Collections - Lists & sets

Java/J2EE Programming Training
Collections – Lists, Sets

Classification: Restricted
Agenda
• List – ArrayList, LinkedList
• Set – HashSet, LinkedHashSet, TreeSet

Collections Framework Diagram
Classification: Restricted Page 4

Collection Interface
• Defines fundamental methods
•int size();
•boolean isEmpty();
•boolean contains(Object element);
•boolean add(Object element); // Optional
•boolean remove(Object element); // Optional
•Iterator iterator();
• These methods are enough to define the basic behavior of a collection
• Provides an Iterator to step through the elements in the Collection

Iterator Interface
• Defines three fundamental methods
•Object next()
•boolean hasNext()
•void remove()
• These three methods provide access to the contents of the
collection
• An Iterator knows position within collection
• Each call to next() “reads” an element from the collection
• Then you can use it or removeit

Example - SimpleCollection
public class SimpleCollection {
public static void main(String[] args) {
Collection c;
c = new ArrayList();
System.out.println(c.getClass().getName()); for (int i=1; i
<= 10; i++) {
c.add(i + " * " + i + " ="+i*i);
}
Iterator iter = c.iterator(); while
(iter.hasNext())
System.out.println(iter.next());
}
}

List Interface Context
Collection
List

ListIterator Interface
• Extends the Iterator interface
• Defines three fundamental methods
• void add(Object o) - before current position
• boolean hasPrevious()
• Object previous()
• The addition of these three methods defines the basic behavior of
an ordered list
• A ListIterator knows position within list

ArrayList and LinkedList Context
ArrayList LinkedList
Collection
List

List as part of Collection

List Implementations
• ArrayList
• 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
• Vector
• Similar to ArrayList, but thread-safe

ArrayList overview
• Constant time positional access (it’s an array)
• One tuning parameter, the initial capacity
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new
IllegalArgumentException(
"Illegal Capacity:
"+initialCapacity);
this.elementData = new Object[initialCapacity];
}

ArrayList methods
• The indexed get and set methods of the List interface are
appropriate to use since ArrayLists are backed by an array
•Object get(int index)
•Object set(int index, Object element)
• Indexed add and remove are provided, but can be costly if used
frequently
•void add(int index, Object element)
•Object remove(int index)
• May want to resize in one shot if adding manyelements
•void ensureCapacity(int minCapacity)

LinkedList overview
• Stores each element in a node
• Each node stores a link to the next and previousnodes
• Insertion and removal are inexpensive
• just update the links in the surrounding nodes
• Linear traversal is inexpensive
• Random access is expensive
• Start from beginning or end and traverse each node while
counting

LinkedList entries
private static class Entry { Object
element;
Entry next; Entry
previous;
Entry(Object element, Entry next, Entry previous) {
this.element = element;
this.next = next;
this.previous = previous;
}
}
private Entry header = new Entry(null, null, null); public
LinkedList() {
header.next = header.previous = header;
}

LinkedList methods
• The list is sequential, so access it that way
•ListIterator listIterator()
• ListIterator knows about position
• use add() from ListIterator to add at a position
• use remove() from ListIterator to remove at a position
• LinkedList knows a few things too
•void addFirst(Object o), void addLast(Object o)
•Object getFirst(), Object getLast()
•Object removeFirst(), Object removeLast()

ArrayList vs. LinkedList vs. Vector
From the hierarchy diagram, they all implement List interface. They are very
similar to use. Their main difference is their implementation which causes
different performance for different operations. ArrayList is implemented as a
resizable array. As more elements are added to ArrayList, its size is increased
dynamically. It's elements can be accessed directly by using the get and set
methods, since ArrayList is essentially an array. LinkedList is implemented as
a double linked list. Its performance on add and remove is better than
Arraylist, but worse on get and set methods. Vector is similar with ArrayList,
but it is synchronized. ArrayList is a better choice if your program is thread-
safe. Vector and ArrayList require space as more elements are added. Vector
each time doubles its array size, while ArrayList grow 50% of its size each
time. LinkedList, however, also implements Queue interface which adds more
methods than ArrayList and Vector, such as offer(), peek(), poll(),
etc. Note: The default initial capacity of an ArrayList is pretty small. It is a
good habit to construct the ArrayList with a higher initial capacity. This can
avoid the resizing cost.

Example: ArrayList
ArrayList al = new ArrayList();
al.add(3);
al.add(2);
al.add(1);
al.add(4);
al.add(5);
al.add(6);
al.add(6);
Iterator iter1 = al.iterator();
while(iter1.hasNext()){
System.out.println(iter1.next());
}

Example: ArrayList (Using Generics + Iterating
using Iterator and for-each loop)
import java.util.*; class
TestCollection1{
public static void main(String args[]){
ArrayList<String> list=new ArrayList<String>();//Creating arraylist
list.add("Ravi");//Adding object in arraylist
list.add("Vijay");
list.add("Ravi");
list.add("Ajay");
//Traversing list through Iterator
Iterator itr=list.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
//Traversing using for-each loop
for(String obj:list)
System.out.println(obj);
}
}

User-defined class objects in Java ArrayList
class Student{
int rollno;
String
name; int
age;
Student(int rollno,String name,int
age){ this.rollno=rollno;
this.name=name
; this.age=age;
}
}
import java.util.*;
public class TestCollection3{
//Creating user-defined class objects Student s1=new
Student(101,"Sonoo",23); Student s2=new
Student(102,"Ravi",21);
Student s2=new Student(103,"Hanumat",25);
//creating arraylist
ArrayList<Student> al=new ArrayList<Student>();
al.add(s1);//adding Student class object al.add(s2);
al.add(s3);
//Getting Iterator Iterator
itr=al.iterator();
//traversing elements of ArrayList object
Student st=(Student)itr.next();
System.out.println(st.rollno+" "+st.name+"
"+st.age);
}
}

ArrayList Constructors

ArrayList Methods

Example of addAll(Collection c) method
import java.util.*;
class TestCollection4{
public static void main(String args[]){ ArrayList<String> al=new
ArrayList<String>(); al.add("Ravi");
al.add("Vijay");
al.add("Ajay");
ArrayList<String> al2=new ArrayList<String>(); al2.add("Sonoo");
al2.add("Hanumat");
al.addAll(al2);//adding second list in first list
Iterator itr=al.iterator();
while(itr.hasNext()){ System.out.println(itr.next());
}
}
}

Example of removeAll() method
TestCollection5{
public static void main(String args[]){ ArrayList<String> al=new
ArrayList<String>(); al.add("Ravi");
al.add("Vijay");
al.add("Ajay");
ArrayList<String> al2=new ArrayList<String>(); al2.add("Ravi");
al2.add("Hanumat");
al.removeAll(al2);
System.out.println("iterating the elements after removing the elements of al2...");
}
}
}

Example of retainAll()
TestCollection6{
ArrayList<String> al=new ArrayList<String>();
al.add("Ravi");
al.add("Vijay");
al.add("Ajay");
ArrayList<String> al2=new ArrayList<String>();
al2.add("Ravi");
al2.add("Hanumat");
al.retainAll(al2);
System.out.println("iterating the elements after retaining the elements of al2...");
}
}
}

Exercise…
• Write a class “Book” that takes as members id, name, author, publisher and
quantity.
• Write a program for creating an ArrayList of Books. Add 5 books to the list.
• Iterate through the list and print all the properties of the books.

Exercise… ListIterator
• What is a ListIterator?
• When we already have Iterator, why do we need ListIterator? What are the
advantages?

Exercise: ArrayList Hierarchy
The ArrayList Hierarchy is shown. Go
through the Java API on Oracle’s site and
draw the hierarchy for all collections
discussed in class today.

LinkedList
• Uses doubly linked list to store the elements. It provides a linked-list data
structure. It inherits the AbstractList class and implements List and Deque
interfaces
• Java LinkedList class can contain duplicate elements.
• Java LinkedList class maintains insertion order.
• Java LinkedList class is non synchronized.
• In Java LinkedList class, manipulation is fast because no shifting needs to
be occurred.
• Java LinkedList class can be used as list, stack or queue.

LinkedList constructors

LinkedList Methods

Example: LinkedList
LinkedList ll = new LinkedList();
ll.add(3);
ll.add(2);
ll.add(1);
ll.add(4);
ll.add(5);
ll.add(6);
ll.add(6);
Iterator iter2 = al.iterator();
while(iter2.hasNext()){
System.out.println(iter2.next());
}

Exercise: LinkedList
Implement all the program examples written for ArrayList with LinkedList.

Performance: ArrayList vs LinkedList
ArrayList arrayList = new ArrayList();
LinkedList linkedList = new LinkedList();
// ArrayList add
long startTime = System.nanoTime();
for (int i = 0; i < 100000; i++) {
arrayList.add(i);
}
long endTime = System.nanoTime();
long duration = endTime - startTime;
System.out.println("ArrayList add: " + duration);
// LinkedList add
startTime = System.nanoTime();
for (int i = 0; i < 100000; i++) {
linkedList.add(i);
}
endTime = System.nanoTime();
duration = endTime - startTime;
System.out.println("LinkedList add: " + duration);
// ArrayList get
for (int i = 0; i < 10000; i++) {
arrayList.get(i);
}
System.out.println("ArrayList get: " + duration);
// LinkedList get
for (int i = 0; i < 10000; i++) {
linkedList.get(i);
}
System.out.println("LinkedList get: " + duration);
// ArrayList remove
for (int i = 9999; i >=0; i--) {
arrayList.remove(i);
}
System.out.println("ArrayList remove: " + duration);
// LinkedList remove
for (int i = 9999; i >=0; i--) {
linkedList.remove(i);
}
System.out.println("LinkedList remove: " + duration);

ArrayList vs LinkedList

List Interface in Java
• public interface List<E> extends Collection<E>

Java ListIterator Interface
• public interface ListIterator<E> extends Iterator<E>
• Can be used to traverse through the list in forward and backwards/reverse
direction

Java ListIterator example
import java.util.*;
public class TestCollection8{
ArrayList<String> al=new ArrayList<String>();
al.add("Amit");
al.add("Vijay");
al.add("Kumar");
al.add(1,"Sachin");
System.out.println("element at 2nd position: "+al.get(2));
ListIterator<String> itr=al.listIterator();
System.out.println("traversing elements in forward direction...");
}
System.out.println("traversing elements in backward direction...");
while(itr.hasPrevious()){
System.out.println(itr.previous());
}
}
}

Set Interface Context
Collection
Set

Set Interface
• Same methods as Collection
• different contract - no duplicate entries
• Defines two fundamental methods
•boolean add(Object o) - rejectduplicates
•Iterator iterator()
• Provides an Iterator to step through the elements in the
Set
• No guaranteed order in the basic Set interface
• There is a SortedSet interface that extends Set

HashSet and TreeSet Context
HashSet TreeSet
Collection
Set

List vs Set
• List can contain duplicate elements whereas Set contains unique elements
only.

HashSet
• Find and add elements very quickly
• uses hashing implementation in HashMap
• Hashing uses an array of linked lists
• The hashCode() is used to index intothe array
• Then equals() is used to determine if element is in the(short) list of
elements at that index
• No order imposed on elements
• The hashCode()method and the equals()method must be compatible
• if two objects are equal, they must have the same hashCode()
value

HashSet constructors

HashSet methods

HashSet Example
import java.util.*;
//Creating HashSet and adding elements HashSet<String> set=new
HashSet<String>(); set.add("Ravi");
set.add("Vijay");
set.add("Ravi");
set.add("Ajay");
//Traversing elements Iterator<String> itr=set.iterator();
}
}
}

HashSet Example : Book
import java.util.*;
class Book {
int id;
String name,author,publisher;
int quantity;
public Book(int id, String name, String author,
String publisher, int quantity) {
this.id = id;
this.name = name;
this.author = author;
this.publisher = publisher;
this.quantity = quantity;
}
}
public class HashSetExample {
HashSet<Book> set=new HashSet<Book>();
//Creating Books
Book b1=new Book(101,"Let us
C","Yashwant Kanetkar","BPB",8);
Book b2=new Book(102,"Data
Communications & Networking","Forouzan","Mc
Graw Hill",4);
Book b3=new Book(103,"Operating
System","Galvin","Wiley",6);
//Adding Books to HashSet
set.add(b1);
set.add(b2);
set.add(b3);
//Traversing HashSet
for(Book b:set){
System.out.println(b.id+" "+b.name+"
"+b.author+" "+b.publisher+" "+b.quantity);
}
}
}

Exercise…
• What is LinkedHashSet?
• When would you use LinkedHashSet?

TreeSet
• Elements can be inserted in any order
• The TreeSet stores them in order
• An iterator always presents them in order
• Default order is defined by natural order
• objects implement the Comparable interface
• TreeSet uses compareTo(Object o) tosort
• Can use a different Comparator
• provide Comparator to the TreeSet constructor

TreeSet constructor

TreeSet Methods

TreeSet Example
import java.util.*;
//Creating and adding elements TreeSet<String> al=new TreeSet<String>();
al.add("Ravi");
al.add("Vijay");
al.add("Ravi");
al.add("Ajay");
//Traversing elements Iterator<String> itr=al.iterator();
}
}
}

TreeSet Example: Book
import java.util.*;
class Book implements Comparable<Book>{
int id;
String name,author,publisher;
int quantity;
public Book(int id, String name, String author,
String publisher, int quantity) {
this.id = id;
this.name = name;
this.author = author;
this.publisher = publisher;
this.quantity = quantity;
}
public int compareTo(Book b) {
if(id>b.id){
return 1;
}else if(id<b.id){
return -1;
}else{
return 0;
}
}
}
public class TreeSetExample {
Set<Book> set=new TreeSet<Book>();
//Creating Books
Book b1=new Book(121,"Let us C","Yashwant
Kanetkar","BPB",8);
Book b2=new Book(233,"Operating
System","Galvin","Wiley",6);
Book b3=new Book(101,"Data Communications &
Networking","Forouzan","Mc Graw Hill",4);
//Adding Books to TreeSet
set.add(b1);
set.add(b2);
set.add(b3);
//Traversing TreeSet
for(Book b:set){
System.out.println(b.id+" "+b.name+"
"+b.author+" "+b.publisher+" "+b.quantity);
}
}
}

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