Now that we’ve learned about the Serializable interface and how to implement it, let’s take a look at how to serialize an object to a file. To do this we’ll need to use the helper classes, FileOutputStream, which will help us write to a file, and ObjectOutputStream, which will help us write a serializable object to an output stream.

Let’s look at how to do this with some code:

public class Person implements Serializable {
  private String name;
  private int age;
  private static final long serialVersionUID = 1L; 

  public Person(String name, int age) {
    this.name = name;
    this.age = age;
  }  

  public static void main(String[] args) throws FileNotFoundException, IOException{
    Person michael = new Person("Michael", 26);
    Person peter = new Person("Peter", 37);

    FileOutputStream fileOutputStream = new FileOutputStream("persons.txt");
    ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);

    objectOutputStream.writeObject(michael);
    objectOutputStream.writeObject(peter);    
  }
} 

In the example above we:

• Added a constructor to the Person class we defined in the previous lesson.
• Defined main() and initialized two Person objects - michael and peter.
• Initialized a FileOutputStream object in main() which will create and write a stream of bytes to a file named person-file.txt.
• Initialized an ObjectOutputStream object in main() which will help serialize an object to a specified output stream.
• Used objectOutputStream.writeObject() in main() to serialize the michael and peter objects to a file.

After the execution of the above program, the persons.txt will contain a stream of bytes that has the type and value information of the fields for the michael and peter objects respectively.

Let’s practice serializing an object to a file.