ObjectInputStream

The Java ObjectInputStream class (java.io.ObjectInputStream) enables you to read Java objects from an InputStream instead of just raw bytes. You wrap an InputStream in a ObjectInputStream and then you can read objects from it. Of course the bytes read must represent a valid, serialized Java object. Otherwise reading objects will fail.

Normally you will use the ObjectInputStream to read objects written (serialized) by a Java ObjectOutputStream . You will see an example of that later.

ObjectInputStream Example

Here is a Java ObjectInputStream example:

ObjectInputStream objectInputStream =
    new ObjectInputStream(new FileInputStream("object.data"));

MyClass object = (MyClass) objectInputStream.readObject();
//etc.

objectInputStream.close();

For this ObjectInputStream example to work the object you read must be an instance of MyClass, and must have been serialized into the file "object.data" via an ObjectOutputStream.

Before you can serialize and de-serialize objects the class of the object must implement java.io.Serializable. For more info, see Java Serializable.

Using an ObjectInputStream With an ObjectOutputStream

I promised earlier to show you an example of using the Java ObjectInputStream with the ObjectOutputStream. Here is that example:

import java.io.*;

public class ObjectInputStreamExample {

    public static class Person implements Serializable {
        public String name = null;
        public int    age  =   0;
    }


    public static void main(String[] args) throws IOException, ClassNotFoundException {

        ObjectOutputStream objectOutputStream =
            new ObjectOutputStream(new FileOutputStream("data/person.bin"));

        Person person = new Person();
        person.name = "Jakob Jenkov";
        person.age  = 40;

        objectOutputStream.writeObject(person);
        objectOutputStream.close();


        ObjectInputStream objectInputStream =
            new ObjectInputStream(new FileInputStream("data/person.bin"));

        Person personRead = (Person) objectInputStream.readObject();

        objectInputStream.close();

        System.out.println(personRead.name);
        System.out.println(personRead.age);
    }
}

This example first creates an ObjectOutputStream connected to a FileOutputStream. Then it creates a Person object and writes it to the ObjectOutputStream, and then closes the ObjectOutputStream.

Then the example creates an ObjectInputStream connected to the same file the ObjectOutputStream was connected to. The example then reads in an object from the ObjectInputStream and casts it to a Person object. After that the ObjectInputStream is also closed, and the values read into the Person object are printed to System.out.

The output printed from running this example should be:

Jakob Jenkov
40