Java ObjectInputStream Class

The ObjectInputStream class of the java.io package can be used to read objects that were previously written by ObjectOutputStream.

It extends the InputStream abstract class.

Before you learn about the ObjectInputStream class, make sure you know about the ObjectOutputStream Class.

Working of ObjectInputStream

The ObjectInputStream is mainly used to read data written by the ObjectOutputStream.

Basically, the ObjectOutputStream converts Java objects into corresponding streams. This is known as serialization. Those converted streams can be stored in files or transferred through networks.

Now, if we need to read those objects, we will use the ObjectInputStream that will convert the streams back to corresponding objects. This is known as deserialization.

Create an ObjectInputStream

In order to create an object input stream, we must import the java.io.ObjectInputStream package first. Once we import the package, here is how we can create an input stream.

// Creates a file input stream linked with the specified file
FileInputStream fileStream = new FileInputStream(String file);

// Creates an object input stream using the file input stream
ObjectInputStream objStream = new ObjectInputStream(fileStream);

In the above example, we have created an object input stream named objStream that is linked with the file input stream named fileStream.

Now, the objStream can be used to read objects from the file.

Methods of ObjectInputStream

The ObjectInputStream class provides implementations of different methods present in the InputStream class.

read() Method

• read() - reads a byte of data from the input stream
• readBoolean() - reads data in boolean form
• readChar() - reads data in character form
• readInt() - reads data in integer form
• readObject() - reads the object from the input stream

Example 1: Java ObjectInputStream

Let's see how we can use the ObjectInputStream class to read objects written by the ObjectOutputStream class.

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

class Main {
    public static void main(String[] args) {

        int data1 = 5;
        String data2 = "This is programiz";

        try {
            FileOutputStream file = new FileOutputStream("file.txt");
            ObjectOutputStream output = new ObjectOutputStream(file);

            // Writing to the file using ObjectOutputStream
            output.writeInt(data1);
            output.writeObject(data2);

            FileInputStream fileStream = new FileInputStream("file.txt");
            // Creating an object input stream
            ObjectInputStream objStream = new ObjectInputStream(fileStream);

            //Using the readInt() method
            System.out.println("Integer data :" + objStream.readInt());

            // Using the readObject() method
            System.out.println("String data: " + objStream.readObject());

            output.close();
            objStream.close();
        }
        catch (Exception e) {
            e.getStackTrace();
        }
    }
}

Output

Integer data: 5
String data: This is programiz

In the above example, we have used the readInt() and readObject() method to read integer data and object data from the file.

Here, we have used the ObjectOutputStream to write data to the file. We then read the data from the file using the ObjectInputStream.

Example 2: Java ObjectInputStream

Let's see another practical example,

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

class Dog implements Serializable {

    String name;
    String breed;

    public Dog(String name, String breed) {
        this.name = name;
        this.breed = breed;
    }
}

class Main {
    public static void main(String[] args) {

        // Creates an object of Dog class
        Dog dog = new Dog("Tyson", "Labrador");

        try {
            FileOutputStream file = new FileOutputStream("file.txt");

            // Creates an ObjectOutputStream
            ObjectOutputStream output = new ObjectOutputStream(file);

            // Writes objects to the output stream
            output.writeObject(dog);

            FileInputStream fileStream = new FileInputStream("file.txt");

            // Creates an ObjectInputStream
            ObjectInputStream input = new ObjectInputStream(fileStream);

            // Reads the objects
            Dog newDog = (Dog) input.readObject();

            System.out.println("Dog Name: " + newDog.name);
            System.out.println("Dog Breed: " + newDog.breed);

            output.close();
            input.close();
        }

        catch (Exception e) {
            e.getStackTrace();
        }
    }
}

Output

Dog Name: Tyson
Dog Breed: Labrador

In the above example, we have created

• ObjectOutputStream named output using the FileOutputStream named file
• ObjectInputStream named input using the FileInputStream named fileStream
• An object dog of the Dog class

Here, we have then used the object output stream to write the object to the file. And, the object input stream to read the object from the file.

Note: The Dog class implements the Serializable interface. It is because the ObjectOutputStream only writes the serializable objects to the output stream.

Other Methods Of ObjectInputStream

To learn more, visit Java ObjectInputStream (official Java documentation).