Java - ObjectOutputStream writeObject(Object obj) method

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Description

The Java ObjectOutputStream writeObject(Object obj) method writes the specified object to the ObjectOutputStream. The class of the object, the signature of the class, and the values of the non-transient and non-static fields of the class and all of its supertypes are written. Default serialization for a class can be overridden using the writeObject and the readObject methods. Objects referenced by this object are written transitively so that a complete equivalent graph of objects can be reconstructed by an ObjectInputStream.

writeObject(Object obj)

• Serializes a Java object to an output stream.

• The object must implement the Serializable interface.

• Supports nested objects, arrays, collections, and class metadata.

• Paired with ObjectInputStream.readObject() to deserialize.

Declaration

Following is the declaration for java.io.ObjectOutputStream.writeObject(Object obj) method.

public void writeObject(Object obj)

Parameters

• obj − The object to be written.

Return Value

This method does not return a value.

Exception

• InvalidClassException − Something is wrong with a class used by serialization.

• NotSerializableException − Some object to be serialized does not implement the java.io.Serializable interface.

• IOException − If I/O errors occur while writing to the underlying stream.

Example - Usage of ObjectOutputStream writeObject(Object obj) method

The following example shows the usage of ObjectOutputStream writeObject(Object obj) method.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) {
      String s = "Hello world!";
      int i = 897648764;
      
      try {
         // create a new file with an ObjectOutputStream
         FileOutputStream out = new FileOutputStream("test.txt");
         ObjectOutputStream oout = new ObjectOutputStream(out);

         // write something in the file
         oout.writeObject(s);
         oout.writeObject(i);

         // close the stream
         oout.close();

         // create an ObjectInputStream for the file we created before
         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("test.txt"));

         // read and print what we wrote before
         System.out.println("" + (String) ois.readObject());
         System.out.println("" + ois.readObject());
      } catch (Exception ex) {
         ex.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Hello world!
897648764

Example - Serialize and deserialize a simple object

The following example shows the usage of ObjectOutputStream writeObject(Object obj) method. We're writing a User object to a file, then read it back.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) {
      String filename = "user1.ser";

      // Write object
      try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(filename))) {
         User user = new User("alice", 25);
         oos.writeObject(user);
         System.out.println("User object serialized.");
      } catch (IOException e) {
         e.printStackTrace();
      }

      // Read object
      try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream(filename))) {
         User user = (User) ois.readObject();
         System.out.println("Deserialized user: " + user.name + ", age " + user.age);
      } catch (IOException | ClassNotFoundException e) {
         e.printStackTrace();
      }
   }

   static class User implements Serializable {
      String name;
      int age;
      User(String name, int age) {
         this.name = name;
         this.age = age;
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

User object serialized.
Deserialized user: alice, age 25

Example - Serialize a list of objects using writeObject()

The following example shows the usage of ObjectOutputStream writeObject(Object obj) method. We're serialize a List<Product> to a file and restore it.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Arrays;
import java.util.List;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) {
      String filename = "products.ser";

      List<Product> products = Arrays.asList(
         new Product("Laptop", 1500),
         new Product("Phone", 800)
      );

      // Serialize list of products
      try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(filename))) {
         oos.writeObject(products);
         System.out.println("Product list serialized.");
      } catch (IOException e) {
         e.printStackTrace();
      }

      // Deserialize list
      try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream(filename))) {
         @SuppressWarnings("unchecked")
         List<Product> restored = (List<Product>) ois.readObject();
         System.out.println("Deserialized products:");
         for (Product p : restored) {
            System.out.println(" - " + p.name + ": $" + p.price);
         }
      } catch (IOException | ClassNotFoundException e) {
         e.printStackTrace();
      }
   }

   static class Product implements Serializable {
      String name;
      double price;
      Product(String name, double price) {
         this.name = name;
         this.price = price;
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Product list serialized.
Deserialized products:
 - Laptop: $1500.0
 - Phone: $800.0