The obj) method writes an "unshared" object to the ObjectOutputStream. This method is identical to writeObject, except that it always writes the given object as a new, unique object in the stream (as opposed to a back-reference pointing to a previously serialized instance).Specifically −

  • An object written via writeUnshared is always serialized in the same manner as a newly appearing object (an object that has not been written to the stream yet), regardless of whether or not the object has been written previously.

  • If writeObject is used to write an object that has been previously written with writeUnshared, the previous writeUnshared operation is treated as if it were a write of a separate object. In other words, ObjectOutputStream will never generate back-references to object data written by calls to writeUnshared.

While writing an object via writeUnshared does not in itself guarantee a unique reference to the object when it is deserialized, it allows a single object to be defined multiple times in a stream, so that multiple calls to readUnshared by the receiver will not conflict. Note that the rules described above only apply to the base-level object written with writeUnshared, and not to any transitively referenced sub-objects in the object graph to be serialized.

ObjectOutputStream subclasses which override this method can only be constructed in security contexts possessing the "enableSubclassImplementation" SerializablePermission; any attempt to instantiate such a subclass without this permission will cause a SecurityException to be thrown.


Following is the declaration for method.

public void writeUnshared(Object obj)


obj − object to write to stream.

Return Value

This method does not return a value.


  • NotSerializableException − If an object in the graph to be serialized does not implement the Serializable interface.

  • InvalidClassException − If a problem exists with the class of an object to be serialized.

  • IOException − If an I/O error occurs during serialization.


The following example shows the usage of method.

package com.tutorialspoint;


public class ObjectOutputStreamDemo {

   public static void main(String[] args) {
      Object s = "Hello World!";
      Object i = 1974;
      try {

         // create a new file with an ObjectOutputStream
         FileOutputStream out = new FileOutputStream("test.txt");
         ObjectOutputStream oout = new ObjectOutputStream(out);

         // write something in the file

         // close the stream

         // 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("" + ois.readUnshared());
         System.out.println("" + ois.readUnshared());

      } catch (Exception ex) {


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

Hello World!


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