The document provides an agenda and overview of collections in Java. It begins with a recap of arrays, noting their fixed length as a limitation. It then introduces the Collections API and framework, including core interfaces like Collection and List. It describes two common List implementations: ArrayList, which uses an array for fast random access but slower insertion/deletion; and LinkedList, which allows faster insertion/deletion but slower random access. The document provides examples of using ArrayList, including to store user-defined objects. It previews topics for the next session, including sorting and searching collections.

1. Java & JEE Training
Day 15 – Collections

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Agenda
• Recap of Arrays
• Introduction to Collections API
• Lists – ArrayList, Vector, LinkedList

3. Recap of Arrays

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Arrays…
• How are arrays defined?
int[] myArray = { 2, 5, -2, 6, -3, 8, 0, -7, -9, 4 };
int[] myArray = new int[10];
myArray[0] = 2;
myArray[1] = 5; …
• Iterating through array: Use for loop
• Sort and search: Use java.util.Arrays
• Demo of arrays…

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Major shortcoming of arrays?
• Length of arrays is fixed when the array is created. It cannot be changed
after that.
• The solution is to use one of the List classes from the Collections API.
• PROGRAM = DATA STRUCTURE + ALGORITHM;

6. Collections API

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Readings and References
• References
• "Collections", Java tutorial
• http://java.sun.com/docs/books/tutorial/collections/index.html

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Collections Framework
• Unified architecture for representing and manipulating collections.
• A collections framework contains three things
• Interfaces
• Implementations
• Algorithms

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Collections Framework Diagram

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Collection Interface
• Defines fundamental methods
• int size();
• boolean isEmpty();
• boolean contains(Object element);
• boolean add(Object element);
• boolean remove(Object element);
• 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

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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 remove it

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Iterator Position

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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());
}
}

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List Interface Context
Collection
List

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ArrayList and LinkedList Context
ArrayList LinkedList
Collection
List

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List as part of Collection

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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

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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];
}

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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 many elements
• void ensureCapacity(int minCapacity)

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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());
}

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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);
}
}

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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{
public static void main(String args[]){
//Creating user-defined class objects
Student s1=new Student(101,"Sonoo",23);
Student s2=new Student(102,"Ravi",21);
Student s3=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
while(itr.hasNext()){
Student st=(Student)itr.next();
System.out.println(st.rollno+" "+st.name+" "+st.age);
}
}
}

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Topics to be covered in next session
• Recap of Arrays, ArrayLists
• Basically, there can be 2 operations that you would want to perform on
Arrays/ArrayLists.. (and maybe, other collections)
• Search: Override equals() and hashCode().
• Sort: provide comparison logic – Two ways
• Comparable interface
• Comparator interface

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Thank you!