Serializable variables
outside Serializable lambdas, local and anonymous classes.
When a local variable is referenced from an anonymous class, its value
is stored in an implicit field of that class. The same happens
for local classes and lambdas. If the variable is of a
non-Serializable type, serialization will fail.
Example:
interface A extends Serializable {
abstract void foo();
}
class B {}
class C {
void foo() {
B b = new B();
A a = new A() {
@Override
public void foo() {
System.out.println(b); // warning
}
};
}
}