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Serializing a non-static inner class will result in an attempt at serializing the outer class as well. If the outer class is not serializable, then serialization will fail, resulting in a runtime error.

Making the inner class static (i.e. "nested") avoids this problem, therefore inner classes should be static if possible. However, you should be aware that there are semantic differences between an inner class and a nested one: 

  an inner class can only be instantiated within the context of an instance of the outer class.
  a nested (static) class can be instantiated independently of the outer class.



Noncompliant Code Example
public class Pomegranate {
  // ...

  public class Seed implements Serializable {  // Noncompliant; serialization will fail
    // ...
  }
} 


Compliant Solution
public class Pomegranate {
  // ...

  public static class Seed implements Serializable {
    // ...
  }
}