# How to write an Object to a File in Java 

By Atta Ur Rehman Shah (https://attacomsian.com/about). Published Dec 11, 2019. Topics: Java.
Canonical URL: https://attacomsian.com/blog/java-write-object-to-file

> A quick tutorial to learn how to serialize a Java Object to a file.

In this quick article, you'll learn **how to write a Java Object to a file** in the local file system. To do this serialization, the class of the object must implement the `Serializable` interface. This will enable us to perform basic I/O operations on the class in Java.

To write an object to a file, all you need to do is the following:

* Create a Java class that implements the `Serializable` interface.
* Open a new or an existing file using `FileOutputStream`.
* Create an instance of `ObjectOutputStream` and pass `FileOutputStream` as an argument to its constructor.
* Use `ObjectOutputStream.writeObject()` method to write the object to the file.

## Create Java Class

Let us first create a simple Java class named `User.java` and implement the `Serializable` interface:

```java
public class User implements Serializable {

    public String name;
    public String email;
    private String[] roles;
    private boolean admin;

    public User() {
    }

    public User(String name, String email, String[] roles, boolean admin) {
        this.name = name;
        this.email = email;
        this.roles = roles;
        this.admin = admin;
    }

    // getters and setters, toString() .... (omitted for brevity)
}
```

## Write Object to File

The following example demonstrates how you can create a `User` object and write it to a file in Java 7 or higher:

```java
try (FileOutputStream fos = new FileOutputStream("object.dat");
     ObjectOutputStream oos = new ObjectOutputStream(fos)) {

    // create a new user object
    User user = new User("John Doe", "john.doe@example.com",
            new String[]{"Member", "Admin"}, true);

    // write object to file
    oos.writeObject(user);

} catch (IOException ex) {
    ex.printStackTrace();
}
```

In older Java versions (Java 6 or below), you have to manually close `ObjectOutputStream` as shown below:

```java
try {
    FileOutputStream fos = new FileOutputStream("object.dat");
    ObjectOutputStream oos = new ObjectOutputStream(fos);

    // create a new user object
    User user = new User("John Doe", "john.doe@example.com",
            new String[]{"Member", "Admin"}, true);

    // write object to file
    oos.writeObject(user);

    // close writer
    oos.close();

} catch (IOException ex) {
    ex.printStackTrace();
}
```

## Further Reading

You may be interested in other Java I/O articles:

* [Reading and Writing Files in Java](https://attacomsian.com/blog/java-read-write-files)
* [How to Read and Write Text Files in Java](https://attacomsian.com/blog/java-read-write-text-files)
* [How to Read and Write Binary Files in Java](https://attacomsian.com/blog/java-read-write-binary-files)
* [Reading and Writing Files using Java NIO API](https://attacomsian.com/blog/java-read-write-files-nio-api)
* [How to read a file line by line in Java](https://attacomsian.com/blog/how-to-read-a-file-line-by-line-in-java)