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Java ObjectOutputStream writeObject() Method with Example

ObjectOutputStream Class writeObject() method: Here, we are going to learn about the writeObject() method of ObjectOutputStream Class with its syntax and example.
Submitted by Preeti Jain, on April 10, 2020

ObjectOutputStream Class writeObject() method

  • writeObject() method is available in java.io package.
  • writeObject() method is used to write the given object to the ObjectOutputStream. It includes the class, signature, values of non-static and non-transient fields of the given object of the class are written. Objects address by this object (o) are written transitively so that graph objects can be reformed by an ObjectInputStream.
  • writeObject() method is a non-static method, it is accessible with the class object only and if we try to access the method with the class name then we will get an error.
  • writeObject() method may throw an exception at the time of writing an object.
    • IOException: This exception may throw when getting any input/output error while writing to the output stream.
    • InvalidClassException: This exception may throw when invalid things with a class used by serialization.
    • NotSerializableException: This exception may throw when the serialized object does not implement the Serializable interface.


    public final void writeObject(Object o);


  • Object o – represents the object to be written.

Return value:

The return type of the method is void, it returns nothing.


// Java program to demonstrate the example 
// of void writeObject(Object o) method of 
// ObjectOutputStream

import java.io.*;

public class WriteObjectOfOOS {
 public static void main(String[] args) throws Exception {
  // Instantiates ObjectOutputStream , ObjectInputStream 
  // FileInputStream and FileOutputStream

  FileOutputStream file_out_stm = new FileOutputStream("D:\\includehelp.txt");
  ObjectOutputStream obj_out_stm = new ObjectOutputStream(file_out_stm);
  FileInputStream file_in_stm = new FileInputStream("D:\\includehelp.txt");
  ObjectInputStream obj_in_stm = new ObjectInputStream(file_in_stm);

  // By using writeObject() method is to
  // write the object to the stream		


  // By using readObject() method is to 
  // read the object
  for (int i = 0; i < 2; ++i) {
   String str = (String) obj_in_stm.readObject();
   System.out.println("obj_in_stm.readObject(): " + str);

  // By using close() method is to 
  // close all the streams 
  System.out.println("Stream Shutdown... ");



obj_in_stm.readObject(): Java
obj_in_stm.readObject(): World
Stream Shutdown... 

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