Explain the purpose of the ObjectInputStream and ObjectOutputStream classes.

The ObjectInputStream and ObjectOutputStream classes in Java are part of the Java Input/Output (I/O) system and are used for reading and writing objects, respectively. These classes are primarily designed for object serialization, which is the process of converting an object's state into a byte stream and reconstructing the object from that byte stream. Serialization is essential for scenarios like storing objects in files, sending objects over a network, or saving object states between different program executions.

1. ObjectOutputStream:

   • Purpose: This class is used to serialize objects. It takes an object and writes its state to a stream in a platform-independent and standardized format.
   • Methods: The key method is writeObject(Object obj), which writes the specified object to the underlying stream.
   import java.io.Serializable;
   
   public class MyClass implements Serializable {
   // Class members and methods
   }
   
   

• ObjectInputStream:

  • Purpose: This class is used to deserialize objects. It reads a byte stream and reconstructs the original object, restoring its state.
  • Methods: The primary method is readObject(), which reads the next object from the input stream and returns it as a generic Object. The caller must cast the returned object to its appropriate type.
  ObjectInputStream ois = new ObjectInputStream(new FileInputStream("data.ser"));
  MyClass obj = (MyClass) ois.readObject();
  ois.close();
  
  

It's important to note that for an object to be serializable, its class must implement the Serializable interface. This interface acts as a marker, indicating that the class can be serialized. If a class does not implement Serializable, an exception (NotSerializableException) will be thrown at runtime.

Here's a simple example of a serializable class:

These classes are powerful tools for handling object persistence and communication in Java, allowing objects to be easily saved, transmitted, and reconstructed.