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<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]
Nysa says
Hi i used the anonymous inner class way to initialize an arrayList but i get the error below:
The type java.util.function.Consumer cannot be resolved. It is indirectly referenced from required .class files
Can you hint me as to what i have missed ?
Thanks
Nysa
tony says
Hello Nysa,
I don’t know if you received an answer to your question. I’m a beginner as well learning Java. I believe you received that error message because you are using a later version of Java than the one the author posted here. If you look at the web address, it looks like he created this page in 2013. I have seen certain commands are different in my school book than what I find on the web, and I believe the version is the reason.
Hope this helps. Even if you do not get this, others may run into this same issue and find my comment helpful.
irfan says
may be u are missing import statement
Shivam says
Hello,
I have a doubt :
//************************************
import java.util.*;
public class Initialization2 {
public static void main(String args[]) {
final ArrayList cities = new ArrayList() {
{
add(“Delhi”);
add(“Agra”);
add(“Chennai”);
}
};
System.out.println(“Content of Array list cities:” + cities);
cities.add(“Goa”);
System.out.println(“Content of Array list cities:” + cities);
}
}
//************************************
In this code I have declared the ArrayList as “final” so it should not allow any modifications, like any integer variable marked as final cannot be changed once it is assigned.
But it allows the modification as I have added one more object “Goa” to cities ArrayList.
Please explain.
Thanks
vishwjeet says
@Shivam in the above example…u r making reference variable as final so,if u want to again assign reference variable cities= new ArrayList();// you will get warning
Shashibhushan says
In the example 1 to initialize , you missed showing the relationship between the arrays.aslist and arraylist object…
arr.addAll(Arrays.asList(“bangalore”,”hyderabad”,”chennai”));
Aditya says
In this method,
ArrayList cities = new ArrayList(){{
add(“Delhi”);
add(“Agra”);
add(“Chennai”);
}};
Why is there 2 openings after object creation I didn’t get that and what is the difference between ArrayList and Array.asList.
In the above example what I have given why not use “cities.add(“”) ” and whats the difference between add and obj.add ?