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Java ObjectOutputStream writeFields() Method with Example

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

ObjectOutputStream Class writeFields() method

  • writeFields() method is available in java.io package.
  • writeFields() method is used to write the buffered fields to the ObjectOutputStream.
  • writeFields() 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.
  • writeFields() method may throw an exception at the time of writing fields.
    • IOException: This exception may throw when getting any input/output error while writing to the output stream.
    • NotActiveException: This exception may throw when writeObject() was not invoked to write the state of the object of classes.


    public void writeFields();


  • It does not accept any parameter.

Return value:

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


// Java program to demonstrate the example 
// of void writeFields() method of ObjectOutputStream

import java.io.*;

public class WriteFieldsOfOOS {
 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 writeInt() method is to write
  // integer to the obj_out_stm stream
  obj_out_stm.writeObject(new ReadFields());


  // By using readObject() method is to read
  //  object and display fields
  ReadFields rf = (ReadFields) obj_in_stm.readObject();
  System.out.println("obj_in_stm.writeFields(): " + rf.val);

 static public class ReadFields implements Serializable {
  static int val = 12685;

  private static final ObjectStreamField[] serialPersistentFields = {
   new ObjectStreamField("val", Integer.TYPE)

  private void readObject(ObjectInputStream oin)
  throws IOException, ClassNotFoundException {

   // Invokes readFields()
   ObjectInputStream.GetField fi = oin.readFields();

   // By using get() is to get var
   val = fi.get("val", 0);

  private void writeObject(ObjectOutputStream oos) throws IOException {
   // By using putFields() method is to
   // write var into ObjectStreamField[]
   ObjectOutputStream.PutField fi = oos.putFields();
   fi.put("val", val);

   // Invokes writeFields()


obj_in_stm.writeFields(): 12685

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