Java Collections Framework:ArrayList-2(Basics)

 

How to initialize an ArrayList

Method 1: Initialization using Arrays.asList

Syntax:

ArrayList<Type> obj = new ArrayList<Type>(
        Arrays.asList(Object o1, Object o2, Object o3, ....so on));

Example:

import java.util.*;
public class InitializationExample1 {
   public static void main(String args[]) {
	   ArrayList<String> obj = new ArrayList<String>(
		Arrays.asList("Pratap", "Peter", "Harsh"));
	  System.out.println("Elements are:"+obj);
   }
}

Output:

Elements are:[Pratap, Peter, Harsh]

Method 2: Anonymous inner class method to initialize ArrayList

Syntax:

ArrayList<T> obj = new ArrayList<T>(){{
		   add(Object o1);
		   add(Object o2);
		   add(Object o3);
                   ...
                   ...
		   }};

Example:

import java.util.*;
public class InitializationExample2 {
   public static void main(String args[]) {
	   ArrayList<String> cities = new ArrayList<String>(){{
		   add("Delhi");
		   add("Agra");
		   add("Chennai");
		   }};
	  System.out.println("Content of Array list cities:"+cities);
   }
}

Output:

Content of Array list cities:[Delhi, Agra, Chennai]

Method3: Normal way of ArrayList initialization

Syntax:

ArrayList<T> obj = new ArrayList<T>();
	   obj.add("Object o1");
	   obj.add("Object o2");
	   obj.add("Object o3");
                        ...
                        ...

Example:

import java.util.*;

public class Details {
   public static void main(String args[]) {
	   ArrayList<String> books = new ArrayList<String>();
	   books.add("Java Book1");
	   books.add("Java Book2");
	   books.add("Java Book3");
	  System.out.println("Books stored in array list are: "+books);
   }
}

Output:

Books stored in array list are: [Java Book1, Java Book2, Java Book3]

Method 4: Use Collections.ncopies

Collections.ncopies method can be used when we need to initialize the ArrayList with the same value for all of its elements. Syntax: count is number of elements and element is the item value

ArrayList<T> obj = new ArrayList<T>(Collections.nCopies(count, element));

Example:

import java.util.*;

public class Details {
   public static void main(String args[]) {
	   ArrayList<Integer> intlist = new ArrayList<Integer>(Collections.nCopies(10, 5));
	  System.out.println("ArrayList items: "+intlist);
   }
}

Output:

ArrayList items: [5, 5, 5, 5, 5, 5, 5, 5, 5, 5]

How to loop ArrayList in Java

There are four ways to loop ArrayList:

1. For Loop
2. Advanced for loop
3. While Loop
4. Iterator

Lets have a look at the below example – I have used all of the mentioned methods for iterating list.

import java.util.*;
public class LoopExample {
   public static void main(String[] args) {

      ArrayList<Integer> arrlist = new ArrayList<Integer>();
      arrlist.add(14);
      arrlist.add(7);
      arrlist.add(39);
      arrlist.add(40);

      /* For Loop for iterating ArrayList */
      System.out.println("For Loop");
      for (int counter = 0; counter < arrlist.size(); counter++) { 		      
          System.out.println(arrlist.get(counter)); 		
      }   		

      /* Advanced For Loop*/ 		
      System.out.println("Advanced For Loop"); 		
      for (Integer num : arrlist) { 		      
           System.out.println(num); 		
      }

      /* While Loop for iterating ArrayList*/ 		
      System.out.println("While Loop"); 		
      int count = 0; 		
      while (arrlist.size() > count) {
	 System.out.println(arrlist.get(count));
         count++;
      }

      /*Looping Array List using Iterator*/
      System.out.println("Iterator");
      Iterator iter = arrlist.iterator();
      while (iter.hasNext()) {
         System.out.println(iter.next());
      }
   }
}

Output:

For Loop
14
7
39
40
Advanced For Loop
14
7
39
40
While Loop
14
7
39
40
Iterator
14
7
39
40

How to iterate arraylist elements using Enumeration interface

import java.util.Enumeration;
import java.util.ArrayList;
import java.util.Collections;
 
public class EnumExample {
 
   public static void main(String[] args) {
      //create an ArrayList object
      ArrayList<String> arrayList = new ArrayList<String>();
 
      //Add elements to ArrayList
      arrayList.add("C");
      arrayList.add("C++");
      arrayList.add("Java");
      arrayList.add("DotNet");
      arrayList.add("Perl");
 
      // Get the Enumeration object
      Enumeration<String> e = Collections.enumeration(arrayList);
 
      // Enumerate through the ArrayList elements
      System.out.println("ArrayList elements: ");
      while(e.hasMoreElements())
      System.out.println(e.nextElement());
   }
}

Output:

ArrayList elements: 
C
C++
Java
DotNet
Perl

______________________________________________

How to find length of ArrayList in Java

By using  size() method of ArrayList class we can easily determine the size of the ArrayList. This method returns the number of elements of ArrayList.

public int size()

Example:

package beginnersbook.com;
import java.util.ArrayList;
public class Details
{
    public static void main(String [] args)
    {
        ArrayList<Integer> al=new ArrayList<Integer>();
        System.out.println("Initial size: "+al.size());
        al.add(1);
        al.add(13);
        al.add(45);
        al.add(44);
        al.add(99);
        System.out.println("Size after few additions: "+al.size());
        al.remove(1);
        al.remove(2);
        System.out.println("Size after remove operations: "+al.size());
        System.out.println("Final ArrayList: ");
        for(int num: al){
            System.out.println(num);
        }
   }
}

Output:

Initial size: 0
Size after few additions: 5
Size after remove operations: 3
Final ArrayList: 
1
45
99