1. Chapter 7 Arrays and Array Lists

2. Assignment:
Read lessons 7.1 thru 7.8, take notes, and complete the self-check questions in your notebook
Written exercises: R7.1, 7.3, 7.4,7.6 – 7.7 R7.11, 7.12, 7.16 – Due December 9, 2013
Programming exercises P7.2 – 7.6, 7.9, 7.10, Due December 17, 2013
For 7.2 – 7.6 You may write one long program that includes each modification to your Purse Class. Comment each method as 7.2, or 7.3, or 7.4 …etc.
Work with one other person and submit one final copy of the all programs to the completed works folder.
FunNumber2 exercises – due December 20, 2013

3. Chapter Goals To become familiar with using arrays and array lists
To learn about wrapper classes, auto-boxing and the generalized for loop
To study common array algorithms
Continued…

4. Chapter Goals To learn how to use two-dimensional arrays
To understand when to choose array lists and arrays in your programs
To implement partially filled arrays

5. Arrays An array is a fixed-length sequence of values of the same type.
You access array elements with an integer index, using the notation a[i].
The number of elements an array can hold is accessed by using the length field of the array: a.length.
(notice no () after length, because it is a public final field)
Arrays can hold primitive values or objects.

6. Arrays Construct array: Store in variable of type double[ ]
Find Length of an array
double[] data = new double[10];
new double[10]
System.out.println (data.length);
Continued…

7. Arrays
Arrays are often partially filled and you need to remember the number of elements that you actually placed in the array.

8. Arrays
Note that length is the number of elements the array can hold….not the number of positions filled.
int[] a = new int[5];
a[0] = 78;
a[1] = 89;
for (int x = 0; x < a.length ; x++) {
System.out.print("\t"+ a[x] + "\t"); }
prints:

9. Arrays
When array is created, all values are initialized depending on array type:
Numbers: 0
Boolean: false
Object References: null

10. Arrays
Figure 1: An Array Reference and an Array

11. Arrays Use [ ] to access an element data[2] = 29.95;
Figure 2: Storing a Value in an Array

12. Arrays Using the value stored:
Get array length as data.length. (Not a method!)
Index values range from 0 to(length - 1)
System.out.println("The value of this data item is " + data[4]);
Array Name
Continued…

13. Arrays
Arrays are often partially filled and you need to remember the number of elements that you actually placed in the array.
// filling array from input
import java.util.Scaner; …
scanner in = new Scanner(System.in);
int count = 0;
boolean done = false;
int maxCount = 10;
int[] arr = new int[maxCount];
while(count < maxCount && !done) {
System.out.println("Enter integer (-1 to quit.): ");
int a = in.nextInt();
if (a != -1)
arr[count] = a;
count++; }
else
done = true;
size = count; // Remember number of filled
// elements

14. Arrays
Arrays are often partially filled and you need to remember the number of elements that you actually placed in the array and carry it around with you!
public static void listForward(int[] tList, int num) {
for (int x = 0; x < num ; x++)
System.out.print("\t"+ tList[x] + "\t"); }
System.out.println();

15. Arrays Accessing a nonexistent element results in a bounds error
Limitation: Arrays have fixed length
double[] data = new double[10]; data[10] = 29.95; // ERROR

16. Self Check
What elements does the data array contain after the following statements?
Ans: 0, 1, 4, 9, 16, 25, 36, 49, 64, 81, but not 100
double[] data = new double[10]; for (int i = 0; i < data.length; i++)
data[i] = i * i;

17. Self Check
What do the following program segments print? Or, if there is an error, describe the error and specify whether it is detected at compile-time or at run-time.
a runtime error: array index out of bounds
3. a compile-time error: c is not initialized
double[] a = new double[10]; System.out.println(a[0]);
double[] b = new double[10]; System.out.println(b[10]);
double[] c; System.out.println(c[0]);

18. Copying Arrays: Copying Array References
Copying an array variable yields a second reference to the same array
double[] data = new double[10]; // fill array double[] prices = data;
Continued…

19. Copying Arrays: Copying Array References
Figure 7: Two References to the Same Array

20. Copying Arrays: Cloning Arrays
Use clone to make true copy
double[] prices = (double[]) data.clone();
Need to cast as a (double) since cloned type is an Object
Continued…

21. Copying Arrays: Cloning Arrays
Figure 8: Cloning an Array

22. Copying Arrays: Copying Array Elements
System.arraycopy(from, fromStart, to, toStart, count);
How many elements to copy
From
Array Name To
Array Name
Continued…

23. Copying Arrays: Copying Array Elements
Figure 9: The System.arraycopy Method

24. Arrays Now, what if you run out of room?
You must allocate a larger array and copy the elements from the original array to the larger array.

25. Growing an Array
If the array is full and you need more space, you can grow the array:
Create a new, larger array.
Copy all elements into the new array
Store the reference to the new array in the array variable
double[] newData = new double[2 * data.length];
System.arraycopy(data, 0, newData, 0, data.length);
data = newData;

26. Arrays
You must allocate a larger array and copy the elements from the original array to the larger array.
data newData
free space
By setting data = newData
the array data now references the copy.

27. Growing an Array
Figure 12: Growing an Array

28. Sample Code
You must allocate a larger array and copy the elements from the original array to the larger array.
int[] copyOfArray = new int[2* arr.length];
for (int i = 0; i < arr.length; i++) {
copyOfArray[i] = arr[i]; }
arr = copyOfArray;

29. Simple Array Algorithms: Finding the Maximum or Minimum
Steps -- Ex.  Find the largest bank acct balance
Initialize a candidate with the starting element
Compare candidate with remaining elements
Update it if you find a larger or smaller value
Continued…

30. Simple Array Algorithms: Finding the Maximum or Minimum
Example:
BankAccount largestYet = accounts[0]; for (int i = 1; i < accounts.length; i++) { BankAccount a = accounts[i]; if (a.getBalance() > largestYet.getBalance()) largestYet = a; } return largestYet;

31. Filling an Array using an initializer list:
Instead of:
int[] primes = new int[3];
primes[0] = 2;
primes[1] = 3;
primes[2] = 5;
Use:
int[]primes = {2, 3, 5};

32. Arrays
It is not very convenient to track array sizes and to grow arrays when they run out of space.
If you are collecting objects, use ArrayList.
If you collect numbers, you must make a choice. Which is the least inconvenient?
Using wrapper classes?
Tracking array size?

33. Dr. Seuss: "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!

34. "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.

35. 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!

36. 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.

37. 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.

38. 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.

39. ArrayList
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.

40. boolean add(Object x) // appends x to the end of list; returns true
ArrayList methods:
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()
//Returns an iterator over the elements in the arraylist in
proper sequence.
ListIterator listIterator()
//Returns a list iterator over the elements in this arraylist
(in proper sequence).
Use to traverse through the list – iterators point between two elements in the list.

41. More ArrayList 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.

42. Array Lists
The ArrayList class is a generic class: ArrayList<T> collects objects of type T:
size method yields number of elements
ArrayList<BankAccount> accounts = new ArrayList<BankAccount>(); accounts.add(new BankAccount(1001)); accounts.add(new BankAccount(1015)); accounts.add(new BankAccount(1022));

43. Retrieving Array List Elements
Use get method
Index starts at 0
Bounds error if index is out of range
BankAccount anAccount = accounts.get(2); // gets the third element of the array list
Continued…

44. Retrieving Array List Elements
Most common bounds error:
int i = accounts.size(); anAccount = accounts.get(i); // Error // legal index values are i-1
Since positions start at zero

45. Adding Elements set overwrites an existing value
add adds a new value before the index
BankAccount anAccount = new BankAccount(1729); accounts.set(2, anAccount);
accounts.add(i, a)
Continued…

46. Adding Elements
Figure 3: Adding an Element in the Middle of an Array List

47. Removing Elements remove removes an element at an index
Accounts.remove(i)
Continued…

48. Removing Elements
Figure 4: Removing an Element in the Middle of an Array List

49. File: ArrayListTester.java - p294-296
01: import java.util.ArrayList;
02:
03: /**
04: This program tests the ArrayList class.
05: */
06: public class ArrayListTester
07: {
08: public static void main(String[] args)
09: {
10: ArrayList<BankAccount> accounts
11: = new ArrayList<BankAccount>();
12: accounts.add(new BankAccount(1001));
13: accounts.add(new BankAccount(1015));
14: accounts.add(new BankAccount(1729));
15: accounts.add(1, new BankAccount(1008));
16: accounts.remove(0);
Continued…

50. File: ArrayListTester.java
17:
18: System.out.println("size=" + accounts.size());
19: BankAccount first = accounts.get(0);
20: System.out.println("first account number="
21: first.getAccountNumber());
22: BankAccount last = accounts.get(accounts.size() - 1);
23: System.out.println("last account number="
24: last.getAccountNumber());
25: }
26: }
Output
size=3 first account number=1008 last account number=1729

51. Self Check How do you construct an array of 10 strings?
An array list of strings?
Ans:
new String[10];
new ArrayList<String>();

52. Self Check Continued…
What is the content of names after the following statements?
Ans: names contains the strings "B" and "C" at positions 0 and 1
ArrayList<String> names = new ArrayList<String>(); names.add("A"); names.add(0, "B"); names.add("C"); names.remove(1);

53. Wrappers You cannot insert primitive types directly into array lists
To treat primitive type values as objects, you must use wrapper classes:
ArrayList<Double> data = new ArrayList<Double>(); data.add(29.95); double x = data.get(0);
Continued…

54. Wrappers
Figure 5: An Object of a Wrapper Class

55. Wrappers
There are wrapper classes for all eight primitive types

56. Auto-boxing
Auto-boxing: Starting with Java 5.0, conversion between primitive types and the corresponding wrapper classes is automatic.
Double d = 29.95; // auto-boxing; same as Double d = new Double(29.95); double x = d; // auto-unboxing; same as double x = d.doubleValue();
Continued…

57. Auto-boxing Auto-boxing even works inside arithmetic expressions
Means:
auto-unbox d into a double
add 1
auto-box the result into a new Double
store a reference to the newly created wrapper object in e
Double e = d + 1;

58. Self Check What is the difference between the types double and Double?
Ans: double is one of the eight primitive types. Double is a class type.
Suppose data is an ArrayList<Double> of size > 0. How do you increment the element with index 0?
Ans: data.set(0, data.get(0) + 1);

59. The Enhanced for Loop Traverses all elements of a collection:
double[] data = . . .; double sum = 0; for (double e : data) // You should read this loop as // "for each e in data" { sum += e; }
Continued…

60. The Enhanced for Loop Traditional alternative:
double[] data = . . .; double sum = 0; for (int i = 0; i < data.length; i++) { double e = data[i]; sum += e; }

61. The Enhanced for Loop Works for ArrayLists too:
ArrayList<BankAccount> accounts = ; double sum = 0; for (BankAccount a : accounts) { sum += a.getBalance(); }

62. The Enhanced for Loop Equivalent to the following ordinary for loop:
double sum = 0; for (int i = 0; i < accounts.size(); i++) { BankAccount a = accounts.get(i); sum = sum + a.getBalance(); }

63. Syntax 7.3: The "for each" Loop
 for (Type variable : collection) statement
Example:
 for (double e : data) sum = sum + e;
Purpose:
To execute a loop for each element in the collection. In each iteration,
the variable is assigned the next element of the collection. Then the
statement is executed.

64. Self Check
Write a "for each" loop that prints all elements in the array data
Ans:
Why is the "for each" loop not an appropriate shortcut for the following ordinary for loop?
Ans: The loop writes a value into data[i]. The "for each" loop does not have the index variable i.
for (double x : data)
System.out.println(x);
for (int i = 0; i < data.length; i++)
data[i] = i * i;

65. Simple Array Algorithms: Counting Matches
Textbook
Page
Check all elements and count the matches until you reach the end of the array list.
public class Bank { public int count(double atLeast) { int matches = 0; for (BankAccount a : accounts) { if (a.getBalance() >= atLeast) matches++; // Found a match } return matches; } private ArrayList<BankAccount> accounts; }

66. Simple Array Algorithms: Finding a Value
Check all elements until you have found a match.
public class Bank {
public BankAccount find(int accountNumber)
for (BankAccount a : accounts)
if (a.getAccountNumber() == accountNumber)
// Found a match return a; }
return null; // No match in the entire array list
. . .

67. Simple Array Algorithms: Finding the Maximum or Minimum
Works only if there is at least one element in the array list
If list is empty, return null
if (accounts.size() == 0) return null; BankAccount largestYet = accounts.get(0); . . .

68. BankAccount largestYet = accounts.get(0);
Initialize a candidate with the starting element
Compare candidate with remaining elements
Update it if you find a larger or smaller value
Example:
BankAccount largestYet = accounts.get(0);
for (int i = 1; i < accounts.size(); i++) {
BankAccount a = accounts.get(i);
if (a.getBalance()> largestYet.getBalance())
largestYet = a; } return largestYet;

69. Two-Dimensional Arrays
When constructing a two-dimensional array, you specify how many rows and columns you need:
You access elements with an index pair a[i][j]
final int ROWS = 3; final int COLUMNS = 3; String[][] board = new String[ROWS][COLUMNS];
board[i][j] = "x";

70. Traversing Two-Dimensional Arrays
It is common to use two nested loops when filling or searching:
See File: TicTacToe.java – text pages
for (int i = 0; i < ROWS; i++) for (int j = 0; j < COLUMNS; j++) board[i][j] = " ";

71. A Tic-Tac-Toe Board
Figure 6: A Tic-Tac-Toe Board

72. Self Check
How do you declare and initialize a 4-by-4 array of integers?
ans: int[][] array = new int[4][4]

73. Regression Testing Save test cases
Use saved test cases in subsequent versions
A test suite is a set of tests for repeated testing
Cycling = bug that is fixed but reappears in later versions
Regression testing: repeating previous tests to ensure that known failures of prior versions do not appear in new versions
See regression/BankTester.java pp. 319 – 320.

74. Self Check 7.15
Suppose you modified the code for a method. Why do you want to repeat tests that already passed with the previous version of the code?
Answer: It is possible to introduce errors when modifying code.

75. Self Check 7.16
Suppose a customer of your program finds an error. What action should you take beyond fixing the error?
Answer: Add a test case to the test suite that verifies that the error is fixed.

76. Arrays/ Array lists
Applications?

77. What to do with arrays…. Input Process Output How? What? Where?
Get “stuff” into our array
Process
Do something with the stuff in our array
Output
Display contents of our array
How?
What?
Where?

78. Input
// filling arrays with consecutive integers for (count = 0;count < 20;count++) { v[count] = count ; } // filling array list with consecutive integers v.add(new Integer(count));

79. Input String pathname = "..h:\\yourFilename\\Data.txt“;
// filling array from file
int count = 0;
String pathname = "..h:\\yourFilename\\Data.txt“;
File file = File(pathname);
Scanner input = null;
try {
input = new Scanner(file); }
catch (FileNotFoundException ex)
System.out.println("*** Cannot open " + pathname " ***");
System.exit(1); // quit the program
try-catch block – page 508 of textbook

80. while(count < maxCount && input.hasNextInt()){
int a = input.nextInt();
arr[count] = a;
count++; }
size = count; //number of filled elements

81. Input // filling array list from file
ArrayList<Integer> arr = new ArrayList<Integer>();
while(input.hasNextInt()) {
int a = input.nextInt();
if (input.hasNextInt())
arr.add(a); }

82. What about filling our 20-element container with random integers from 1 to 100?
final int MAXELTS = 20; Random generator = new Random(); int[] a = new int[MAXELTS]; for(int k = 0; k < MAXELTS; k++) { a[k] = generator.nextInt(100) + 1; }

83. What about filling our 20-element container with random integers from 1 to 100?
final int MAXELTS = 20; Random generator = new Random(); int[] a = new int[MAXELTS]; for(int k = 0; k < MAXELTS; k++) { a[k] = generator.nextInt(100) + 1; } How does this change with an ArrayList?

84. What about filling our 20-element container with random integers from 1 to 100 -- ArrayList?
final int MAXELTS = 20; Random generator = new Random(); ArrayList<Integer> a = new ArrayList<Integer>; for (int k = 0; k < MAXELTS; k++) { int num = generator.nextInt(100)+1; a.add(num); }

85. PROBLEM : Fill a 20-element 'container' with random integers from inclusive ensuring that there are no duplicate values in array.

86. We will look at several possible solutions.
PROBLEM : Fill a 20-element 'container' with random integers from inclusive ensuring that there are no duplicate values in array.
We will look at several possible solutions.
You will be asked for others.
You will be asked your preference and a justification!
Note : solutions are not given in Java code!

87. Solution 1 : for (int k = 0; k < 20; k++) do4 8 6 23 *
for (int k = 0; k < 20; k++) do
Generate random integer(1-100)
Check all elements in array that are filled already until random number is not a dupe
while (random num is a dupe);
Put the non-dupe in array 8

88. Solution 1 : for (int k = 0; k < 20; k++) do4 8 6 23 *
for (int k = 0; k < 20; k++) do
Generate random integer(1-100)
Check all elements in array that are filled already until random number is not a dupe
while (random num is a dupe);
Put the non-dupe in array
space efficiency?
time efficiency? 8

89. Solution 1 : for (int k = 0; k < 20; k++) do4 8 6 23 *
for (int k = 0; k < 20; k++) do
Generate random integer(1-100)
Check all elements in array that are filled already until random number is not a dupe
while (random num is a dupe);
Put the non-dupe in array
space efficiency? OK
time efficiency? not very 8

90. Solution 2: Choose a random number, r
Create an additional array, b, with 101 elements (random choices are from 1-100) and fill it with 0s.
for (int k = 0; k < 20; k++) do
Choose a random number, r
while b[r] = 0
Put r in your array
Put 1 in b[r] to mark it “taken”

91. Solution 2 : a picture b a 1 4 8 Choose a random number, r 8
b[1]
b[2]
b[3]
b[4] 1
b[5]
b[6]
b[7]
b[8]
b[9]
b[10]
b[11] 4 8
for (int k=0; k < 20; k++) do
Choose a random number, r 8
while b[r] = 0
Put r in your array
Put 1 in b[r] to mark it “taken”

92. Solution 3:
Create an additional array, b, with 101 elements (random choices are from 1-100) and fill it with indices of b (b[1] = 1, b[5]= 5). Set n = 100;
for (int k=0;k<20;k++)
Choose a random number, r, in the range [1, n]
Put r in your array
b[n] replaces b[r]
Decrement n

93. Solution 3 : a picture A 4 1 2 3 4100 5 6 7 8 9 10 11 n= 100
b[0]
b[1] 1
b[2] 2
b[3] 3
b[4]
4100
b[5] 5
b[6] 6
b[7] 7
b[8] 8
b[9] 9
b[10] 10
b[11] 11
n= 100
for (int k=0; k < 20; k++)
Choose a random number,
r, in the range [1, n] 4
Put b[r] in your array
b[n] replaces b[r]
Decrement n n = 99

94. Solution 4: Choose a random number, r, in the range [0, 99]
Create an array, b, with 100 elements (random choices are from 1-100) and fill it with (index + 1) (b[0] = 1, b[4]= 5).
for (int k=0;k< 20;k++)
Choose a random number, r, in the range [0, 99]
Swap b[k] with b[r]

95. Solution 4 : a picture
b[0] 1
b[1] 2
b[2] 3
b[3] 4
b[4] 5
b[5] 6
b[6] 7
b[7] 8
b[8] 9
b[9] 10
b[10] 11
b[11] 12 5
for (int k=0; k < 20; k++)
Choose a random number,
r, in the range [0, 99] 4
Swap b[r] with b[k]
copy the first 20
elements of b to an array
of size 20 and
assign b to this reference.
to swap:
int temp = a[i];
a[i] = a[j];
a[j] = temp; 1

96. Solution 5 :
Assign to each of the first 100 elements of an ArrayList, b, the value of index + 1.
Create a new empty ArrayList (or 20-element array), a.
for(int x = 1; x < = 20; x++)
Choose a random integer in the range [0, b.size()]
Remove this element from b and add it to a.

97. Solution 5 : A picture
Assign to each of the first 100 elements of an ArrayList, b, the value of index + 1.
Create a new empty ArrayList, a.
for(int x = 1; x < = 20; x++)
Choose a random integer in the range [0, b.size()]
Remove this element from b and add it to a. 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 9 10 1 3 4 5 6 7 9 10 8 2

98. Other solutions ???

99. You should be able to
Give advantages and disadvantages of each solution
Discuss time and space considerations
Commit to and defend one of the solutions

100. Now………. Our array is filled Let’s process……. (by some obscure method)
Find the sum.
Find the mean.
Find the largest/smallest value.
Many other things we can do….

101. Find the largest value in the array/array list
int large = a[0]; for(int k = 0; k < a.length; k++) { if (a[k] > large) large = a[k]; } System.out.println("The largest value is: " + large);

102. Find the largest value in the array/array list
ArrayList<Integer>nums = new ArrayList<Integer>(); int large = nums.get(0); for (int i = 0; i < nums.size(); i++) { int tempLarge = nums.get(i); if(tempLarge.compareTo(large) > 0) large = tempLarge; }
compareTo
returns a value < 0 if tempLarge is less than large
returns a value = 0 if tempLarge is equal to large
returns a value > 0 if tempLarge is greater than large

103. PROBLEM/Example code: Report the number that has the longest sequence of consecutive repeated values in an array/array list. If the array contains  , the number 3 should be reported)
public static int longSequence(int[] v) { int seqIndex = 0; // holds index of number in seq int longSeq= 1; // holds longest sequence so far int count = 1; // current sequence length

104.
for ( int k = 1; k < v.length; k++ ) {
if ( v[k] == v[k-1] )
count++; //current seq length
if ( count > longSeq )
longSeq = count;
seqIndex = k; }
else
count = 1;
return v[seqIndex]; // ANSWER
k = 1;
int seqIndex = 0;
int longSeq = 1;
int count = 1

105. How else can we process ??? Find the largest number in array
Determine if there are duplicates
Find the mean (how?)
Find the median (how?)
Find the mode (how?)
Sort the array (more on this later)

106. Declaring multiple arrays
int[] a, b; // Declares two arrays – uninitialized now a = new int[10]; b = new int [10]; =============================== int a[], b; // Declares one array and one int int a, b[]; // Declares one array and one int

107. Initializer Lists int[] arr = {10, 20, 30, 40, 50, 60, 70, 80};
System.out.println("Printing arr");
for (int i = 0; i < arr.length; i++)
// arr's length is 8 {
System.out.print(arr[i] + " "); } //

108. A word about efficiency
Problem:
Insert an element into an array (that is not full).
Insert it as the first element.
Insert it as the last element.
Insert it in the middle.
Delete an element from an array.
Delete the first element.
Delete the last element.
Delete a middle element.

109. A word about efficiency (using “big-Oh” notation)
Problem:
Insert an element into an array (that is not full).
Insert it as the first element. O(n)
Insert it as the last element. O(1)
Insert it in the middle. O(n)
Delete an element from an array.
Delete the first element. O(n)
Delete the last element. O(1)
Delete a middle element. O(n)

110. A word about efficiency
Problem:
Insert it as the first element. O(n)

111. A word about efficiency
Problem:
Insert it as the first element. O(n)
Will this work?
for (int i = 1; i < arrayName.length; i++)
arrayName[i] = arrayName[i - 1];
arrayName[0] = newValue;
newValue = 90;
__ __ __

112. A word about efficiency
Problem:
Insert it as the first element. O(n)
Will this work?
for (int i = 1; i < arrayName.length; i++)
arrayName[i] = arrayName[i - 1]; WRONG
arrayName[0] = newValue;
newValue = 90;
__ __ __

113. A word about efficiency
Problem:
Insert it as the first element. O(n)
Will this work?
for (int i = arrayName.length ; i > 0; i--)
arrayName[i] = arrayName[i - 1];
arrayName[0] = newValue;
newValue = 90;
__ __ __

114. A word about efficiency
Problem:
Insert it as the first element. O(n)
Will this work?
for (int i = arrayName.length - 1; i > 0; i--)
arrayName[i] = arrayName[i - 1]; CORRECT
arrayName[0] = newValue;
newValue = 90;
__ __ __

115. A word about efficiency
Problem:
What about deleting an element?
Algorithm?
What about number of elements in the array?
__ __ __

116. equals
Using equals with array lists will use the default Object equals and will compare references not contents -- . equals returns true if two ArrayList references are the same, false otherwise.
When you use equals with arrays, you are also comparing references. array1.equals(array2) is asking if array1 and array2 reference the same array.

117. Arrays and Array Lists as Parameters to Methods:
A method cannot change the size of an array parameter. Why not?
A method can change the length of an array list that is passed as a parameter. Why?
You can change the contents of the array/array list but you cannot change the reference.

118. Make Parallel Arrays into Arrays of Objects
// Don't do this int[] accountNumbers; double[] balances;
Figure 13: Avoid Parallel Arrays

119. Make Parallel Arrays into Arrays of Objects
Avoid parallel arrays by changing them into arrays of objects:
BankAccount[] = accounts;
Figure 14: Reorganizing Parallel Arrays into Arrays of Objects

120. java.io How do I read an int from a file?
BufferedInputStream BufferedOutputStream BufferedReader BufferedWriter ByteArrayInputStream ByteArrayOutputStream CharArrayReader CharArrayWriter DataInputStream DataOutputStream File FileDescriptor FileInputStream FileOutputStream FilePermission FileReader FileWriter FilterInputStream FilterOutputStream FilterReader FilterWriter
InputStream InputStreamReader LineNumberInputStream LineNumberReader ObjectInputStream ObjectInputStream.GetField ObjectOutputStream ObjectOutputStream.PutField ObjectStreamClass ObjectStreamField OutputStream OutputStreamWriter PipedInputStream PipedOutputStream PipedReader PipedWriter PrintStream PrintWriter PushbackInputStream PushbackReader
RandomAccessFile Reader SequenceInputStream SerializablePermission StreamTokenizer StringBufferInputStream StringReader StringWriter Writer
How do I read an int from a file?
Java IO is not for the faint at heart. It looks like Java developers went all out to confuse programmers. That’s why programmers are such well-paid specialists.

121. java.io (cont’d)
Uses four hierarchies of classes rooted at Reader, Writer, InputStream, OutputStream.
InputStream/OutputStream hierarchies deal with bytes. Reader/Writer hierarchies deal with chars.
Has a special stand-alone class RandomAccessFile.
The Scanner class has been added to java.util in Java 5 to facilitate reading numbers and words.
The developer of the RandomAccessFile class probably was not a “team player,” as they say in the industry. Or this part of the project was assigned to a different group. Scanner is an afterthought (Java 5). Scanner got rid of checked exceptions (except in the constructor). It still won’t let you read one character.

122. java.io.File
The File class represents a file (or folder) in the file directory system.
Methods:
String getName() // Returns the name of the file or directory
String getAbsolutePath() // Returns the absolute pathname string
long length() // Returns the length of the file
boolean isDirectory() //tests if file is a directory
File[ ] listFiles() // Returns an array of abstract pathnames denoting
//the files in the directory
String pathname = "../Data/words.txt“;
File file = new File(pathname);
A File object does not have to be associated with an open file. In fact, it can represent a directory entry for a file that does not exist yet.

123. Reading from a Text File
String pathname = "words.txt";
File file = new File(pathname);
Scanner input = null;
try {
input = new Scanner(file); }
catch (FileNotFoundException ex)
System.out.println("*** Cannot open " + pathname
+ " ***");
System.exit(1); // quit the program
Tries to open the file
You have used the same Scanner class to read from the keyboard.

124. Scanner Methods boolean hasNextLine() String nextLine()
boolean hasNextInt()
int nextInt()
boolean hasNextDouble()
double nextDouble()
void close()
Reads one word
You can check whether a particular token (an int, a word) exists in the stream, then read it.

125. Writing to a Text File String pathname = "output.txt";
File file = new File(pathname);
PrintWriter output = null;
try {
output = new PrintWriter(file); }
catch (FileNotFoundException ex)
System.out.println("Cannot create " + pathname);
System.exit(1); // quit the program
output.println(...);
output.printf(...);
output.close();
Once a PrintWriter has been created, you can write to it using print, println, and printf methods, the same way you write to the console screen.
Required to flush the output buffer

126. Summary We can create arrays/ array lists We can fill them
We can process them
We can output their contents
Which do we use…..
Array lists?
Arrays?

127. Programming Exercises Continued…
Fun Number Lab2 – FunNumber2.java on s:\drive
Read in FNUM1, FNUM2, and FNUM3 lists from the s:\ drive in Assignments/AP Java/data(See chapter 11.1 for file reading using Scanner class)
Find:
Number of elements
Maximum
Minimum
Mode
Median
Mean