2. An Overview of the Collections Framework
Why do we need collections of objects?
When we are only creating a few objects, we can afford to declare individualized reference variables for these objects:
E.g. Students s1, s2, s3,
E.g. Professors profA, profB, profC.
At other times, individualized reference variables are impractical.
Too large no. of objects to create:
E.g. university course catalog might have hundreds of courses
We do not know until runtime how many objects to create
So we can’t predefine no. of reference variables at compile time
Most collection needn’t be assigned an explicit capacity at the time that they are instantiated
Collections can grow and shrink as needed.

3. Fortunately, Java solve this problem by collection
Special category of object
What are Collections?
Way to gather up objects as they are created
We can manage them as a group
We can operate on them collectively
We can refer to them individually when necessary
Collections hold and organize References to Other Objects
Not objects themselves!

4. A collection organizes references to objects that live in memory outside of the collection:
BankAccount Object
BankAccount Object
BankAccount Object
An ArrayList Object

5. An Overview of the Collections Framework
Java collections framework:
a hierarchy of interface types and classes for collecting objects.
Each interface type is implemented by one or more classes
Figure 1 Interfaces and Classes in the Java Collections Framework

6. An Overview of the Collections Framework
The Collection interface is at the root
All Collection classes implement this interface
Refer javadoc api
So all have a common set of methods
Adding objects : Collections automatically expand as new items are added.
Removing objects: Collections automatically shrink when items are removed.
Retrieving specific individual objects
Iterating through the objects in some predetermined order
Getting a count of the number of objects presently referenced by the collection
Answering a true/false question as to whether a particular object’s reference is in the collection or not

9. Three Generic Types of Collection:
Ordered lists
Dictionaries
Sets
Ordered Lists
Sets
Dictionaries/Maps

10. Ordered Lists: Allows us to insert items in a particular order
Allow later retrieving them in that same order
Specific objects can also be retrieved based on their position in the list
By default, items are added at the end of an ordered list
E.g. a student waiting list:
Order maintenance is important to be fair in selecting students from waiting list
Ordered lists are realized in java using :
List interface
Queue interface

11. List Interface Implementations
ArrayList - FIFO
low cost random access
high cost insert and delete
array that resizes if need be
LinkedList
sequential access
low cost insert and delete
high cost random access
Stack
LIFO
Vector

12. Interface List

13. ArrayList Resizable-array implementation of the List interface.
To declare an array list of strings
ArrayList<String> names = new ArrayList<String>();
Angle brackets denote a type parameter
Replace String with any other class to get a different array list type
E.g.
ArrayList<Student> waitinglist = new ArrayList<Student>();
Refer javadoc API ArrayList class page

14. Declaring and Using Array Lists
ArrayList<String> is first constructed, it has size 0:
ArrayList<String> names = new ArrayList<String>();
Use the add method to add an object to the end of the array list:
names.add("Emily"); // Now names has size 1 and element "Emily”
names.add("Bob"); // Now names has size 2 and elements "Emily", "Bob”
names.add("Cindy"); //names has size 3 and elements "Emily", "Bob",
// and "Cindy”
The size method gives the current size of the array list.
Size is now 3
Figure 17 Adding an Array List Element with add

15. Array Lists
Refer javadoc API ArrayList class page

16. Declaring and Using Array Lists
To obtain an array list element, use the get method
E.g. To retrieve the name with index 0:
String name = names.get(0);
The last valid index is:
names.size() - 1
To set an array list element to a new value, use the set method:
names.set(2, "Carolyn");
To add a new element at middle of array list :
names.add(1, "Ann"); //add element “Ann” at index==1
moves all elements with index 1 or larger by one position.

17. Declaring and Using Array Lists
Figure 18 Adding and Removing Elements in the Middle of an Array List

18. Declaring and Using Array Lists
The remove method,
removes the element at a given position
moves all elements after the removed element down by one position
and reduces the size of the array list by 1.
names.remove(1);

19. Programming Question
Write a tester class ArrayListDemo to display the words of a file (words.txt) as a list:
Read file content to an arrayList allWords
Hint: Use Scanner class:
Scanner input = new Scanner(new File("words.txt"));
Print elements in allWords
Print elements in allWords in reverse order
Note: create and save words.txt with following content. Template for ArrayListDemo is given:
A sample run is sown below:
words.txt
ArrayListDemo.java It is a
beautiful
day
public class ArrayListDemo{
public static void main(String args[]) throws Exception {
//TODO }

20. Answer import java.util.ArrayList; import java.util.Scanner;
import java.io.File;
public class ArrayListDemo {
public static void main(String args[])throws Exception
//populate allwords
ArrayList<String> allWords = new ArrayList<String>();
Scanner input = new Scanner(new File("words.txt"));
while (input.hasNext()) {
String word = input.next();
allWords.add(word); }
//print allwords
System.out.println(allWords);
//print in reverse order
for(int i=allWords.size()-1; i>=0;i--) {
System.out.print(allWords.get(i)+",");

21. Using the Enhanced for Loop with Array Lists
E.g. print elements in arraylist names:
for (String name : names){
System.out.println(name); }
This is equivalent to:
for (int i = 0; i < names.size(); i++){
String name = names.get(i);

22. Copying Array Lists
Copying an array list reference yields two references to the same array list.
E.g. After the code below is executed
ArrayList<String> friends = names;
friends.add("Harry");
Figure 19 Copying an Array List Reference

23. Copying Array Lists
To make a true copy of an array list, construct the copy and pass the original list into the constructor:
ArrayList<String> newNames = new ArrayList<String>(names);
Read: Deep copy vs shallow copy
Shallow copies duplicate as little as possible. A shallow copy of a collection is a copy of the collection structure, not the elements. With a shallow copy, two collections now share the individual elements.
Deep copies duplicate everything. A deep copy of a collection is two collections with all of the elements in the original collection duplicated.

24. Working With Array Lists

25. Wrapper Classes
You cannot directly insert primitive type values into array lists (or any other collection type).
Use the matching wrapper class.
E.g. To collect double values in an array list, you use an
ArrayList<Double>.

26. Storing Input Values in an Array List
To collect an unknown number of inputs, use array lists :
ArrayList<Double> inputs = new ArrayList<Double>();
while (in.hasNextDouble()) {
inputs.add(in.nextDouble()); }
> run LinkedListDemo2
list before:[Diana, Harry, Romeo, Tom]
list after:[Diana, Harry, Romeo]
> import java.util.*
> Collection<Integer> c = new ArrayList<Integer>()
> c.add(1);
> List<Integer> c = new ArrayList<Integer>()
>

27. Question Removing Matches: What is the purpose of the code?
ArrayList<String> words = ...;
for (int i = 0; i < words.size(); i++) {
String word = words.get(i);
if (word.length() < 4)
words.remove(i); }
What is the purpose of the code?
What is wrong with the code?

28. Answer Purpose: remove all words with length<4 from list. Error:
When element is removed indexes automatically change for following elements
So, should not increment i when an element is removed
Correct Pseudocode:
If the element at index i matches the condition
Remove the element.
Else
Increment i.
How do you correct previous code?

29. Removing Matches Use a while loop, not a for loop int i = 0;
while (i < words.size()){
String word = words.get(i);
if (word.length() < 4) { words.remove(i); }
else { i++; } }

30. Choosing Between Array Lists and Arrays
For most programming tasks, array lists are easier to use than arrays
Array lists can grow and shrink.
Arrays have a nicer syntax.
Recommendations
If the size of a collection never changes, use an array.
If you collect a long sequence of primitive type values and you are concerned about efficiency, use an array.
Otherwise, use an array list.

31. Choosing Between Array Lists and Arrays

32. Programming Question
Write ArrayListDemo2 class to use an ArrayList called values to read and store user input numbers (user can quit any time by typing Q).
Write code that find and print the largest in the values array list marking the largest.
A sample program run is shown:
Please enter values, Q to quit: Q 35 80
115 <== largest value
44.5
Find template in next slide:

33. Use following template to get started:
public class ArrayListDemo2 {
public static void main(String[] args)
//declare arraylist values
// Read inputs to values
// Find the largest
// Print all values, marking the largest }

34. Answer ArrayListDemo2.java import java.util.ArrayList;
import java.util.Scanner;
public class ArrayListDemo2 {
public static void main(String[] args) {
//create arraylist
ArrayList<Double> values = new ArrayList<Double>();
// Read inputs
System.out.println("Please enter values, Q to quit:");
Scanner in = new Scanner(System.in);
while (in.hasNextDouble()) {
values.add(in.nextDouble()); }
// Find the largest
double largest = values.get(0);
for (int i = 1; i < values.size(); i++) {
if (values.get(i) > largest) {
largest = values.get(i);
// Print all values, marking the largest
for (double element : values) {
System.out.print(element);
if (element == largest) { System.out.print(" <== largest value"); }
System.out.println();

35. Question
What does the array list names contain after the following statements execute?
ArrayList<String> names = new ArrayList<String>;
names.add("Bob");
names.add(0, "Ann");
names.remove(1);
names.add("Cal");

36. Answer
What does the array list names contain after the following statements?
ArrayList<String> names = new ArrayList<String>;
names.add("Bob");
names.add(0, "Ann");
names.remove(1);
names.add("Cal");
Answer: "Ann", "Cal"

37. Linked Lists Doubly-linked list implementation of the List interface.
A linked list consists of a number of nodes
Each node stores element + has references to the next node and previous node.
Visiting the elements of a linked list in sequential order is efficient.
Random access is NOT efficient.

38. Linked Lists When inserting/adding or removing a node:
Adding and removing elements in the middle of a linked list is efficient.
When inserting/adding or removing a node:
Only the neighboring node references need to be updated (Unlike arrays!)
Adding a new node with element=“Romeo”
Removing node with element=“Diana”

39. Linked Lists When to use a linked list:
You are concerned about the efficiency of inserting or removing elements
You rarely need element access in random order

40. The LinkedList Class of the Java Collections Framework
Generic class
Specify type of elements in angle brackets:
LinkedList<Product> l= new LinkedList<Product>();
LinkedList<Integer> l= new LinkedList<Integer>();
Some methods:
l.add("C");
l.addLast("Z"); //add as last element
l.addFirst("A"); //add as first element
l.add("B",3);
l.remove(2);
l.removeFirst(); //remove first element
ll.removeLast(); //remove last element

41. The LinkedList Class of the Java Collections Framework
Some additional LinkedList methods:
Refer javadoc api

42. List Iterator
Use a list iterator to access elements inside a linked list.
To get a list iterator, use the listIterator method of the LinkedList class.
LinkedList<String> employeeNames = new LinkedList<String>();
ListIterator<String> iterator = employeeNames.listIterator();

43. List Iterator
To traverse all elements in a linked list of strings, use next() method:
Using while loop:
while (iterator.hasNext()) {
String name = iterator.next();
//Do something with name }
Smilar to “for each” loop:
for (String name : employeeNames)
foreach loop uses an Iterator behind the scenes

44. List Iterator The nodes of the LinkedList class store two links:
One to the next element
One to the previous element
Called a doubly-linked list
To move the list position forward, use methods in ListIterator:
hasNext
next - return next element and moves the iterator position past it
To move the list position backwards, use methods in ListIterator :
hasPrevious
Previous - return previous element and moves the iterator position past it

45. Example import java.util.LinkedList; import java.util.ListIterator;
public class ListIteratorDemo {
public static void main(String args[])
LinkedList<String> l = new LinkedList<String>();
l.add("A");
l.add("B");
l.add("C");
ListIterator<String> iterator = l.listIterator(); // |ABC
while (iterator.hasNext())
String name = iterator.next(); // A|BC  AB|C  ABC|
System.out.println(name); }
//TODO: how to iterate list backward?
import java.util.LinkedList;
import java.util.ListIterator;
public class ListIteratorDemo {
public static void main(String args[])
LinkedList<String> l = new LinkedList<String>();
l.add("A");
l.add("B");
l.add("C");
ListIterator<String> iterator = l.listIterator();
while (iterator.hasNext())
String name = iterator.next();
System.out.println(name); }
while (iterator.hasPrevious()) //if you comment previous while loop,this while loop will not print anything
String name = iterator.previous();

46. List Iterator adds an object after the iterator.
iterator points between two elements:
The add method:
adds an object after the iterator.
Then moves the iterator position past the new element.
iterator.add("Juliet");
Figure 8 A Conceptual View of the List Iterator
iterator.next()
iterator.add(“J”)

47. List Iterator The remove method:
Removes object that was returned by the last call to next or previous
To remove all names that fulfill a certain condition:
while (iterator.hasNext()) {
String name = iterator.next();
if (condition is fulfilled for name)
iterator.remove(); }
Be careful when calling remove:
It can be called only ONCE after calling next or previous
You CANNOT call it immediately after a call to add
If you call it improperly, it throws an IllegalStateException

48. List Iterator Refer javadoc api
ListIterator interface extends Iterator interface.
Methods of the Iterator and ListIterator interfaces
Refer javadoc api

49. Programming Question Write a tester class LinkedListDemo that :
Create a linked list staff to maintain names of staff of a company.
Inserts 4 names into the end of the list (Diana, Harry, Romeo, Tom)
Create a list iterator for stafflist
Iterates through the list (use ListIterator):
Iterate first two elements
After iterating the second element, add two new names (Juliet, Nina)
Remove the last traversed element
Prints the list
Program template in next slide

50. public class LinkedListDemo {
public static void main(String[] args) {
//create linked list staff
//add 4 elements: Diana, Harry, Romeo, Tom
//create a list iterator for staff list
//iterate first two elements
// Add elements Juliet and Nina after second element
// Remove last traversed element
// Print all elements
System.out.println("Expected: [Diana, Harry, Juliet, Nina, Tom]"); }

51. Answer LinkedListDemo.java import java.util.LinkedList;
import java.util.ListIterator;
public class LinkedListDemo {
public static void main(String[] args) {
LinkedList<String> staff = new LinkedList<String>();
staff.addLast("Diana");
staff.addLast("Harry");
staff.addLast("Romeo");
staff.addLast("Tom");
// | in the comments indicates the iterator position
ListIterator<String> iterator = staff.listIterator(); // |DHRT
iterator.next(); // D|HRT
iterator.next(); // DH|RT
// Add more elements after second element
iterator.add("Juliet"); // DHJ|RT
iterator.add("Nina"); // DHJN|RT
iterator.next(); // DHJNR|T
// Remove last traversed element
iterator.remove(); // DHJN|T
// Print all elements
System.out.println(staff);
System.out.println("Expected: [Diana, Harry, Juliet, Nina, Tom]"); }
iterator.next(); // DHJNR|T //comment this line to see IllegalStateException
// Remove last traversed element
iterator.remove(); // DHJN|T

52. Programming Question
Write LinkedListDemo2 class by writing a loop that removes all names with length less than 5 from staff list. (Hint: use a list iterator)
Use following template:
public class LinkedListDemo2{
public static void main(String[] args) {
LinkedList<String> staff = new LinkedList<String>();
staff.addLast("Diana");
staff.addLast("Harry");
staff.addLast("Romeo");
staff.addLast("Tom");
System.out.println("list before:"+staff);
//TODO: remove names with length<5
System.out.println("list after:"+staff); }

53. Answer LinkedListDemo.java import java.util.LinkedList;
import java.util.ListIterator;
public class LinkedListDemo2 {
public static void main(String[] args)
LinkedList<String> staff = new LinkedList<String>();
staff.addLast("Diana");
staff.addLast("Harry");
staff.addLast("Romeo");
staff.addLast("Tom");
System.out.println("list before:"+staff);
ListIterator<String> iter = staff.listIterator(); // |DHRT
while (iter.hasNext())
String str = iter.next();
if (str.length() < 5) { iter.remove(); } }
System.out.println("list after:"+staff);

54. Queue A queue To visualize a queue, think of people lining up.
Lets you add items to one end of the queue (the tail)
Remove items from the other end of the queue (the head)
Items are removed in the same order in which they were added
First-in, first-out or FIFO order
To visualize a queue, think of people lining up.
Typical application: a print queue.

55. Queue The Queue interface in the standard Java library has:
an add method to add an element to the tail of the queue,
A remove method to remove the head of the queue, and
A peek method to get the head element of the queue without removing it.
The LinkedList class implements the Queue interface.
When you need a queue, initialize a Queue variable with a LinkedList object:
Queue<String> q = new LinkedList<String>();
q.add("A");
q.add("B");
q.add("C");
while (q.size() > 0) { System.out.print(q.remove() + " "); }
// Prints A B C

56. Queue

57. Queue Animator: Queues: Array Implementation
Queues: Linked List Implementation

58. Question
Why would you want to declare a variable as Queue<String> q = new LinkedList<String>(); instead of simply declaring it as a linked list?

59. Answer
This way, we can ensure that only queue operations can be invoked on the q object.

60. 2. Sets An unordered collection
i.e. you CANNOT ask for a particular item by number/position once it has been inserted into the set.
We can iterate though elements one by one
But, order is not predetermined
Duplicate entries aren’t allowed in a set
Unlike lists
E.g. group employees by department
Inserting and removing elements is more efficient with a set than with a list.

61. Question
A gradebook application stores a collection of quizzes. Should it use a list or a set?

62. Answer
A list is a better choice because the application will want to retain the order in which the quizzes were given.

63. Question
A student information system stores a collection of student records for a university. Should it use a list or a set?

64. Answer
A set is a better choice. There is no intrinsically useful ordering for the students. For example, the registrar's office has little use for a list of all students by their GPA.
By storing them in a set, adding, removing, and finding students can be efficient.

65. Set Realized using Set interface Question: Refer javadoc api
What are classes implementing Set interface?
(use java API to find out)

66. Sets Two implementing classes :
HashSet
based on hash table
TreeSet
based on binary search tree
A Set implementation arranges the elements so that it can locate them quickly.

67. Sets
HashSet:
Elements are internally grouped according to a hashcode
A hashCode digests input data into a single hash value (a 32-bit signed integer).
E.g. Try MD5 algorithm hash generation:
echo -n 'text to be encrypted' | md5sum -
E.g. Try SHA1 algorithm hash generation:
echo -n "yourpassword" | openssl sha1
E.g.
HashSet<String> set1 = new HashSet<String>();
Set<Rectangle> set2 = new HashSet<Rectangle>();
HashSet<HashSet<Integer>> = new HashSet<HashSet<Integer>>();
HashMap LoadFactor:
An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.
As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.
In Java, every class provides a hashCode() method, which digests the data stored in an instance of the class into a single hash value (a 32-bit signed integer).
2 ~]$ echo -n 'text to be encrypted' | md5sum -
8df639b301a1e10c36cc2f03bbdf8863
5 ~]$ echo -n 'text to be encrypt' | md5sum -
a57f7943d c75598fce8dbb2da -

68. Sets TreeSet Elements are kept in sorted order
The nodes are arranged in a tree shape, not in a linear sequence
You can form tree sets for any class that implements the Comparable interface (must implement compareTo method):
Example: String or Integer.
Use a TreeSet if you want to visit the set's elements in sorted order.
Otherwise choose a HashSet
It is a bit more efficient — if the hash function is well chosen
public class BankAccount implements Comparable<BankAccount>  {      . . .      /**          Compares two bank accounts.      */      public int compareTo(BankAccount other) {
if (balance < other.balance) return -1;
if (balance == other.balance) return 0;
return 1; }
    private double balance;  }

69. Sets
Store the reference to a TreeSet or HashSet in a Set<String> variable:
Set<String> names = new HashSet<String>(); Or
Set<String> names = new TreeSet<String>();

70. Working with Sets Adding and removing elements:
names.add("Romeo");
names.remove("Juliet");
Sets don't have duplicates.
Adding a duplicate is ignored.
Attempting to remove an element that isn't in the set is ignored.
The contains method tests whether an element is contained in the set:
if (names.contains("Juliet")) . . .
The contains method uses the equals method of the element type

71. Working with Sets To process all elements in the set, get an iterator.
A set iterator visits the elements in the order in which the set implementation keeps them.
Iterator<String> iter = names.iterator();
while (iter.hasNext()) {
String name = iter.next();
//Do something with name }
You can also use the “for each” loop
for (String name : names)

72. Working with Sets

73. Programming Question Write a class called SetDemo with a method solve:
public static void solve(List<String> list1, String[]a2)
This method, given a list of Strings list1, and an array of String a2 as arguments, find two sets s1 and s2 formed from list1 and a2 , respectively , by removing duplicate elements . Then find their union s3 = s1  s2 , intersection s4 = s1 s2 and symmetric difference s5 = ( s1 – s2 )(s2–s1) and print them.
 Note that s1 and s2 must not be changed during the computation of s3, s4 and s5.
Sample run (given list1=[“Harry”, “Diana”, “Romeo”], a2 ={“Jim”, “Harry”, “Diana”}):
> run SetDemo
s1 = [Diana, Harry, Tom, Romeo]
s2 = [Diana, Harry, Jim]
s1 union s2 = [Diana, Harry, Jim, Tom, Romeo]
s1 intersection s2 = [Diana, Harry]
( s1 – s2 )union (s2–s1) = [Jim, Tom, Romeo]
Find program template in next slide

74. public class SetDemo{
public static void main(String[] args) {
List<String> list = new ArrayList<String>();
list.add("Diana");
list.add("Harry");
list.add("Romeo");
list.add("Tom");
String[] ary = {"Diana", "Harry", "Jim"};
solve(list, ary); }
public static void solve(List<String> list1, String[]a2) {
//TODO: create set s1 from list1
// TODO: create set s2 from a2
// TODO: create and print s3 = s1 union s2
// TODO: create and print s4 = s1 intersection s2
// TODO: create and print s5 = ( s1 – s2 )union (s2–s1).

75. Answer SetDemo.java import java.util.Set; import java.util.List;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Arrays;
public class SetDemo{
public static void main(String[] args) {
List<String> list = new ArrayList<String>();
list.add("Harry ");
list.add(" Diana ");
list.add("Romeo");
list.add("Tom");
String[] ary = {"Diana", "Harry", "Jim"};
solve(list, ary); }
public static void solve(List<String> list1, String[]a2) {
Set<String> s1 = new HashSet<String>(list1) ;
Set<String> s2 = new HashSet<String>( Arrays.asList(a2) ) ;
//s3 = s1 union s2
Set<String> s3 = new HashSet<String>(s1);
s3.addAll(s2);
System.out.println("s1 union s2 = "+s3);
//s4 = s1 intersection s2
Set<String> s4 = new HashSet<String>(s1);
s4.retainAll(s2);
System.out.println("s1 intersection s2 = "+s4);
//s5 = ( s1 – s2 )union (s2–s1).
Set<String> s1Subs2= new HashSet<String>(s1); //s1-s2
s1Subs2.removeAll(s2);
Set<String> s2Subs1 = new HashSet<String>(s2); //s2-s1
s2Subs1.removeAll(s1);
Set<String> s5= new HashSet<String>();
s5.addAll(s1Subs2 );
s5.addAll(s2Subs1 );
System.out.println("( s1 – s2 )union (s2–s1) = "+s5);

76. Programming Question
Save and run following program to see output. Change HashSet to Treeset. How does your output change?
import java.util.Set;
import java.util.TreeSet;
import java.util.HashSet;
public class SetDemo2 {
public static void main(String args[]) {
Set<String> s = new HashSet<String>();
s.add("C");
s.add("A");
s.add("B");
s.add("E");
s.add("F");
s.add("D");
System.out.println(s);
for(String str:s)
System.out.print(str+",");
System.out.println(); }

77. Question Can you declare a TreeSet of any type?
E.g. Given Employee class (with name and id attributes), is it possible to create a TreeSet of Employees in Java?
SetDemo3.java
Employee.java
import java.util.Set;
import java.util.TreeSet;
import java.util.HashSet;
public class SetDemo3 {
public static void main(String args[]) {
Set<Employee> s = new TreeSet<Employee>();
// Add elements to the tree set
s.add(new Employee("Jim", 1));
s.add(new Employee("Andy", 2));
s.add(new Employee("Brandon", 4));
s.add(new Employee("Sam", 3));
System.out.println(s); }
public class Employee {
private String name;
private int id;
public Employee(String name,int id){
this.name = name;
this.id = id; }
public String toString(){
return "[Employee:name="+name+" id="+id+"]";

78. Answer
For a class to be used as element type in a TreeSet, class must implement Comparable interface
Some java library classes implement Comparable. E.g.:
String
Integer
(all wrapper classes)
Date

79. Comparable Interface
A class implementing Comparable interface should implement compareTo method
For two object obj1 , obj2 of same type, a call of obj1.compareTo( obj2) should return:
a value < 0 if obj1 comes "before" obj2 in the ordering (obj1 < obj2)
usually return -1
a value > 0 if obj1 comes "after" obj2 in the ordering, (obj1 > obj2)
usually return 1
exactly 0 if obj1 and obj2 are considered "equal" in the ordering (obj1 = obj2)
return 0

80. Example class implementing Comparable Interface
public class Country implements Comparable<Country> {
int area;
public int compareTo(Country otherCountry)
if (this.area < otherCountry.area) { return -1; }
else if (this.area == otherCountry.area) { return 0; }
else { return 1; } }

81. Programming Question Write and Save program SerDemo3.java
Modify given Employee class to implement Comparable interface. Implement compareTo method to compare based on employee id.
SetDemo3.java
Employee.java
import java.util.Set;
import java.util.TreeSet;
import java.util.HashSet;
public class SetDemo3 {
public static void main(String args[]) {
Set<Employee> s = new TreeSet<Employee>();
// Add elements to the tree set
s.add(new Employee("Jim", 1));
s.add(new Employee("Andy", 2));
s.add(new Employee("Brandon", 4));
s.add(new Employee("Sam", 3));
System.out.println(s); }
public class Employee {
private String name;
private int id;
public Employee(String name,int id){
this.name = name;
this.id = id; }
public String toString(){
return "[Employee:name="+name+" id="+id+"]";
Output:
[[Employee: name=Jim id=1], [Employee: name=Andy id=2], [Employee: name=Sam id=3], [Employee: name=Brandon id=4]]

82. Answer
Employee.java
public class Employee implements Comparable<Employee> {
private String name;
private int id;
public Employee(String name,int id){
this.name = name;
this.id = id; }
public int compareTo(Employee other) {
if (id < other.id) { return -1; }
else if (id == other.id) { return 0; }
else { return 1; }
public String toString(){
return "[Employee: name="+name+" id="+id+"]";

83. Programming Question
Modify compareTo to compare based on employee name
Output:
[[Employee: name=Andy id=2], [Employee: name=Brandon id=4], [Employee: name=Jim id=1], [Employee: name=Sam id=3]]

84. Answer public class Employee implements Comparable<Employee> {
private String name;
private int id;
public Employee(String name,int id){
this.name = name;
this.id = id; }
public int compareTo(Employee other) {
return name.compareTo(other.name);
public String toString(){
return "[Employee: name="+name+" id="+id+"]";

85. 3. Dictionaries/ Maps
Provides a means for storing each object reference along with a unique lookup key that can later be used to quickly retrieve the object
The key is often selected based on one or more of the object’s attribute values.
E.g. a Student object’s student ID number would make an excellent key, because its value is inherently unique for each Student.

86. Example map (set of <key, value> pairs )

87. Browse project specification for maps

88. Maps are realized in java using Map interface
Set of <key,Value> pairs.
Refer javadoc api
Key provide easy/faster lookup of objects based on key
E.g. you can lookup a student object based on student id
Key must be unique to value
Map has no duplicate keys

90. Some predefined Java classes that implement the notion of a dictionary are:
HashMap
TreeMap
The TreeMap is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time
guaranteed log(n) time cost for the containsKey, get, put and remove operations

91. Maps
A map allows you to associate elements from a key set with elements from a value collection.
Use a map when you want to look up objects by using a key.
No duplicate keys allowed
Figure 10 A Map

92. Maps Store the reference to the map object in a Map reference:
Map<String, Color> favoriteColors = new HashMap<String, Color>();
Map<String, Color> favoriteColors = new TreeMap<String, Color>();
Key type
Key represent Person Name
Value type
Value represent favorite color of person

94. Maps
Use the put method to add an association/ a <key,value> pair :
favoriteColors.put("Juliet", Color.RED);
You can change the value of an existing association by calling put again:
favoriteColors.put("Juliet", Color.BLUE);
The get method returns the value associated with a key:
Color favorite = favorite.get("Juliet");
If you ask for a key that isn't associated with any values, the get method returns null.
To remove a <key,value> pair, call the remove method with the key:
favoriteColors.remove("Juliet");

95. Working with Maps

96. Maps The keySet method yields the set of keys.
To iterate through <key,value> pairs in a map m:
Set<String> keySet = m.keySet(); //get set of keys
for (String key : keySet)//for each key {
Color value = m.get(key); //get value associated with key
System.out.println(key + "->" + value); //print key,value pair }

97. Programming Question
Implement the tester class MapDemo. In the main method, create a map called favoriteColors with person name as the key and favorite color of the person as value. Then add <key,value> pairs based on following diagram:
Finally print all <key,value> pairs in the map.
Program Run:
Juliet : java.awt.Color[r=0,g=0,b=255]
Adam : java.awt.Color[r=255,g=0,b=0]
Eve : java.awt.Color[r=0,g=0,b=255]
Romeo : java.awt.Color[r=0,g=255,b=0]

98. Answer
MapDemo.java
import java.awt.Color; import java.util.HashMap; import java.util.TreehMap; import java.util.Map; import java.util.Set; public class MapDemo { public static void main(String[] args) Map<String, Color> favoriteColors = new HashMap<String, Color>(); favoriteColors.put("Juliet", Color.BLUE); favoriteColors.put("Romeo", Color.GREEN); favoriteColors.put("Adam", Color.RED); favoriteColors.put("Eve", Color.BLUE); // Print all keys and values in the map Set<String> keySet = favoriteColors.keySet(); for (String key : keySet) Color value = favoriteColors.get(key); System.out.println(key + " : " + value); }
How does the output change if you change HashMap to TreeMap?

99. Question
Why is the collection of the keys of a map a set and not a list?

100. Answer
The ordering does not matter, and you cannot have duplicates

101. Question
Why is the collection of the values of a map not a set?

102. Answer
Because it might have duplicates.

103. Question
Suppose you want to track how many times each word occurs in a document. Declare a suitable map variable.

104. Answer
Answer:
Map<String, Integer> wordFrequency;

105. Question
Suppose you want to maintain a thesaurus that lists synonyms for a given word as a collection.
For example, the key "improve" might have as its synonyms: ["ameliorate", "better", "enhance", "enrich", "perfect", "refine"].
How do we define a suitable collection for this?

106. Answer
Map<String, HashSet<String>>
word
synonyms

107. Programming Question
Write a program WordCount to read a text file (poem.txt) and print count of each word in the text. Use a map to keep track of count of each word.
Sample output:
poem.txt
We were stepping out of a reading
in October, the first cold night,
and we were following this couple,
were they at the reading? and because
we were lost, I called out to them,
“Are you going to the after party?”
The woman laughed and said no
and the man kept walking, and she
was holding his hand like I hold yours,
though not exactly, she did not
need him for balance. Then what
got into me? I said, “How long
have you been married?” and she said
“Almost 30 years” and because
we were walking in public, no secret,
tell everyone now it’s official,
I said, “How’s marriage?” The man
kept walking. The woman said,
“It gets better but then it gets different.”
The man kept walking.
Find program template in next slide
Create and save poem.txt
with above content

108. import java.util.*;
import java.io.*;
public class WordCount {
public static void main(String args[]) throws IOException {
//TODO: create map : wordFrequency
//create a scanner object to read file
Scanner sc = new Scanner(new File("poem.txt"));
sc.useDelimiter("[^a-zA-Z]+"); // Use any characters other than a-z or A-Z as delimiters
//TODO: update count of words in map
//TODO: print map (word along with its frequency) }

109. Answer WordCount.java import java.util.*; import java.io.*;
public class WordCount {
public static void main(String args[]) throws IOException {
//create map: key=word, value=count of word
Map<String, Integer> wordFrequency = new HashMap<String, Integer>();
//create a scanner object to read file
Scanner sc = new Scanner(new File("poem.txt"));
sc.useDelimiter("[^a-zA-Z]+");
//update count of words in map
while (sc.hasNext()) {
String word = sc.next();
if (wordFrequency.get(word) != null) {
int count = wordFrequency.get(word);
count++;
wordFrequency.put(word, count); }
else {
wordFrequency.put(word, 1);
//print map (word along with its frequency)
//System.out.println("wordFrequency: " + wordFrequency);
Set<String> keySet = wordFrequency.keySet();
for (String key : keySet) {
Integer value = wordFrequency.get(key);
System.out.println(key + " : " + value);

110. Stacks A stack lets you insert and remove elements only at one end:
Called the top of the stack.
Removes items in the opposite order than they were added
Last-in, first-out or LIFO order
Add and remove methods are called push and pop.

111. Stacks Example Stack<String> s = new Stack<String>();
s.push("A");
s.push("B");
s.push("C");
System.out.print(s.pop()); // Prints C

112. Stack Animator: Array implementation: List Implementation:
List Implementation:

113. Stacks Many applications for stacks in computer science.
Consider: Undo function of a word processor
The issued commands are kept in a stack.
When you select “Undo”, the last command is popped off the stack and undone
Run-time stack that a processor or virtual machine:
Stores the values of variables in nested methods.
When a new method is called, its parameter variables and local variables are pushed onto a stack.
When the method exits, they are popped off again.

114. Stack in the Java Library
Stack class provides push, pop and peek methods.
Refer javadoc api

115. Programming Question
Implement a tester class StackDemo. The main method should do following:
create a stack to hold integers.
Add values 1,2,3 to the stack.
Print stack content
Remove top element
Print stack after removal
A sample program run is shown:

116. Answer StackDemo.java import java.util.*; public class StackDemo {
public static void main(String args[]) {
// creating stack
Stack<Integer> st = new Stack<Integer>();
// populating stack
st.push(Integer.valueOf(1));
st.push(Integer.valueOf(2));
st.push(Integer.valueOf(3));
//elements before remove
System.out.println("Elelments before removal: "+st);
// removing top object
System.out.println("Removed object is: "+st.pop());
// elements after remove
System.out.println("Elements after remove: "+st); }

117. Programming Question
Printing a Sentence in Reverse Using a Stack: Write a program StackDemo2.java that takes a line of text and uses a stack to display the words of the line in reverse order. For example, given the string= “My name is Tom” output is “Tom is name My”:
Find program template in next slide

118. public class StackDemo2{
public static void main( String[] args )
//TODO: create stack
// get input sentence
Scanner scanner = new Scanner( "My name is Tom" );
//TODO: take each word from input and push on stack
//TODO: print reverse string by popping words from stack. }

119. Answer StackDemo2.java import java.util.Scanner;
import java.util.Stack;
public class StackDemo2 {
public static void main( String[] args )
//create stack
Stack< String > stack = new Stack< String >();
// get input text
Scanner scanner = new Scanner( "My name is Tom" );
// take each word from input and push on stack
while ( scanner.hasNext())
stack.push( scanner.next() );
//System.out.println("st="+stack); }
System.out.println( "Input string in reverse order:" );
// build reverse string by popping words from stack.
while ( !stack.isEmpty() )
Object removedObject = stack.pop();
System.out.printf( "%s ", removedObject );
} // end while
System.out.println(); // print trailing newline

120. References
From : Beginning Java Objects, JACQUIE BARKER