Collections Array list

Java/J2EE Programming Training
Collections

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

Arrays…
Classification: Restricted Page 3
• 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…

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;

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

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

Collections Framework Diagram

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

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

Iterator Position

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

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 many elements
•void ensureCapacity(int minCapacity)

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{
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);
}
}

Collections Array list

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