Arraylist in Java

Example

import java.util.*;
public class ArrayListDemo {
   public static void main(String args[]) {
      // create an array list
      ArrayList al = new ArrayList();
      System.out.println("Initial size of al: " + al.size());
      // add elements to the array list
      al.add("C");
      al.add("A");
      al.add("E");
      al.add("B");
      al.add("D");
      al.add("F");
      al.add(1, "A2");
      System.out.println("Size of al after additions: " + al.size());
      // display the array list
      System.out.println("Contents of al: " + al);
      // Remove elements from the array list
      al.remove("F");
      al.remove(2);
      System.out.println("Size of al after deletions: " + al.size());
      System.out.println("Contents of al: " + al);
   }
}

Output

Initial size of al: 0
Size of al after additions: 7
Contents of al: [C, A2, A, E, B, D, F]
Size of al after deletions: 5
Contents of al: [C, A2, E, B, D]

Arraylist Methods

• Arraylist add: This is used to add elements to the Array List. If an ArrayList already contains elements, the new element gets added after the last element unless the index is specified.
• Arraylist remove: The specified element is removed from the list and the size is reduced accordingly. Alternately, you can also specify the index of the element to be removed.
• Java array size:  This will give you the number of elements in the Array List. Just like arrays, here too the first element starts with index 0.
• Arraylistcontains:  This method will return true if the list contains the specified element.

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);  
 }  
 }  
}

import java.util.ArrayList;
 

List<String> names;
 names = new ArrayList<String>();
 

names.add("Martin");
 names.add("Galloway");
 

for(int i = 0; i < names.size(); i++) {
     System.out.println(names.get(i));
 }

Implementing the ArrayList in Java

    private Object [] list;
    final int maxObjects = 100;
    public ListOfObjects()
    {
        list= new Object[maxObjects];
    }
    public ListOfObjects(Object[]o)
    {
        list= o;
    }
    public void add(Object element,int index)
    {
        Object[] newData = new Object[list.length+1];
        for(int i =0; i < index; i++)
        {
            newData[i] = list[i];
            newData[list] = element;
        }
        for(int i = index; i < list.length; i++)
        {
     newData[i+1] = list[i];
        }
    }