The inner workings of ArrayList in Java

ArrayList is a class that implements the List interface in the Java Collection Framework. It is a linear structure that stores and accesses each element sequentially, this is because it internally uses a dynamic array to store its elements. Like arrays, it also allows storing duplicate elements. A dynamic array here refers to an array that can grow and shrink as needed. In this article, we'll explore the inner workings of an ArrayList to show how it stores its elements and resizes itself during operations.

How does ArrayList work internally in Java?

Most of us know that standard arrays are of fixed length. Once arrays are declared and initialized, they cannot grow or shrink, which means we cannot add more elements after specifying their size. However, in some corner cases, the developer may not know the required array size until runtime. In this case, ArrayList is a better alternative to array, which can dynamically increase or decrease the number of elements.

Constructor of ArrayList

To use ArrayList in our program, we first need to create an instance of the ArrayList class. For this purpose, Java provides three different constructors mentioned below:

• ArrayList(): It will create an empty ArrayList.

• ArrayList(intcapacityInitial): In this constructor we can pass the initial capacity of the number of elements that the ArrayList can hold. Once the number of elements reaches the initial capacity, the capacity is automatically increased.

• ArrayList(Collection collectionName): We can also use this constructor to create an ArrayList from another collection.

Why is ArrayList dynamic?

When we add an element to an ArrayList using the built-in method "add()", it first checks if there is enough space in the array to store the new element. If present, it assigns the new element to the next available index in the array. If there is not enough room for the new element, it creates a new array with a larger capacity, usually 1.5 times the current capacity, then copies all existing elements from the old array to the new array and assigns the new elements to the next The available index of an element into the newly created array.

When we remove an element from the ArrayList using the built-in method "remove()", it moves all the elements after the removed element one index to the left. However, it does not shrink the array immediately, but it may shrink later if the size is too small compared to the capacity.

Let us discuss a few examples to better understand the points discussed above.

Example 1

The following example shows how to create and print elements of an ArrayList in Java.

import java.util.*;
public class Example1 {
   public static void main(String[] args) {
	   // Creating arraylist 
      ArrayList<Integer> araylist = new ArrayList<Integer>();
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>// Adding elements in arraylist<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(8);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(5);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(2);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(9);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(2);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(4);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(9);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(7);
<span style="font-size: 11.4844px;">      </span>System.out.println("List of elements: " + araylist);
   }
}

Output

List of elements: [8, 5, 2, 9, 2, 4, 9, 7]

Example 2

In the following example, we will create an ArrayList with an initial capacity and try to add elements beyond the specified capacity to show that the ArrayList can grow dynamically.

import java.util.*;
public class Example2 {
   public static void main(String[] args) {
      // Creating arraylist with initial capacity of 5
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>ArrayList<Integer> araylist = new ArrayList<Integer>(5);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>// Adding elements in arraylist
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(80); // 1
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(15); // 2
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(23); // 3
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(9); // 4
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(12); // 5
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>// adding more elements 
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(14);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(91);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>araylist.add(74);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>System.out.println("List of elements: " + araylist);
   }
}

Output

List of elements: [80, 15, 23, 9, 12, 14, 91, 74]

Example 3

In this example, we will create an ArrayList from a HashSet to demonstrate that elements of a collection can be added to an ArrayList. Internally, it first checks the size of the passed collection and then stores the elements in the given ArrayList using the "Arrays.copyOf()" method.

import java.util.*;
public class Example3 {
   public static void main(String[] args) {
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>// Creating a Set 
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>HashSet<Integer> setH = new HashSet<>();
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>// Adding elements in the Set
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>setH.add(80); 
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>setH.add(15); 
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>setH.add(23); 
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>setH.add(9); 
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>setH.add(12); 
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>setH.add(14);<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>setH.add(91);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>setH.add(74);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>// Passing the Set to the ArrayList
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>ArrayList<Integer> araylist = new ArrayList<>(setH);
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>// printing the result
<span style="font-size: 0.875em; font-weight: var(--bs-body-font-weight);">      </span>System.out.println("List of elements: " + araylist);
   }
}

Output

List of elements: [80, 23, 9, 74, 91, 12, 14, 15]

in conclusion

We started this article by defining ArrayList, and in the next section, we discuss its inner workings. ArrayList is a resizable array with a default capacity of 10 which can be increased when we add more elements to it. We can also specify the initial capacity in the constructor of the ArrayList when creating it.

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