How to serialize and deserialize ArrayList in Java

In Java, ArrayList class is serializable by default. It essentially means that we do not need to implement Serializable interface explicitly in order to serialize ArrayList.

We can directly use ObjectOutputStream to serialize ArrayList, and ObjectInputStream to deserialize an arraylist object.

Note – The elements stored in arraylist should also be serializable, else program will throw NotSerializableException.

1. Example to serialize ArrayList of strings

Given below is an example Java program to persist an arraylist of strings.

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.ArrayList;

public class ArrayListExample 
{
    public static void main(String[] args) throws Exception 
    {
        ArrayList<String> namesList = new ArrayList<String>();
        
        namesList.add("alex");
        namesList.add("brian");
        namesList.add("charles");

        try 
        {
            FileOutputStream fos = new FileOutputStream("listData");
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(namesList);
            oos.close();
            fos.close();
        } 
        catch (IOException ioe) 
        {
            ioe.printStackTrace();
        }
    }
}

Program output.

The list is serialized in project root folder.

2. Example to serialize ArrayList of objects

Given below is an example Java program to save an arraylist of Employee objects. Employee class implement Serializable interface.

import java.io.Serializable;

public class Employee implements Serializable {

    String id;
    String firstName;
    String lastName;

    //Getters and setters

    public Employee(String id, String firstName, String lastName) {
        super();
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
    }

    @Override
    public String toString() {
        return "Employee [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + "]";
    }
}

public class ArrayListExample 
{
    public static void main(String[] args) throws Exception 
    {
        ArrayList<Employee> employees = new ArrayList<>();
        
        employees.add(new Employee("1", "lokesh", "gupta"));
        employees.add(new Employee("2", "brian", "motto"));

        try 
        {
            FileOutputStream fos = new FileOutputStream("employeeData");
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(employees);
            oos.close();
            fos.close();
        } 
        catch (IOException ioe) 
        {
            ioe.printStackTrace();
        }
    }
}

Notice if Employee class does not implement Serializable interface, we will recieve this error.

java.io.NotSerializableException: com.howtodoinjava.demo.model.Employee
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
	at java.util.ArrayList.writeObject(ArrayList.java:766)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1128)
	at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
	at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
	at com.howtodoinjava.demo.ArrayListExample.main(ArrayListExample.java:23)

3. Example to deserialize ArrayList

3.1. Deserialize list of strings

Java program to deserialize list of strings and verify list content.

import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.util.ArrayList;

public class ArrayListExample 
{
    public static void main(String[] args) throws Exception 
    {
        ArrayList<String> namesList = new ArrayList<String>();
        
        try 
        {
            FileInputStream fis = new FileInputStream("listData");
            ObjectInputStream ois = new ObjectInputStream(fis);

            namesList = (ArrayList) ois.readObject();

            ois.close();
            fis.close();
        } 
        catch (IOException ioe) 
        {
            ioe.printStackTrace();
            return;
        } 
        catch (ClassNotFoundException c) 
        {
            System.out.println("Class not found");
            c.printStackTrace();
            return;
        }
        
        //Verify list data
        for (String name : namesList) {
            System.out.println(name);
        }
    }
}

Program output.

alex
brian
charles

3.1. Deserialize list of objects

Java program to deserialize list of objects and verify list content.

import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.util.ArrayList;

public class ArrayListExample 
{
    public static void main(String[] args) throws Exception 
    {
        ArrayList<Employee> employees = new ArrayList<>();
        
        try 
        {
            FileInputStream fis = new FileInputStream("employeeData");
            ObjectInputStream ois = new ObjectInputStream(fis);

            employees = (ArrayList) ois.readObject();

            ois.close();
            fis.close();
        } 
        catch (IOException ioe) 
        {
            ioe.printStackTrace();
            return;
        } 
        catch (ClassNotFoundException c) 
        {
            System.out.println("Class not found");
            c.printStackTrace();
            return;
        }
        
        //Verify list data
        for (Employee employee : employees) {
            System.out.println(employee);
        }
    }
}

Program output.

Employee [id=1, firstName=lokesh, lastName=gupta]
Employee [id=2, firstName=brian, lastName=motto]

Happy Learning !!

Read More:

A Guide to Java ArrayList
ArrayList Java Docs

Further Reading:

Gson – Serialize and deserialize HashMap containing custom objects GSON – Serialize and deserialize JSON to Set Lombok Serialize and Deserialize @Builder Class