The Serializable interface is a marker interface–in other words, it contains no methods. Its purpose is to categorize the classes that may be serialized using Java’s serialization API. Only the objects that implement the Serializable interface can be serialized. This means that any object that needs to be serialized can contain only Serializable objects. For example, consider this class:

 1. public class TestSerializable implements Serializable {2.   public String s = "Hello";3.   public NonSerializableObject o = new NonSerializableObject(s);	4. }

The class NonSerializableObject is defined as:

 1. public class NotSerializableObject {2.   public String str_ = null;3.   4.   public NonSerializableObject(String str) { str_ = str; }5. 6.   public String getStr () { return str_; }7. }

If you attempt to run this code, which uses the TestSerializable class, you will get a java.io.NotSerializableException:

 1. TestSerializable ts = new TestSerializable();2. FileOutputStream fos = new FileOutputStream("myObject.ser");3. ObjectOutputStream oos = new ObjectOutputStream(fos);4. oos.writeObject(ts);5. oos.flush();6. oos.close();

The exception will be thrown on Line 4, where you attempt to write out the object. The reason is that the class TestSerializable contains a variable of type NonSerializableObject, which is not serializable (because it does not implement the Serializable interface). To get around this problem, you simply need to modify the class NonSerializableObject so that it implements the Serializable interface.

Suppose you can’t change the definition of the class NonSerializableObject. In that case, you have two options. One is to eliminate serializing the object of type NonSerializableObject (see Tip “Avoid Serialization Using the Transient Modifier”). The other option is to override the default serialization behavior for the class TestSerialization (see Tip “Customizing Serialization”).