1. Java IO Tutorial
2. Java IO Overview
3. Java IO: Files
4. Java IO: Streams
5. Java IO: Pipes
6. Java IO: Networking
7. Java IO: Byte & Char Arrays
8. Java System.in, System.out, and System.error
9. Java IO: Input Parsing
10. Java IO: Readers and Writers
11. Java IO: Concurrent IO
12. Java IO: Exception Handling
13. Java InputStream
14. Java OutputStream
15. Java FileInputStream
16. Java FileOutputStream
17. Java RandomAccessFile
18. Java File
19. Java IO: PipedInputStream
20. Java IO: PipedOutputStream
21. Java ByteArrayInputStream
22. Java ByteArrayOutputStream
23. Java IO: FilterInputStream
24. Java IO: FilterOutputStream
25. Java BufferedInputStream
26. Java BufferedOutputStream
27. Java PushbackInputStream
28. Java IO: SequenceInputStream
29. Java DataInputStream
30. Java IO: DataOutputStream
31. Java IO: PrintStream
32. Java IO: ObjectInputStream
33. Java IO: ObjectOutputStream
34. Java IO: Serializable
35. Java Reader
36. Java Writer
37. Java InputStreamReader
38. Java OutputStreamWriter
39. Java FileReader
40. Java FileWriter
41. Java IO: PipedReader
42. Java IO: PipedWriter
43. Java IO: CharArrayReader
44. Java IO: CharArrayWriter
45. Java BufferedReader
46. Java BufferedWriter
47. Java IO: FilterReader
48. Java IO: FilterWriter
49. Java PushbackReader
50. Java IO: LineNumberReader
51. Java StreamTokenizer
52. Java IO: PrintWriter
53. Java IO: StringReader
54. Java IO: StringWriter

• Serializable Example
• serialVersionUID
• Object Serialization Today
• More Information About Serialization

The Java Serializable interface (java.io.Serializable is a marker interface your classes must implement if they are to be serialized and deserialized. Java object serialization (writing) is done with the ObjectOutputStream and deserialization (reading) is done with the ObjectInputStream.

That Serializable is a marker interface means that it contains no methods. Therefore, a class implementing Serializable does not have to implement any specific methods. Implementing Serializable thus just tells the Java serialization classes that this class is intended for object serialization.

Serializable Example

Here is an example of a class that implements the Java Serializable interface:

import java.io.Serializable;

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

As you can see, the Person class implements the Serializable interface, but does not actually implement any methods. As mentioned earlier, the Java Serializable interface is just a marker interface so there are no methods to implement.

To see a full example of serializing and deserializing an object that implements Serializable, see my ObjectInputStream tutorial.

serialVersionUID

In addition to implementing the Serializable interface, a class intended for serialization should also contain a private static final long variable named serialVersionUID.

Here is the Person class from before, with a serialVersionUID variable added:

import java.io.Serializable;

public static class Person implements Serializable {

    private static final long serialVersionUID = 1234L;

    public String name = null;
    public int    age  =   0;
}

The serialVersionUID variable is used by Java's object serialization API to determine if a deserialized object was serialized (written) with the same version of the class, as it is now attempting to deserialize it into.

Imagine that a Person object is serialized to disk. Then a change is made to the Person class. Then you try to deserialize the stored Person object. Now the serialized Person object may not correspond to the new version of the Person class.

To detect such problems a class implementing Serializable should contain a serialVersionUID field. If you make big changes to the class, you should also change its serialVersionUID value.

The Java SDK and many Java IDEs contains tools to generate the serialVersionUID so you don't have to.

Object Serialization Today

In today's world (2015 and forward) many Java projects serialize Java objects using different mechanisms than the Java serialization mechanism. For instance, Java objects are serialized into JSON, BSON or other more optimized binary formats. This has the advantage of the objects also being readable by non-Java applications. For instance, JavaScript running in a web browser can natively serialize and deserialize objects to and from JSON.

These other object serialization mechanisms typically do not require your Java classes to implement Serializable, by the way. They are typically using Java Reflection to inspect your class, so implementing the Serializable interface would be superflous - it would not add any useful information.

I have a separate tutorial about serializing and deserializing Java objects to and from JSON here: Java JSON Tutorial .

More Information About Serialization

Object serialization is kind of a topic in its own right. This Java IO tutorial is mostly focused on the streams and readers / writers. Therefore I will not get into deeper detail about object serialization at this point. In addition,a lot has already been written online about Java object serialization. Instead of repeating it, I'll give you a link to deeper explanations of the subject.