Which Data Type Cannot be Stored in Java ArrayList?

Last Updated : 17 Dec, 2020

The ArrayList class implements a growable array of objects. ArrayList cannot hold primitive data types such as int, double, char, and long. With the introduction to wrapped class in java that was created to hold primitive data values. Objects of these types hold one value of their corresponding primitive type(int, double, short, byte). They are used when there is a usage of the primitive data types in java structures that require objects such as JLists, ArrayLists. Now, in order to hold primitive data such as int and char in ArrayList are explained.

Primitive data types cannot be stored in ArrayList but can be in Array. ArrayList is a kind of List and List implements Collection interface. The Collection container expects only Objects data types and all the operations done in Collections, like iterations, can be performed only on Objects and not Primitive data types. An ArrayList cannot store ints. To place ints in ArrayList, we must convert them to Integers. This can be done in the add() method on ArrayList. Each int must be added individually.

Cases:

Integers in ArrayList
Character in ArrayList

Case 1: Integers in ArrayList

To place int in ArrayList, First, they are converted into Integers. This can be done in the add method on ArrayList. Each int must be added individually. For example, consider an int array. It has 3 values in it. We create an ArrayList and add those int as Integers in a for-loop.

The add() method receives an Integer. And we can pass an int to this method—the int is cast to an Integer.

Java

// Java Program that uses ArrayList of Integer Values

// Importing ArrayList class from
// java.util package
import java.util.ArrayList;

// Class
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Create and insert custom elements in array
        int[] ids = { -3, 0, 100 };

        // Create ArrayList of Integer type
        ArrayList<Integer> values = new ArrayList<>();

        // For- each loop to iterate over ArrayList
        for (int id : ids) {

            // Add all the ints as Integers with add()
            // method, here the ints are cast to Integer
            // implicitly.
            values.add(id);
        }
      
        System.out.println(values);

        // The collections have the same lengths
        System.out.println(values.size());

        System.out.println(ids.length);
    }
}

Output

[-3, 0, 100]
3
3

Using toArray method - The toArray method copies an ArrayList's elements to an array.

One returns an Object array and requires casting. This version, though, returns a typed array.

Java

// Importing List and ArrayList class of
// java.util package 
import java.util.ArrayList;
import java.util.List;

public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating and initializing ArrayList
        ArrayList<Integer> list = new ArrayList<>();

        // Adding elements to ArrayList
        // Custom inputs
        list.add(7);
        list.add(8);
        list.add(9);

        // Create an empty array and pass to toArray.
        Integer[] array = {};

        // Converting above array to ArrayList
        // using inbuilt method - list.toArray(array)
        array = list.toArray(array);

        // Iterating over elements using for-each loop over
        // elements of ArrayList derived from initial array
        for (int elem : array) {

            // Printing elements of converted ArrayList
            System.out.println(elem);
        }
    }
}

Output

7
8
9

Case 2: Character in ArrayList

In Java ArrayList char character use-case is:

Convert them into Character
Convert string value into the character ArrayList.

Example: Java Program to convert a string to a list of characters