Java - ObjectInputStream Class

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Introduction

The Java ObjectInputStream class deserializes primitive data and objects previously written using an ObjectOutputStream. Following are the important points about BufferedInputStream −

• It is used to recover those objects previously serialized. It ensures that the types of all objects in the graph created from the stream match the classes present in the Java Virtual Machine.

• Classes are loaded as required using the standard mechanisms.

Class declaration

Following is the declaration for Java.io.ObjectInputStream class −

public class ObjectInputStream
   extends InputStream
      implements ObjectInput, ObjectStreamConstants

Class constructors

Class methods

Methods inherited

This class inherits methods from the following classes −

• Java.io.InputStream
• Java.io.Object
• Java.io.ObjectInput

Example - Usage of ObjectInputStream available() method

The following example shows the usage of Java ObjectInputStream available() method.

ObjectInputStreamDemo.java

package com.tutorialspoint;

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

public class ObjectInputStreamDemo {
   public static void main(String[] args) {
      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.writeUTF("Hello World");
         oout.flush();

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

         // check how many bytes are available
         System.out.println("" + ois.available());
      } catch (Exception ex) {
         ex.printStackTrace();
      }
      
   }
}

Output

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

13

Example - Closing an ObjectInputStream After Reading an Object

The following example shows the usage of Java ObjectInputStream close() method. This example writes an object to a file, reads it using ObjectInputStream, and then closes the stream.

ObjectInputStreamDemo.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;

class Person implements Serializable {
   private static final long serialVersionUID = 1L;
   String name;
   int age;

   public Person(String name, int age) {
      this.name = name;
      this.age = age;
   }

   @Override
   public String toString() {
      return "Person{name='" + name + "', age=" + age + "}";
   }
}

public class ObjectInputStreamDemo {
   public static void main(String[] args) {
      try {
         // Serialize object to file
         ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("person.dat"));
         oos.writeObject(new Person("Alice", 30));
         oos.close();

         // Read object from file
         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("person.dat"));
         Person person = (Person) ois.readObject();
         System.out.println("Read Object: " + person);

         // Close the ObjectInputStream
         ois.close();

         // Trying to read again after closing
         System.out.println("Trying to read again...");
         ois.readObject(); // This will throw an IOException

      } catch (IOException | ClassNotFoundException e) {
         System.out.println("Exception: " + e.getMessage());
      }
   }
}

Output

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

Read Object: Person{name='Alice', age=30}
Trying to read again...
Exception: Stream Closed

Explanation

• Writes a Person object to a file (person.dat).

• Reads the object using ObjectInputStream.

• Closes the stream using ois.close().

• Attempts to read again after closing, which throws an IOException: Stream Closed.

Example - Using defaultReadObject() for Basic Serialization

The following example shows the usage of Java ObjectInputStream defaultReadObject() method. This example demonstrates default serialization using defaultReadObject()

ObjectInputStreamDemo.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;

class Person implements Serializable {
   private static final long serialVersionUID = 1L;
   String name;
   int age;

   public Person(String name, int age) {
      this.name = name;
      this.age = age;
   }
   
   private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
      ois.defaultReadObject(); // Restores default serialized fields
   }

   @Override
   public String toString() {
      return "Person{name='" + name + "', age=" + age + "}";
   }
}

public class ObjectInputStreamDemo {
   public static void main(String[] args) {
      try {
         // Serialize object
         ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("person.dat"));
         oos.writeObject(new Person("Alice", 30));
         oos.close();

         // Deserialize object
         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("person.dat"));
         Person person = (Person) ois.readObject();
         ois.close();

         System.out.println("Deserialized Object: " + person);
      } catch (IOException | ClassNotFoundException e) {
         e.printStackTrace();
      }
   }
}

Output

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

Deserialized Object: Person{name='Alice', age=30}

Explanation

• The Person object is serialized and written to person.dat.

• During deserialization, defaultReadObject() restores the name and age fields.

• The deserialized object is printed correctly.