Collections in Java

All the operations that you perform on a data such as searching, sorting, insertion, manipulation, deletion etc. can be performed by Java Collections.Collections in java is a framework that provides an architecture to store and manipulate the group of objects.

Java Collection simply means a single unit of objects. Java Collection framework provides many interfaces (Set, List, Queue, Deque etc.) and classes (ArrayList, Vector, LinkedList, PriorityQueue, HashSet, LinkedHashSet, TreeSet etc).

Java ArrayList class:

Java ArrayList class uses a dynamic array for storing the elements. It inherits AbstractList class and implements List interface.

The important points about Java ArrayList class are:

Java ArrayList class can contain duplicate elements.
Java ArrayList class maintains insertion order.
Java ArrayList class is non synchronized.
Java ArrayList allows random access because array works at the index basis.
In Java ArrayList class, manipulation is slow because a lot of shifting needs to be occurred if any element is removed from the array list.

ava ArrayList Example

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());
}
}
}

       Ravi
       Vijay
       Ravi
       Ajay

Two ways to iterate the elements of collection in java

There are two ways to traverse collection elements:

By Iterator interface.
By for-each loop.

In the above example, we have seen traversing ArrayList by Iterator. Let's see the example to traverse ArrayList elements using for-each loop.

Iterating Collection through for-each loop

import java.util.*;
class TestCollection2{
public static void main(String args[]){
ArrayList<String> al=new ArrayList<String>();
al.add("Ravi");
al.add("Vijay");
al.add("Ravi");
al.add("Ajay");
for(String obj:al)
System.out.println(obj);
}
}

       Ravi
       Vijay
       Ravi
       Ajay

User-defined class objects in Java ArrayList

Let's see an example where we are storing Student class object in array list.

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 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
while(itr.hasNext()){
Student st=(Student)itr.next();
System.out.println(st.rollno+" "+st.name+" "+st.age);
}
}
}

output

       101 Sonoo 23
       102 Ravi 21
       103 Hanumat 25

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());
}
}
}

output:

       Ravi
       Vijay
       Ajay
       Sonoo
       Hanumat

Example of removeAll() method

import java.util.*;
class 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...");
Iterator itr=al.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
}
}

       iterating the elements after removing the elements of al2...
       Vijay
       Ajay

Example of retainAll() method

import java.util.*;
class TestCollection6{
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.retainAll(al2);
System.out.println("iterating the elements after retaining the elements of al2...");
Iterator itr=al.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
}
}

       iterating the elements after retaining the elements of al2...
       Ravi

Java ArrayList Example: Book

Let's see an ArrayList example where we are adding books to list and printing all the books.

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 ArrayListExample {
public static void main(String[] args) {
//Creating list of Books
List<Book> list=new ArrayList<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 list
list.add(b1);
list.add(b2);
list.add(b3);
//Traversing list
for(Book b:list){
System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity);
}
}
}

Output:

101 Let us C Yashwant Kanetkar BPB 8
102 Data Communications & Networking Forouzan Mc Graw Hill 4
103 Operating System Galvin Wiley 6

JAVA DEVELOPMENT Training By Om Sir