Custom ArrayList in Java

In Java, array and ArrayList are the well-known data structures. An array is a basic functionality provided by Java, whereas ArrayList is a class of Java Collections framework. It belongs to java.util package. The limitation of the ArrayList is that it allows us to store data of the same data type. To overcome this problem, we can customize the ArrayList and can store data of different primitive types. In this section, we are going to learn custom ArrayList in Java and how to implement custom ArrayList in Java. Before moving to the custom ArrayList, let's have a look at ArrayList in Java.

Java ArrayList Class

In Java, ArrayList is a class of Java Collections framework. It implements List<E>, Collection<E>, Iterable<E>, Cloneable, Serializable, and RandomAccess interfaces. It extends the AbstractList<E> class. It uses a dynamic array for storing the elements.

We can create an instance of ArrayList by using the following statement:

ArrayList is internally backed by the array in Java. The resize operation in ArrayList slows down the performance as it involves a new array and copying content from an old array to a new array. It calls the native implemented method System.arraycopy(sec, srcPos, dest, destPos, length).

We cannot store primitive type in ArrayList. So, it stores only objects. It automatically converts primitive type to object. For example, we have created an ArrayList object:

ArrayList <Integer> list=new ArrayList<Integer>();  //object of ArrayList  
arrayObj.add(12);   //trying to add integer primitive to the ArrayList  

The JVM converts it into Integer object through auto-boxing.

ArrayList arrayObj=new ArrayList() //object of ArrayList  
arrayObj(new Integer(12)); //converts integer primitive to Integer object and added to ArrayList object  

Custom ArrayList in Java

A list that contains more than one type of data (elements) is known as custom ArrayList. It is based on user requirements. In the custom ArrayList, the data is provided by the custom inner class that is formed by a combination of various primitive object datatypes.

Consider a situation, if we want to take input of product detail. The detail includes pid, pname, pavailability, pprice, prating. Typically to input these attributes in the ArrayList, we need to create a separate ArrayList for each attribute, as follows:

//product id ArrayList
ArrayList<Integer> pid= new ArrayList<>(); 

//product name ArrayList
ArrayList<String> pname = new ArrayList<>();

//product availability ArrayList
ArrayList<Character> pavailability = new ArrayList<>(); 

//product price ArrayList
ArrayList<Float> pprice = new ArrayList<>();

//product rating ArrayList
ArrayList<Double> prating = new ArrayList<>();

//and n number of products
for(int i = 0; i < n; i++)
{
//adding elements to each ArrayList
pid.add(id_i);
pname.add(name_i);
pavailability.add(availability_i);
pprice.add(price_i);
prating.add(rating_i);
}

The above process is lengthy and takes lots of time to input the elements. It uses separate ArrayList to store each attribute. To overcome the problem, we use custom ArrayList. It stores more than one data type in a single ArrayList. For example, consider the following figure:

In the above figure, we have created an ArrayList. In all the indices of ArrayList, we have stored different data such as accno, name, and interest.

Implementation of Custom ArrayList in Java

To implement the custom ArrayList in Java, follow the steps given below:

Create an object of the ArrayList class.
Place its data type as the class data.
Define a class.
Create a constructor and put the required entities in it.
Link those entities to global variables.
The data received from the ArrayList is of the class that stores multiple data.

Let's follow the above steps and implement the custom ArrayList.

class CustomArrayList 
{ 
// custom class which has data type 
// class has defined the type of data ArrayList 
// size of ArrayList n 
int size=n; 
//custom data type class
class Student 
{ 
//global variables of the class to be added in ArrayList
int sid; 
String sname; 
int smarks; 
double spercentage; 
//constructor of the class that contains types of data which are required 
Student (int sid, String sname, int smarks, double spercentage) 
{ 
//initializing the variables from main 
//function to the global variable of the class 
this.sid = sid; 
this.sname = sname; 
this.smarks = smarks; 
this.spercentage = spercentage; 
} 
} 

Example of Custom ArrayList

CustomArrayListExample.java

import java.util.ArrayList; 
public class CustomArrayListExample 
{ 
// custom class which has data type 
// class has defined the type of data ArrayList 
// size of ArrayList 6 
int size=6; 
//custom data type class
class Product 
{ 
//global variables of the class to be added in ArrayList
int pid; 
String pname; 
char pavailability; 
double pprice; 
float prating;
//constructor of the class that contains types of data which are required 
Product(int pid, String pname, char pavailability, double pprice, float prating) 
{ 
//initializing the variables from main 
//function to the global variable of the class 
this.pid = pid; 
this.pname = pname; 
this.pavailability = pavailability; 
this.pprice = pprice; 
this.prating = prating;
} 
} 
public static void main(String args[]) 
{ 
//data that we want to input
int pid[] = {21823, 89774, 34007, 12098, 45098, 10067}; 
String pname[] = {"HP Laptop", "Inverters", "Volats AC", "iPhone 11", "CanonDSLR", "SonyLEDTV"}; 
char pavailability[] = {'Y', 'Y', 'N', 'Y', 'Y', 'N'}; 
double pprice[] = {31987, 16900, 28000, 99000, 67570, 60500}; 
float prating[] = {4.5f, 4.0f, 4.7f, 4.5f, 3.9f, 4.0f};
//Creating an object of the class 
CustomArrayListExample calobj = new CustomArrayListExample(); 
//invoking the addElements() method that adds the elements in the ArrayList
calobj.addElements(pid, pname, pavailability, pprice, prating); 
} 
public void addElements(int pid[], String pname[], char pavailability[], double pprice[],  float prating[]) 
{ 
//local customized ArrayList of type Data having (int, String, char double, float) data type from the class
ArrayList<Product> arraylist=new ArrayList<>(); 
for (int i = 0; i < size; i++) 
{ 
//send values to the constructor to be saved in the Product class
arraylist.add(new Product (pid[i], pname[i], pavailability[i], pprice[i], prating[i])); 
} 
//calling the method to print the elements of the ArrayList
displayElements(arraylist); 
} 
public void displayElements(ArrayList<Product> arraylist) 
{ 
System.out.println("Product ID" + "   Product Name" + "     Product Availability" + "    Product Price" + "   Product Rating");	
System.out.println("-------------------------------------------------------------------------------------");	
//iteration over the ArrayList for accessing the elements
for (int i = 0; i < size; i++) 
{ 
//invoking the get() method of the ArrayList class to get the elements  
Product product = arraylist.get(i); 
//printing the elemnts to the console
System.out.println(   product.pid+"    "+  "    "  +product.pname+"               "+"  "+product.pavailability+"                "+product.pprice+ "           "+String.format("%.01f", product.prating)); 
} 
} 
}

Output: