oop lecture framework,list,maps,collection

Outline
 An Overview of the Java Collections Framework
 Linked Lists
 Sets
 Maps
 Stacks Queues and Priority Queues
 Stack and Queue Applications

OBJECTIVES
 To learn how to use the collection classes supplied in the
Java library
 To use iterators to traverse collections
 To choose appropriate collections for solving programming
problems
 To study applications of stacks and queues
Page 3

Java Collections Framework
 The Java Collections Framework is a collection of
interfaces and classes which helps in storing and
processing the data efficiently. This framework has several
useful classes which have tons of useful functions which
makes a programmer task super easy

 When you need to organize multiple objects in your
program, you can place them into a collection
 The ArrayList class as an example. It is one of many
collection classes that the standard Java library supplies
 Each interface type is implemented by one or more classes

A collections framework is a unified architecture for representing
and manipulating collections.
 Interfaces − These are abstract data types that represent
collections. Interfaces allow collections to be manipulated
independently of the details of their representation.
 Implementations − These are the concrete implementations of
the collection interfaces.
 Algorithms − Methods that perform useful computations,
(searching and sorting), on objects that implement collection
interfaces. Algorithms said to be polymorphic: the same
method can be used on many different implementations of the
appropriate collection interface.

Collections Framework hierarchy
Each collection
class
implements an
interface from a
hierarchy
Each class is
designed for a
specific type of
storage

Java Collection Framework supports two
types of containers:
- collections: for storing a collection of
elements
- map: for storing key/value pairs

The collection interfaces
 The Collection Interface
This enables you to work with groups of objects; it is at the
top of the collections hierarchy.
 The List Interface
This extends Collection and an instance of List stores an
ordered collection of elements.
 The Set
This extends Collection to handle sets, which must contain
unique elements.
 The SortedSet
This extends Set to handle sorted sets.

 The Map
This maps unique keys to values.
 The Map.Entry
This describes an element (a key/value pair) in a map. This
is an inner class of Map.
 The SortedMap
This extends Map so that the keys are maintained in an
ascending order.
 Iterator
Iterator enables you to cycle through a collection, obtaining
or removing elements.

The Collection interface is the foundation upon which the
collections framework is built. It declares the core methods that
all collections will have.
Methods and Description:
 boolean add(Object obj)
Adds obj to the invoking collection. Returns true if obj was
added to the collection. Returns false if obj is already a
member of the collection, or if the collection does not allow
duplicates.
 boolean addAll(Collection c)
Adds all the elements of c to the invoking collection. Returns
true if the operation succeeds (i.e., the elements were added).
Otherwise, returns false.

 void clear( )
Removes all elements from the invoking collection.
 boolean contains(Object obj)
Returns true if obj is an element of the invoking collection. Otherwise, returns false.
 boolean containsAll(Collection c)
Returns true if the invoking collection contains all elements of c. Otherwise, returns false.
 boolean equals(Object obj)
Returns true if the invoking collection and obj are equal. Otherwise, returns false.
 int hashCode( )
Returns the hash code for the invoking collection.
 boolean isEmpty( )
Returns true if the invoking collection is empty. Otherwise, returns false.
 Iterator iterator( )
Returns an iterator for the invoking collection.

 boolean remove(Object obj)
Removes one instance of obj from the invoking collection. Returns true if the element
was removed. Otherwise, returns false.
 boolean removeAll(Collection c)
Removes all elements of c from the invoking collection. Returns true if the collection
changed.
 boolean retainAll(Collection c)
Removes all elements from the invoking collection except those in c. Returns true if the
collection changed.
 int size( )
Returns the number of elements held in the invoking collection.
 Object[ ] toArray( )
 Returns an array that contains all the elements stored in the invoking collection. The
array elements are copies of the collection elements.
 Object[ ] toArray(Object array[ ])
 Returns an array containing only those collection elements whose type matches that of
array

The List Interface
The List interface extends Collection and declares
the behavior of a collection that stores a sequence
of elements.
 Elements can be inserted or accessed by their position in
the list, using a zero-based index.
 A list may contain duplicate elements.

Lists and Sets
 Ordered Lists
 ArrayList
 Stores a list of items in a dynamically sized array
 LinkedList
 Allows speedy insertion and removal of items from the list
Page 16
A list is a collection that maintains
the order of its elements.

Lists and Sets
 Unordered Sets
 HashSet
 Uses hash tables to speed up finding, adding, and removing elements
 TreeSet
 Uses a binary tree to speed up finding, adding, and removing elements
Page 17
A set is an unordered collection
of unique elements.

Stacks and Queues
Another way of gaining efficiency in a collection
is to reduce the number of operations available
Two examples are:
Stack
Remembers the order of its elements, but it does not
allow you to insert elements in every position
You can only add and remove elements at the top
Queue
Add items to one end (the tail)
Remove them from the other end (the head)
Example: A line of people waiting for a bank teller
Page 18

Maps
A map stores keys, values, and the associations
between them
Example:
 Barcode keys and books
Keys
Provides an easy way to represent an object (such as a
numeric bar code)
Values
The actual object that is associated with the key
Page 19
A map keeps associations
between key and value objects.

The Collection Interface (1)
List, Queue and Set are specialized interfaces
that inherit from the Collection interface
All share the following commonly used methods
Page 20

The Collection Interface (2)
Page 21

15.2 Linked Lists
Linked lists use references to maintain an ordered
lists of ‘nodes’
The ‘head’ of the list references the first node
Each node has a value and a reference to the next
node
They can be used to implement
 A List Interface
 A Queue Interface
Page 22

Linked Lists Operations
Efficient Operations
Insertion of a node
Find the elements it goes between
Remap the references
Removal of a node
Find the element to remove
Remap neighbor’s references
Visiting all elements in order
 Inefficient Operations
 Random access
Page 23
Each instance variable is declared just
like other variables we have used.

LinkedList: Important Methods
Page 24

Generic Linked Lists
The Collection Framework uses Generics
Each list is declared with a type field in < > angle brackets
Page 25
LinkedList<String>
LinkedList<Employee>
LinkedList<String> employeeNames = . . .;

List Iterators
 When traversing a LinkedList, use a ListIterator
 Keeps track of where you are in the list.
 Use an iterator to:
 Access elements inside a linked list
 Visit other than the first and the last nodes
Page 26
LinkedList<String> employeeNames = . . .;
ListIterator<String> iter = employeeNames.listIterator()

Using Iterators
Think of an iterator as pointing between two
elements
Page 27
 Note that the generic type for the listIterator
must match the generic type of the LinkedList
iterator.next();
iterator.add(“J”);
ListIterator<String> iter = myList.listIterator()

Iterator and ListIterator Methods
Iterators allow you to move through a list easily
Similar to an index variable for an array
Page 28

Iterators and Loops
Iterators are often used in while and “for-each” loops
hasNext returns true if there is a next element
next returns a reference to the value of the next element
Where is the iterator in the “for-next” loop?
It is used ‘behind the scenes’
Page 29
while (iterator.hasNext())
{
String name = iterator.next();
// Do something with name
} for (String name : employeeNames)
{
}

while (iterator.hasNext())
{
String name = iterator.next();
if (condition is true for name)
{
iterator.remove();
}
}
Adding and Removing with Iterators
Adding
A new node is added AFTER the Iterator
The Iterator is moved past the new node
Removing
Removes the object that was returned with the last call to
next or previous
It can be called only once after next or previous
You cannot call it immediately after a call to add.
Page 30
iterator.add("Juliet");
If you call the remove
method improperly, it throws
an IllegalStateException.

ListDemo.java (1)
Illustrates adding, removing and printing a list
Page 31

The Set Interface
 A Set is a Collection that cannot contain duplicate elements.
It models the mathematical set abstraction.
 The Set interface contains only methods inherited from
Collection and adds the restriction that duplicate elements
are prohibited.
 Set has its implementation in various classes like HashSet,
TreeSet, LinkedHashSet.

Sets
 A set is an unordered collection
 It does not support duplicate elements
 The collection does not keep track of the order in which
elements have been added
 Therefore, it can carry out its operations more efficiently than an
ordered collection
Page 35
The HashSet and TreeSet classes both
implement the Set interface.

Sets
HashSet: Stores data in a Hash Table
TreeSet: Stores data in a Binary Tree
Both implementations arrange the set
elements so that finding, adding, and
removing elements is efficient
Page 36
Set implementations arrange the elements
so that they can locate them quickly

The SortedSet Interface
The SortedSet interface extends Set and declares
the behavior of a set sorted in an ascending order.
SortedSet have its implementation in various
classes like TreeSet.

Hash Table Concept
Set elements are grouped into smaller collections
of elements that share the same characteristic
It is usually based on the result of a mathematical
calculation on the contents that results in an integer
value
In order to be stored in a hash table, elements must
have a method to compute their integer values
Page 39
100
101
102

hashCode
 The method is called hashCode
 If multiple elements have the same hash code, they are stored in a
Linked list
 The elements must also have an equals method for checking
whether an element equals another like:
 String, Integer, Point, Rectangle, Color, and all collection classes
Page 40
Set<String> names = new HashSet<String>();

Tree Concept
 Set elements are kept in sorted order
 Nodes are not arranged in a linear sequence
but in a tree shape
 In order to use a TreeSet, it must be possible to compare the elements and determine
which one is “larger”
Page 41

TreeSet
Use TreeSet for classes that implement
the Comparable interface
String and Integer, for example
The nodes are arranged in a ‘tree’
fashion so that each ‘parent’ node has
up to two child nodes.
The node to the left always has a ‘smaller’
value
The node to the right always has a ‘larger’
value
Page 42
Set<String> names = new TreeSet<String>();

Iterators and Sets
Iterators are also used when processing sets
hasNext returns true if there is a next element
next returns a reference to the value of the next element
add via the iterator is not supported for TreeSet and HashSet
Note that the elements are not visited in the order in which you
inserted them.
They are visited in the order in which the set keeps them:
Seemingly random order for a HashSet
Sorted order for a TreeSet
Page 43
Iterator<String> iter = names.iterator();
while (iter.hasNext())
{
String name = iter.next();
}
for (String name : names)
{
}

Programming Tip
Use Interface References to Manipulate Data
Structures
It is considered good style to store a reference to a
HashSet or TreeSet in a variable of type Set.
This way, you have to change only one line if you decide to use
a TreeSet instead.
Page 48
Set<String> words = new HashSet<String>();

Programming Tip (continued)
Unfortunately the same is not true of the
ArrayList, LinkedList and List
classes
 The get and set methods for random access are very inefficient
Also, if a method can operate on arbitrary
collections, use the Collection interface
type for the parameter:
Page 49
public static void removeLongWords(Collection<String> words)

The Map Interface
 The Map interface maps unique keys to values. A key is an
object that you use to retrieve a value at a later date.
 Given a key and a value, you can store the value in a Map
object. After the value is stored, you can retrieve it by using
its key.
 Map has its implementation in various classes like
HashMap.

Maps
A map allows you to associate elements from a key
set with elements from a value collection.
 The HashMap and TreeMap classes both implement the Map interface.
 Use a map to look up objects by using a key.
Page 52

Maps
Page 53
Map<String, Color> favoriteColors = new HashMap<String, Color>();
Key Value
Key Value

Working with Maps (Table 5)
Page 54

Key Value Pairs in Maps
 Each key is associated with a value
Page 55
Map<String, Color> favoriteColors = new HashMap<String, Color>();
favoriteColors.put("Juliet", Color.RED);
favoriteColors.put(“Romeo", Color.GREEN);
Color julietsFavoriteColor = favoriteColors.get("Juliet");
favoriteColors.remove("Juliet");

Iterating through Maps
To iterate through the map, use a keySet to get the list
of keys:
Page 56
Set<String> keySet = m.keySet();
for (String key : keySet)
{
Color value = m.get(key);
System.out.println(key + "->" + value);
}
To find all values in a map,
iterate through the key set
and find the values that
correspond to the keys.

The Map.Entry Interface
 The Map.Entry interface enables you to work with a map
entry.
 The entrySet( ) method declared by the Map interface
returns a Set containing the map entries. Each of these set
elements is a Map.Entry object.

The SortedMap Interface
The SortedMap interface extends Map. It ensures
that the entries are maintained in an ascending key
order.
SortedMap has its implementation in various
classes like TreeMap.

Steps to Choosing a Collection
1) Determine how you access values
Values are accessed by an integer position. Use an
ArrayList
Go to Step 2, then stop
Values are accessed by a key that is not a part of the
object
Use a Map.
It doesn’t matter. Values are always accessed “in bulk”,
by traversing the collection and doing something with
each value
2) Determine the element types or key/value types
For a List or Set, a single type
For a Map, the key type and the value type
Page 62

3) Determine whether element or key order
matters
 Elements or keys must be sorted
 Use a TreeSet or TreeMap. Go to Step 6
 Elements must be in the same order in which they
were inserted
 Your choice is now narrowed down to a LinkedList or
an ArrayList
 It doesn’t matter
 If you chose a map in Step 1, use a HashMap and go to
Step 5
Page 63

4) For a collection, determine which operations must be fast
 Finding elements must be fast
 Use a HashSet and go to Step 5
 Adding and removing elements at the beginning or the middle must
be fast
 Use a LinkedList
 You only insert at the end, or you collect so few elements that you
aren’t concerned about speed
 Use an ArrayList.
Page 64

5) For hash sets and maps, decide if you
need to implement the equals and
hashCode methods
If your elements do not support them, you
must implement them yourself.
6) If you use a tree, decide whether to
supply a comparator
If your element class does not provide it,
implement the Comparable interface for your
element class
Page 65

Special Topic: Hash Functions
 Hashing can be used to find elements in a set data structure quickly,
without making a linear search through all elements.
 A hashCode method computes and returns an integer value: the hash
code.
 Should be likely to yield different hash codes
 Because hashing is so important, the Object class has a hashCode method
that computes the hash code of any object x.
Page 66
int h = x.hashCode();

Computing Hash Codes
 To put objects of a given class into a HashSet or use
the objects as keys in a HashMap, the class should
override the default hashCode method.
 A good hashCode method should work such that
different objects are likely to have different hash codes.
 It should also be efficient
 A simple example for a String might be:
Page 67
int h = 0;
for (int i = 0; i < s.length(); i++)
{
h = h + s.charAt(i);
}

Computing Hash Codes
But Strings that are permutations of
another (such as "eat" and "tea") would
all have the same hash code
Better:
 From the Java Library!
Page 68
final int HASH_MULTIPLIER = 31;
int h = 0;
for (int i = 0; i < s.length(); i++)
{
h = HASH_MULTIPLIER * h + s.charAt(i);
}

Sample Strings and HashCodes
 The String class implements a good example of a
hashCode method
 It is possible for two or more distinct objects to have the
same hash code: This is called a collision
A hashCode function should minimizes collisions
Page 69

Computing Object Hash Codes
 You should have a good hashCode method for your
own objects to store them efficiently
 Override hashCode methods in your own classes by
combining the hash codes for the instance variables
 Then combine the hash codes using a prime-number
hash multiplier:
Page 70
public int hashCode()
{
int h1 = name.hashCode();
int h2 = new Double(area).hashCode();
. . .
final int HASH_MULTIPLIER = 29;
int h = HASH_MULTIPLIER * h1 + h2;
return h;
}

hashCode and equals methods
hashCode methods should be compatible with
equals methods
If two objects are equal, their hashCodes should match
a hashCode method should use all instance variables
The hashCode method of the Object class uses the
memory location of the object, not the contents
Page 71

hashCode and equals
methods
Do not mix Object class hashCode or equals
methods with your own:
Use an existing class such as String. Its
hashCode and equals methods have already
been implemented to work correctly.
Implement both hashCode and equals.
 Derive the hash code from the instance variables that
the equals method compares, so that equal objects
have the same hash code
Implement neither hashCode nor equals. Then
only identical objects are considered to be equal
Page 72

15.5 Stacks, Queues and Priority Queues
Queues and Stacks are specialized lists
Only allow adding and removing from the ends
Page 73
Insert At Remove At Operation
Stack Start(top) Start(top) List in, first out (LIFO)
Queue End (tail) Start (head) First in, first out (FIFO)
Priority Queue By Priority Highest
Priority
(Lowest #)
Prioritized list of tasks

Stacks, Queues and Priority
Queues
Stacks are used for undo features
(most recent first)
Queues are like lines at the bank
or store
Priority Queues remove lowest
number first
Page 74

Stack Example
The Java library provides a Stack class that
implements the abstract stack type’s push and
pop operations.
The Stack is not technically part of the Collections
framework, but uses generic type parameters
Page 76
Stack<String> s = new Stack<String>();
s.push("A");
s.push("B");
s.push("C");
// The following loop prints C, B, and A
while (s.size() > 0)
{
System.out.println(s.pop());
}
The stack class
provides a size method

Queues and Priority Queues
Page 77

Priority Queues
A priority Queue collects elements, each of which
has a priority
Example: a collection of work requests, some of which
may be more urgent than others
It is NOT a FIFO Queue
Lowest value priority (1) will be removed first
Page 78
PriorityQueue<WorkOrder> q = new PriorityQueue<WorkOrder>();
q.add(new WorkOrder(3, "Shampoo carpets"));
q.add(new WorkOrder(1, "Fix broken sink"));
q.add(new WorkOrder(2, "Order cleaning supplies"));
WorkOrder next = q.remove(); // removes “Fix broken sink”

15.6 Stack and Queue Applications
 Balancing Parenthesis
Section 2.5, showed how to balance parenthesis by
adding 1 for each left ( and subtracting for each right )
A stack can be used to keep track of ‘depth’:
When you see an opening parenthesis, push it on the stack.
When you see a closing parenthesis, pop the stack.
If the opening and closing parentheses don’t match
The parentheses are unbalanced. Exit.
If at the end the stack is empty
The parentheses are balanced.
Else
The parentheses are not balanced.
Page 79

Using a Stack (Example)
 Here is a walkthrough of the sample expression
 We will use the mathematical version (three types of parenthesis)
Page 80

Reverse Polish Expressions
 The first handheld calculator used a notation that was easily implemented with a
stack: Reverse Polish
 No parenthesis required if you…
Input both operands first, then the operator:
Page 81
Algebra Reverse Polish
(3 + 4) x 5 3 4 + 5 x
(3 + 4) x (5 + 6) 3 4 + 5 6 + x

Reverse Polish Expressions
If you read a number
Push it on the stack.
Else if you read an operand
Pop two values off the stack.
Combine the values with the operand.
Push the result back onto the stack.
Else if there is no more input
Pop and display the result.
Page 82

Reverse Polish Calculator
 Walkthrough with 3 4 5 + x
Page 83

Evaluating Algebraic Expressions
Can be done with two stacks:
1) Numbers
2) Operators
First Example: 3 + 4
Page 86

Expression Example 2
Second Example: 3 x 4 + 5
Must use precedence (multiply before adding)
Page 87

Precedence and Expressions (1)
Third Example: 3 + 4 x 5
Keep operators on the stack until they are ready to be
evaluated
Page 88

Precedence and Expressions (2)
Third Example: 3 + 4 x 5
Evaluate top 2 numbers with top operator
Store result on top of stack
Evaluate until operator stack is empty
Page 89

Expressions with Parenthesis (1)
Fourth Example: 3 x ( 4 + 5 )
If (, don’t evaluate. If ), evaluate
Page 90

Expressions with Parenthesis (2)
Fourth Example: 3 x ( 4 + 5 )
Don’t put ) on stack – evaluate now
Ignore (
Page 91

Precedence Algorithm
If you read a number
Push it on the number stack.
Else if you read a (
Push it on the operator stack.
Else if you read an operator op
While the top of the stack has a higher precedence than op
Evaluate the top.
Push op on the operator stack.
Else if you read a )
While the top of the stack is not a (
Evaluate the top.
Pop the ).
Else if there is no more input
While the operator stack is not empty
Evaluate the top.
Page 92
Evaluate the Top:
Pop two numbers off the
number stack.
Pop an operator off the
operator stack.
Combine the numbers with
that operator.
Push the result on the
number stack.

Backtracking (1)
Uses a stack to solve a maze
Stack current location, arrow
for each possible path
1. We pop off the topmost one,
traveling north from (3, 4).
Following this path leads to
position (1, 4).
2. We now push two choices
on the stack, going west or
east.
Page 93

Backtracking (2)
3. Pop off (1, 4) east. It is a
dead end.
4. Pop off (1, 4) west. It is a
dead end.
5. Now we pop off the path
from (3, 4) going east. That
too is a dead end.
This leaves us with (3, 4) south
and (3, 4) west to try
Page 94

Backtracking (3)
6. Next is the path from (3, 4)
going south. At (5, 4), it comes
to an intersection. Both choices
are pushed on the stack.
7. (5, 4) south is a dead end.
8. (5, 4) west is a dead end.
9. Finally, the path from (3, 4)
going west leads to an exit
Page 95

Maze Solving Pseudocode
Push all paths from the point on which you are
standing on a stack.
While the stack is not empty
Pop a path from the stack.
Follow the path until you reach an exit,
intersection, or dead end.
If you found an exit
Congratulations!
Else if you found an intersection
Push all paths meeting at the intersection, except the current one,
onto the stack.
Page 96

Summary: Collections
A collection groups together elements
and allows them to be retrieved later
A list is a collection that remembers the
order of its elements
A set is an unordered collection of unique
elements
A map keeps associations between key
and value objects
Page 97

Summary: Linked Lists
A linked list consists of a number of nodes, each of
which has a reference to the next node
Adding and removing elements in the middle of a linked
list is efficient
Visiting the elements of a linked list in sequential order
is efficient, but random access is not
You use a list iterator to access elements of a linked list
Page 98

Summary: Choosing a Set
 The HashSet and TreeSet classes both implement the Set
interface.
 Set implementations arrange the elements so that they can
locate them quickly.
 You can form hash sets holding objects of type String,
Integer, Double, Point, Rectangle, or Color.
 You can form tree sets for any class that implements the
Comparable interface, such as String or Integer.
 Sets don’t have duplicates. Adding a duplicate of an element
that is already present is silently ignored.
 A set iterator visits the elements in the order in which the set
implementation keeps them.
 You cannot add an element to a set at an iterator position.
Page 99

Summary: Maps
 Maps associate keys with values
 The HashMap and TreeMap classes both implement the Map interface
 To find all keys and values in a Map, iterate through the key set and find
the values that correspond to the keys
 A hash function computes an integer value from an object.
 A good hash function minimizes collisions—identical hash codes for
different objects.
 Override hashCode methods in your own classes by combining the hash
codes for the instance variables.
 A class’s hashCode method must be compatible with its equals method.
Page 100

Summary: Stacks and Queues
A stack is a collection of elements with
“last-in, first-out” retrieval.
A queue is a collection of elements with
“first-in, first-out” retrieval.
When removing an element from a priority
queue, the element with the most urgent
priority is retrieved.
Page 101

Summary: Problem Solving
A stack can be used to check whether parentheses in
an expression are balanced.
Use a stack to evaluate expressions in reverse Polish
notation.
Using two stacks, you can evaluate expressions in
standard algebraic notation.
Use a stack to remember choices you haven’t yet
made so that you can backtrack to them.
Page 102

Useful links
 https://beginnersbook.com/java-collections-tutorials/

oop lecture framework,list,maps,collection

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