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A reasonable serialized form for StringList is simply the number of strings in the list, followed by the strings themselves This constitutes the logical data represented by a StringList, stripped of the details of its physical representation Here is a revised version of StringList containing writeObject and readObject methods implementing this serialized form As a reminder, the transient modifier indicates that an instance field is to be omitted from a class's default serialized form:
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//StringList with a reasonable custom serialized form public class StringList implements Serializable { private transient int size = 0; private transient Entry head = null; // No longer Serializable! private static class Entry { String data; Entry next; Entry previous; } // Appends the specified string to the list public void add(String s) { } /** * Serialize this <tt>StringList</tt> instance * * @serialData The size of the list (the number of strings * it contains) is emitted (<tt>int</tt>), followed by all of * its elements (each a <tt>String</tt>), in the proper * sequence */ private void writeObject(ObjectOutputStream s) throws IOException { sdefaultWriteObject(); swriteInt(size); // Write out all elements in the proper order for (Entry e = head; e != null; e = enext) swriteObject(edata); } private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException { sdefaultReadObject(); int size = sreadInt(); // Read in all elements and insert them in list for (int i = 0; i < size; i++) add((String)sreadObject());
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// Remainder omitted
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Note that the writeObject method invokes defaultWriteObject and the readObject method invokes defaultReadObject, even though all of StringList's fields are transient If all instance fields are transient, it is technically permissible to dispense with invoking defaultWriteObject and defaultReadObject, but it is not recommended Even if all
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instance fields are transient, invoking defaultWriteObject affects the serialized form, resulting in greatly enhanced flexibility The resulting serialized form makes it possible to add nontransient instance fields in a later release while preserving backward and forward compatibility If an instance is serialized in a later version and deserialized in an earlier version, the added fields will be ignored Had the earlier version's readObject method failed to invoke defaultReadObject, the deserialization would fail with a StreamCorruptedException Note that there is a documentation comment on the writeObject method, even though it is private This is analogous to the documentation comment on the private fields in the Name class This private method defines a public API, the serialized form, and that public API should be documented Like the @serial tag for fields, the @serialData tag for methods tells the Javadoc utility to place this documentation on the serialized forms page To lend some sense of scale to the earlier performance discussion, if the average string length is ten characters, the serialized form of the revised version of StringList occupies about half as much space as the serialized form of the original On my machine, serializing the revised version of StringList is about two and one half times as fast as serializing the original version, again with a string length of ten Finally, there is no stack overflow problem in the revised form, hence no practical upper limit to the size of a StringList that can be serialized While the default serialized form would be bad for StringList, there are classes for which it would be far worse For StringList, the default serialized form is inflexible and performs badly, but it is correct in the sense that serializing and deserializing a StringList instance yields a faithful copy of the original object with all of its invariants intact This is not the case for any object whose invariants are tied to implementation-specific details For example, consider the case of a hash table The physical representation is a sequence of hash buckets containing key-value entries Which bucket an entry is placed in is a function of the hash code of the key, which is not, in general, guaranteed to be the same from JVM implementation to JVM implementation In fact, it isn't even guaranteed to be the same from run to run on the same JVM implementation Therefore accepting the default serialized form for a hash table would constitute a serious bug Serializing and deserializing the hash table could yield an object whose invariants were seriously corrupt Whether or not you use the default serialized form, every instance field that is not labeled
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