.
ArrayList athletes = new ArrayList();
ArrayList csci6 = new ArrayList();
ArrayList accounts = new ArrayList();
The ArrayList method add adds an Object.
The ArrayList method get returns an Object.
Fran Trees
Drew University

16. An ArrayList is a sequence of objects.
boolean add(Object x)
Can I do this???
ArrayList aList = new ArrayList();
aList.add(new Athlete("Dan","Smith","SkiJumping"));
aList.add(new Athlete("George","Harris","Swimming"));
aList.add(new Athlete("Mary","Jones","Tennis"));
Fran Trees
Drew University

17. An ArrayList is a sequence of objects.
boolean add(Object x)
Can I do this???
ArrayList aList = new ArrayList();
aList.add(new Athlete("Dan","Smith","SkiJumping"));
aList.add(new Athlete("George","Harris","Swimming"));
aList.add(new Athlete("Mary","Jones","Tennis"));
Yes…an Athlete IS-A Object.
Fran Trees
Drew University

18. An ArrayList is a sequence of objects.
Object get(int x)
Can I do this???
ArrayList aList = new ArrayList();
aList.add(new Athlete("Dan","Smith","SkiJumping"));
aList.add(new Athlete("George","Harris","Swimming"));
aList.add(new Athlete("Mary","Jones","Tennis"));
for(int x = 0; x < aList.size(); x++) {
System.out.println(aList.get(x).getName()); }
Fran Trees
Drew University

19. An ArrayList is a sequence of objects.
Object get(int x)
Can I do this???
ArrayList aList = new ArrayList();
aList.add(new Athlete("Dan","Smith","SkiJumping"));
aList.add(new Athlete("George","Harris","Swimming"));
aList.add(new Athlete("Mary","Jones","Tennis"));
for(int x = 0; x < aList.size(); x++) {
System.out.println(aList.get(x).getName()); }
NO… getName() is NOT a method of Object.
Fran Trees
Drew University

20. An ArrayList is a sequence of objects.
Object get(int x)
Fix it.
ArrayList aList = new ArrayList();
aList.add(new Athlete("Dan","Smith","SkiJumping"));
aList.add(new Athlete("George","Harris","Swimming"));
aList.add(new Athlete("Mary","Jones","Tennis"));
for(int x = 0; x < aList.size(); x++) {
System.out.println(aList.get(x).getName()); }
Fran Trees
Drew University

21. An ArrayList is a sequence of objects.
Object get(int x)
Fix it.
ArrayList aList = new ArrayList();
aList.add(new Athlete("Dan","Smith","SkiJumping"));
aList.add(new Athlete("George","Harris","Swimming"));
aList.add(new Athlete("Mary","Jones","Tennis"));
for(int x = 0; x < aList.size(); x++) {
System.out.println (((Athlete)aList.get(x)).getName()
+ "\n"); }
Fran Trees
Drew University

22. An ArrayList is a sequence of objects.
Object get(int x)
I prefer…..
ArrayList aList = new ArrayList();
aList.add(new Athlete("Dan","Smith","SkiJumping"));
aList.add(new Athlete("George","Harris","Swimming"));
aList.add(new Athlete("Mary","Jones","Tennis"));
for(int x = 0; x < aList.size(); x++) {
Athlete fit = (Athlete)aList.get(x);
System.out.println(fit.getName() + "\n"); }
Fran Trees
Drew University

23. Some notes about ArrayList
An ArrayList is a sequence of objects.
Each object in an array list has an integer position (index).
get returns an Object. You need to cast the object to the element class.
Position numbers in an array list are
0..size() – 1.
Adding or removing elements from an array list is more work than it seems!
Fran Trees
Drew University

24. A bit more about….
Casting
Fran Trees
Drew University

25. private int catNumber; Cat (int i) catNumber = i; }
public class Cat {
private int catNumber;
Cat (int i)
catNumber = i; }
public void print()
System.out.println("Cat #" + catNumber);
Fran Trees
Drew University

26. private int dogNumber; Dog(int i) dogNumber = i; } public void print()
public class Dog {
private int dogNumber;
Dog(int i)
dogNumber = i; }
public void print()
System.out.println("Dog #" + dogNumber);
Fran Trees
Drew University

27. import java.util.ArrayList; public class CatsAndDogs {
public static void main(String[] args)
ArrayList cats = new ArrayList();
for (int i = 0; i < 7; i++)
cats.add(new Cat(i)); }
cats.add(new Dog(7)); //Can I do this?
for(int i = 0; i < cats.size(); i++)
System.out.println(cats.get(i));
// Does this print anything?
// If so, what?
Fran Trees
Drew University

28. What happened? What is this? ArrayList cats = new ArrayList();
for (int i = 0; i < 7; i++) {
cats.add(new Cat(i)); }
cats.add(new Dog(7));
for(int i = 0; i < cats.size(); i++){
System.out.println(cats.get(i));
What happened?
What is this?
Fran Trees
Drew University

29. ArrayList cats = new ArrayList(); for (int i = 0; i < 7; i++) {
cats.add(new Cat(i)); }
cats.add(new Dog(7));
for(int i = 0; i < cats.size(); i++)
((Cat) cats.get(i)).print();
// Does this print anything?
// If so, what?
Fran Trees
Drew University

30. Prints….. WHY? Cat #0 Cat #1 Cat #2 Cat #3 Cat #4 Cat #5 Cat #6
Exception in thread "main" java.lang.ClassCastException: Dog
at CatsAndDogs.main(CatsAndDogs.java:15)
Press any key to continue...
WHY?
Fran Trees
Drew University

31. Hmmmm…. Object obj = new Cat(3); obj.print(); //Can I do this?
Fran Trees
Drew University

32. Hmmmm…. Object obj = new Cat(3); *obj.print(); // Nope!
// The compiler can determine only that the object is an Object and could possible refer to a Dog object or a Rectangle object, so it doesn't allow the reference *.
Fran Trees
Drew University

33. But…. Object obj = new Cat(3); ((Cat)obj).print();
// But I can do THIS.
// What about the parenthesis?
Fran Trees
Drew University

34. And…. Cat aCat = new Cat(3); Object obj = aCat; //Can I do THIS?
Fran Trees
Drew University

35. And…. Cat aCat = new Cat(3); Object obj = aCat;
//Yep! I can assign a reference variable to the class that it extends.
aCat can be assigned to the class that it extends(Object).
This is Java's one-way assignment compatibility.
aCat IS A Object.
Fran Trees
Drew University

36. What about…. Cat aCat = new Cat(3); Object obj = aCat; obj.print();
Fran Trees
Drew University

37. What about…. Cat aCat = new Cat(3); Object obj = aCat; obj.print();
//nope !
Fran Trees
Drew University

38. Must…. Cat aCat = new Cat(3); Object obj = aCat; ((Cat)obj).print();
//yep !
Fran Trees
Drew University

39. Let's try more…
Fran Trees
Drew University

40. Can I do this? Object anObject = null; String stringA = "hello";
anObject = stringA;
Fran Trees
Drew University

41. Object anObject = null; String stringA = "hello";
anObject = stringA; //OK
Fran Trees
Drew University

42. Can I do this? Object anObject = null; String stringA = "hello";
stringA = anObject;
Fran Trees
Drew University

43. Object anObject = null; String stringA = "hello"; stringA = anObject;
needs cast
Fran Trees
Drew University

44. Can I do this? Object anObject = null; String stringA = "hello";
Comparable c = new Comparable();
Fran Trees
Drew University

45. Object anObject = null; String stringA = "hello";
Comparable c = new Comparable();
No, Comparable is an interface.
Fran Trees
Drew University

46. Can I do this? Object anObject = null; String stringA = "hello";
Comparable c = new String();
Fran Trees
Drew University

47. Object anObject = null; String stringA = "hello";
Comparable c = new String();
Yes, String implements Comparable
Fran Trees
Drew University

48. A bit more about
toString()
Fran Trees
Drew University

49. What happens? public class StringTest {
public static void main(String[] args)
ArrayList stringList = new ArrayList();
stringList.add("Fran");
stringList.add("Marie");
stringList.add("Joe");
System.out.println(stringList); }
Fran Trees
Drew University

50. WHY???? public class StringTest {
public static void main(String[] args)
ArrayList stringList = new ArrayList();
stringList.add("Fran");
stringList.add("Marie");
stringList.add("Joe");
System.out.println(stringList); }
[Fran, Marie, Joe] Let's look at the API!
Fran Trees
Drew University

51. What happens? public class Strings {
public static void main(String[] args){
ArrayList stringList = new ArrayList();
stringList.add("Fran");
stringList.add("Marie");
stringList.add("Joe");
String aStringInList = stringList.get(0);
// is this OK? }
Fran Trees
Drew University

52. no…but public class Strings { public static void main(String[] args)
ArrayList stringList = new ArrayList();
stringList.add("Fran");
stringList.add("Marie");
stringList.add("Joe");
String aStringInList = (String)stringList.get(0);
//NOW this is OK.
} //What about parentheses? }
Fran Trees
Drew University

53. What happens? public class Strings {
public static void main(String[] args){
ArrayList stringList = new ArrayList();
stringList.add("Fran");
stringList.add("Marie");
stringList.add("Joe");
System.out.println(stringList.get(0));
//Can I do this?????? }
Fran Trees
Drew University

54. What happens? public class Strings {
public static void main(String[] args)
ArrayList stringList = new ArrayList();
stringList.add("Fran");
stringList.add("Marie");
stringList.add("Joe");
System.out.println(stringList.get(0));
//YEP….what is printed? }
Fran Trees
Drew University

55. What happens? public class Strings {
public static void main(String[] args)
ArrayList stringList = new ArrayList();
stringList.add("Fran");
stringList.add("Marie");
stringList.add("Joe");
System.out.println(stringList.get(0));
//Fran }
Fran Trees
Drew University

56. AGAIN… An ArrayList is a sequence of objects.
Array lists can hold any kind of object.
The ArrayList method add adds an Object.
The ArrayList method get returns an Object.
What is we want our ArrayList to hold ints or doubles?
Fran Trees
Drew University

57. Wrapper Classes Another look at the API Integer(int value)
class java.lang.Integer implements java.lang.Comparable
Integer(int value)
int intValue()
boolean equals(Object other)
String toString()
int compareTo(Object other)
Another look at the API
Fran Trees
Drew University

58. Wrapper Classes Another look at the API Double(double value)
class java.lang.Double implements java.lang.Comparable
Double(double value)
double doubleValue()
boolean equals(Object other)
String toString()
int compareTo(Object other)
Another look at the API
Fran Trees
Drew University

59. Filling the ArrayList for (int k = 1; k <= 20; k++) {
nbrs.add(new Integer(k)); }
Fran Trees
Drew University

60. Printing the ArrayList
for (int k = 0; k < nbrs.size(); k++) {
System.out.print("\t" + nbrs.get(k)); }
System.out.println();
Fran Trees
Drew University

61. Printing the sum of the integer values in the ArrayList
int sum = 0;
Integer temp;
for (int k = 0; k < nbrs.size(); k++) {
temp = (Integer)nbrs.get(k);
sum += temp.intValue(); }
System.out.println("sum equals: " + sum);
Fran Trees
Drew University

62. ArrayList Applications
Finding values, counting, max and min, average, median, mode…..
But first….LOTTERY
Fran Trees
Drew University

63. CRC cards
This program will simulate the TV Lottery drawing for the Pick-6 New Jersey Lottery game. In this game, balls are placed in a container that keeps them popping around in some random movements.
The "LotteryPicker" picks a ball from the container (6 times).
The chosen balls are displayed.
Fran Trees
Drew University

64. Candidate classes
Ball
Game
Picker
Hopper
Fran Trees
Drew University

65. A lottery game Judy Hromcik
3 classes
Ball Class
the numbered item
Picker Class
the person that gets the ball from the Hopper
Hopper Class
the jug-like container that holds the balls
Fran Trees
Drew University

66. Ball Class public class Ball {
public Ball() //default constructor-sets value to 0
public Ball(int val) //sets value to val
public int value() //returns value on ball
public String toString() //displays ball
private int myValue; }
Fran Trees
Drew University

67. Hopper Class public class Hopper { public Hopper()
//adds default # balls to ArrayList myBalls
public Hopper(int numBalls)
//adds numBalls # balls to ArrayList my Balls
public int size()
//returns # balls in Hopper
public Ball getBall(int j)
removes the jth ball from Hopper
private ArrayList myBalls;
private static int DEFAULT_NUM_BALLS =
<some default int goes here>; }
Fran Trees
Drew University

68. Picker Class public class Picker { public Picker()
//gets a Hopper (default)
public Picker(int numBalls)
//gets a Hopper with numBalls balls
public Ball chooseBall()
//chooses a random ball from Hopper
private static Random generator = new Random();
Hopper myHopper; }
Fran Trees
Drew University

69. Two implementations: Text version Graphics version
Both use SAME three classes
Fran Trees
Drew University