More Library Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 253

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Puzzle 83: Dyslexic Monotheism
Once upon a time, there was a man who thought there was only one exceptional dog, so he wrote
the following class, which he took to be a singleton [Gamma95] :
public class Dog extends Exception {
public static final Dog INSTANCE = new Dog();
private Dog() { }
public String toString() {
return "Woof";
}
}
It turns out that this man was wrong. Can you create a second Dog instance from outside this class
without using reflection?
Solution 83: Dyslexic Monotheism
This class may look like a singleton, but it isn't. The problem is that Dog extends Exception and
Exception implements java.io.Serializable . This means that Dog is serializable, and
deserialization constitutes a hidden constructor. If you serialize Dog.INSTANCE and deserialize the
resulting byte sequence, you will end up with another Dog , as demonstrated by the following
program. It prints false , indicating that the new Dog instance is distinct from the original, and Woof ,
indicating that the new Dog instance is functional:
import java.io.*;
public class CopyDog { // Not to be confused with copycat
public static void main(String[] args) {
Dog newDog = (Dog) deepCopy(Dog.INSTANCE);