问题
The Java Tutorial says that since an inner class is associated with an instance of the enclosing class, it (the inner class) cannot define any static members itself.
It's interesting for me why can't inner classes declare static members for instance, some private static field, which the instance of this inner class could possibly share with the other instances of the same inner class? is this just an implementation of things in Java that has to be taken for granted or something else?
回答1:
Basically just an arbitrary decision. there's no reason it couldn't be supported, but there is also not really any good reason to support it. just declare the static field in the outer class.
also, that quote may not be entirely correct: i believe you can declare a static serialVersionUID in an inner class.
回答2:
Because the Java Language Specification says so:
An inner class is a nested class that is not explicitly or implicitly declared static. Inner classes may not declare static initializers (§8.7) or member interfaces. Inner classes may not declare static members, unless they are compile-time constant fields (§15.28).
As for why it was specified that way, I do not know. My guess is that inner classes were designed as small helper classes that should be very limited in complexity.
回答3:
An inner class may not declare static fields unless they are compile-time constants. Hence, if you declare the static field as final it will work.
class Foo {
class Test {
final static int i = 10;
}
}
will compile and run perfectly
static fields can only be declared in static or top-level types. Hence, a (pure) static variable can be declared only in a static class.
回答4:
That would result in a conflict of interest to have a static member variable inside an inner class. Generally speaking, an inner class needs to have an object instance of the outer or enclosing class before it may be instantiated. A static member variable suggests that you don't even need an object instance for that particular class, the inner class in this case, but that class the inner class is dependent upon and can only coexist along with the outer class instance. Do you see where the conflict of interest arises in the argument? However, you can make a static member variable inside an inner class by declaring the inner class as static which means the inner class no longer needs to coexist with an outer class object.
public class A {
public static class B {
private static JPanel myJPanel;
}
}
来源：https://stackoverflow.com/questions/8554813/why-cant-inner-classes-declare-static-members