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How to initialize an ArrayList


In the last post we discussed about class ArrayList in Java and it’s important methods. Here we are sharing multiple ways to initialize an ArrayList with examples.

Method 1: Initialization using Arrays.asList

Syntax:

ArrayList obj = new ArrayList(
        Arrays.asList(Object o1, Object o2, Object o3, ....so on));
Example:

import java.util.*;
public class InitializationExample1 {
   public static void main(String args[]) {
       ArrayList obj = new ArrayList(
        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 obj = new ArrayList(){{
           add(Object o1);
           add(Object o2);
           add(Object o3);
                   ...
                   ...
           }};
Example:

import java.util.*;
public class InitializationExample2 {
   public static void main(String args[]) {
       ArrayList cities = new ArrayList(){{
           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 obj = new ArrayList();
       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 books = new ArrayList();
       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 obj = new ArrayList(Collections.nCopies(count, element));
Example:

import java.util.*;

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

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

asked Jun 11, 2015 in Java Interview Questions by rajesh
0 votes
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