Non-static inner classes contain a reference to an instance of the outer class. Hence, 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, serialization will fail, resulting in a runtime error.
Making the inner class static
(i.e., "nested") avoids this problem, as no reference to an instance of the outer class is required.
Serializing the inner class can be done independently of the outer class. Hence, inner classes implementing Serializable
should be
static
if the outer class does not implement Serializable
.
Be aware of the semantic differences between an inner class and a nested one:
static
) class can be instantiated independently of the outer class. Make the inner class static
or make the outer class Serializable
.
public class Pomegranate { // ... public class Seed implements Serializable { // Noncompliant, serialization will fail due to the outer class not being serializable // ... } }
public class Pomegranate { // ... public static class Seed implements Serializable { // Compliant, the outer class will not be serialized and hence cannot be the cause for a failure at runtime // ... } }