Java IO.pdf - Nguyen Dang Binh

}
}
Vector v = (Vector) oin.readObject();
Enumeration e = v.elements();
while (e.hasMoreElements()) {
System.out.println(e.nextElement());
}
Java I/O
You may argue with my choice of name here; ExternalizableVector may seem more
accurate. However, from the perspective of a programmer using a class, it doesn't matter
whether a class is serializable or externalizable. In either case, instances of the class are
merely passed to the writeObject() method of an object output stream or read by the
readObject() method of an object input stream. The difference between Serializable and
Externalizable is hidden from the end user.
The writeExternal() method first writes capacityIncrement and elementCount onto the
stream using writeInt(). It then loops through all the elements in the vector, testing each
one with instanceof to see whether or not it's serializable. If the element is serializable, it's
written with writeObject(); otherwise, null is written instead. The readExternal()
method simply reads in the data and sets the appropriate fields in Vector.
capacityIncrement is read first, then elementCount. The elementData array was not
directly written onto the output in writeExternal(); instead, its individual elements were
written. Thus, a new elementData array is created with length elementCount. Finally, the
individual elements are read out and stored in elementData in the same order they were
written.
The main() method tests the program by serializing and deserializing a
SerializableVector that contains assorted serializable and nonserializable elements. Its
output is:
D:\JAVA>java SerializableVector
Element 1
9
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null
Element 1
9
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Other schemes are possible and might be useful in some circumstances. Since elementData
itself isn't stored but only recreated from its length, one obvious possibility is to omit the
nonserializable elements when writing the vector and to adjust the elementCount
accordingly. For example:
public void writeExternal(ObjectOutput out) throws IOException {
out.writeInt(capacityIncrement);
int numSerializable = 0;
for (int i = 0; i < elementCount; i++) {
if (elementData[i] instanceof Serializable) {
numSerializable++;
}
}
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