Example

public static class Object1 implements Externalizable{

private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
s.defaultReadObject();
}
}

Solution
A class implementing Externalizable should always have constructor with zero arguments


public static class Object1 implements Externalizable{

private Object1 (){

}

private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
s.defaultReadObject();
}
}