Java Collection

Java ArrayList class

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

ArrayList al=new ArrayList();//creating old non-generic arraylist  

ArrayList<String> al=new ArrayList<String>();//creating new generic arraylist  

import java.util.*;  class TestCollection1{   public static void main(String args[]){    ArrayList<String> al=new ArrayList<String>();//creating arraylist    al.add("Ravi");//adding object in arraylist    al.add("Vijay");    al.add("Ravi");    al.add("Ajay");    Iterator itr=al.iterator();  //getting Iterator from arraylist to traverse elements    while(itr.hasNext()){     System.out.println(itr.next());    }   }  } 

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);   }  }  

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 s2=new Student(103,"Hanumat",25);    ArrayList<Student> al=new ArrayList<Student>();//creating arraylist    al.add(s1);//adding Student class object    al.add(s2);    al.add(s3);    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);    }   }  }  

compareto and comparable when do you use?
reverse,
collection and map
unique key, we cannot have duplicate key
sort, code compareto and comparable
tree traverse collections