I know that nested class has not had static member,
class OuterClass {
class InnerClass {
private static int x = 0; // this is error
}
}
But when we declared it with final
class OuterClass {
class InnerClass {
private static final int x = 0; // ok
}
}
is it related with the final keyword, because variable can't be changed anymore? Or is there another reason for this?
This behavior is mandated by the Java Language Specification:
8.1.3. Inner Classes and Enclosing Instances
An inner class is a nested class that is not explicitly or implicitly declared static.
An inner class may be a non-static member class (§8.5), a local class (§14.3), or an anonymous class (§15.9.5). A member class of an interface is implicitly static (§9.5) so is never considered to be an inner class.
It is a compile-time error if an inner class declares a static initializer (§8.7).
It is a compile-time error if an inner class declares a member that is explicitly or implicitly static, unless the member is a constant variable (§4.12.4).
An inner class may inherit static members that are not constant variables even though it cannot declare them.
A nested class that is not an inner class may declare static members freely, in accordance with the usual rules of the Java programming language.
Example 8.1.3-1. Inner Class Declarations and Static Members
class HasStatic { static int j = 100; } class Outer { class Inner extends HasStatic { static final int x = 3; // OK: constant variable static int y = 4; // Compile-time error: an inner class } static class NestedButNotInner{ static int z = 5; // OK: not an inner class } interface NeverInner {} // Interfaces are never inner }
Note that this changed with Java 16:
All of the rules that apply to nested classes apply to inner classes. In particular, an inner class may declare and inherit
staticmembers (§8.2), and declare static initializers (§8.7), even though the inner class itself is notstatic.
Because private static final int x = 0; is compile time constant.
According java doc,
Inner classes may not declare static members, unless they are compile-time constant fields
