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    <p>serialversionuid is not automatically generated because it is dangerous. When serialversionuid is set, it implies that two versions of a class are compatible with respect to serialization.</p> <p>Imagine you have a class called Foo, and it has <strong>no serialversionuid</strong> (the default), and you serialize an instance of Foo to a file. Later, you add some new members to the Foo class. If you try to deserialize the Foo object from the file, you will get a serialization failure stating that the objects are incompatible. They <em>are</em> incompatible, this is <em>what you want</em> and is the default. They are incompatible because new members in the Foo class cannot be initialized from the old serialized instance of Foo.</p> <p>Now, you might say, "I don't care, in my application it is acceptable for those fields to be uninitialized". If that <em>really</em> is the case, you can set the serialversionuid of the <em>new</em> Foo class to be the same as the <em>old</em> Foo class. This will tell Java that the objects are compatible with respect to serializablity, and Java will not complain when you deserialize the old Foo instance into the new Foo class (but the new fields will still be uninitialized).</p> <p>If you are creating a new class for the first time, and you set the serialversionuid, <strong><em>you are entering a contract</em></strong>. That contract is, <em>"For all future versions of this class with the same serialversionuid, I will guarantee they are compatible with respect to state and serialization"</em>.</p> <p>If you change a class, and you <em>explicitly</em> want to <em>disallow</em> deserialization of old versions, you can change the serialversionuid to a new value. This will cause an exception to be thrown if an old object is attempted to be deserialized into a new class instance.</p>
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    1. POWhy isn't the serialVersionUID automatically generated?
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    1. POWhy isn't the serialVersionUID automatically generated?
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    1. COThat makes sense. That is something else for me to think about.
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    2. CONot sure why this is marked as the correct answer since serialVersionUID *is* automatically generated. Furthermore, depending on the situation, there can be value in automatically generated IDs, since they ensure a new number when class is modified, instead of someone changing the code and then forgetting. But obviously not for all cases, as this post correctly points out.
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    3. COThis is so simple, and yet so few people understand it (or can be bothered to understand it?). I am guessing from the amount of caps lock you use that you are also frustrated by this.
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      1. USTom Anderson
 

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