Java Examples in a Nutshell, 3rd Edition

Externalizable Classes

The Externalizable interface extends Serializable and defines the writeExternal( ) and readExternal( ) methods. An Externalizable object may be serialized as other Serializable objects are, but the serialization mechanism calls writeExternal( ) and readExternal( ) to perform the serialization and deserialization. Unlike the readObject( ) and writeObject( ) methods in Example 10-2, the readExternal( ) and writeExternal( ) methods can’t call the defaultReadObject( ) and defaultWriteObject( ) methods: they must read and write the complete state of the object by themselves.

It is useful to declare an object Externalizable when the object already has an existing file format or when you want to accomplish something that is simply not possible with the standard serialization methods. Example 10-3 defines the CompactIntList class, an Externalizable subclass of the IntList class of Example 2-7. CompactIntList makes the assumption that it is typically used to store small integers that fit in two bytes instead of four; it implements Externalizable so it can define a serialized form that is more compact than the format used by ObjectOutputStream and ObjectInputStream.

Example 10-3. CompactIntList.java

package je3.serialization; import je3.classes.IntList; import java.io.*; /** * This subclass of IntList assumes that most of the integers it contains are * less than 32,000. It implements Externalizable so that it can define a * compact serialization format that takes ...