Trying to serialize inner class and getting various errors.

Question

Hi, I'm working on a program that contains the inner class User. user implements serializable, and writes just fine. For some strange reason, though, after I read it, I keep getting NullPointerExceptions in my code.
I also tried it using externalization. The User class is essentially built around a HashMap. I have the following methods:
public void writeExternal(ObjectOutput out) throws IOException {
out.writeObject(mainMap);
public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
mainMap = (Map) in.readObject();
String test = (String) in.readObject();
Whenever inputstream.readObject() is called, though, I get an InvalidClassException; "no valid constructor." The sad part is that I have the method:
public User () {
main = new HashMap();
Does anyone know what the problem is??? Thanks a lot!

Answer 1

Does anyone know what the problem is???The inner class is not a static inner class. Therefore your constructor User, is actually User(OuterClass). Solution - make the class static OR use read and/or write resolution to reconstruct the relationship.