Why does Java prohibit static fields in inner classes?

InnerClass cannot have static members because it belongs to an instance (of OuterClass). If you declare InnerClass as static to detach it from the instance, your code will compile.

class OuterClass {
static class InnerClass {
static int i = 100; // no compile error
static void f() { } // no compile error
}
}

BTW: You'll still be able to create instances of InnerClass. static in this context allows that to happen without an enclosing instance of OuterClass.

The idea behind inner classes is to operate in the context of the enclosing instance. Somehow, allowing static variables and methods contradicts this motivation?

8.1.2 Inner Classes and Enclosing Instances

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).

what I want to know is why java forbids static fields/methods inside inner classes

Because those inner classes are "instance" inner classes. That is, they are like an instance attribute of the enclosing object.

Since they're "instance" classes, it doesn't make any sense to allow static features, for static is meant to work without an instance in the first place.

It's like you try to create a static/instance attribute at the same time.

Take the following example:

class Employee {
public String name;
}

If you create two instances of employee:

Employee a = new Employee();
a.name = "Oscar";


Employee b = new Employee();
b.name = "jcyang";

It is clear why each one has its own value for the property name, right?

The same happens with the inner class; each inner class instance is independent of the other inner class instance.

So if you attempt to create a counter class attribute, there is no way to share that value across two different instances.

class Employee {
public String name;
class InnerData {
static count; // ??? count of which ? a or b?
}
}

When you create the instance a and b in the example above, what would be a correct value for the static variable count? It is not possible to determine it, because the existence of the InnerData class depends completely on each of the enclosing objects.

That's why, when the class is declared as static, it doesn't need anymore a living instance, to live itself. Now that there is no dependency, you may freely declare a static attribute.

I think this sounds reiterative but if you think about the differences between instance vs. class attributes, it will make sense.

Actually, you can declare static fields if they are constants and are written in compile time.

class OuterClass {
void foo() {
class Inner{
static final int a = 5; // fine
static final String s = "hello"; // fine
static final Object o = new Object(); // compile error, because cannot be written during compilation
}
}
}

Here is the motivation that I find best suitable for this "limit": You can implement the behavior of a static field of an inner class as an instance field of the outer object; So you do not need static fields/methods. The behaviour I mean is that all inner class instances of some object share a field(or method).

So, suppose you wanted to count all the inner class instances, you would do:

public class Outer{
int nofInner; //this will count the inner class
//instances of this (Outer)object
//(you know, they "belong" to an object)
static int totalNofInner; //this will count all
//inner class instances of all Outer objects
class Inner {
public Inner(){
nofInner++;
totalNofInner++;
}
}
}

I guess it's for consistency. While there doesn't seem to be any technical limitation for it, you wouldn't be able to access static members of the internal class from the outside, i.e. OuterClass.InnerClass.i because the middle step is not static.

1. class Initialization sequence is a critical reason.

As inner classes are dependent on the instance of enclosing/Outer class, so Outer class need to be initialized before the initialization of the Inner class.
This is JLS says about class Initialization. The point we need is, class T will be initialize if

• A static field declared by T is used and the field is not a constant variable.

So if inner class have an static field accessing that will cause initializing the inner class, but that will not ensure that the enclosing class is initialized.

2. It would violate some basic rules. you can skip to the last section (to two cases) to avoid noob stuff

One thing about static nested class, when some nested class is static it will behave just like a normal class in every way and it is associated with the Outer class.

But the concept of Inner class/ non-static nested class is it will be associated with the instance of outer/enclosing class. Please note associated with instance not the class. Now associating with instance clearly means that (from the concept of instance variable) it will exist inside a instance and will be different among instances.

Now, when we make something static we expect it will be initialized when the class is being loaded and should be shared among all instances. But for being non-static, even inner classes themselves (you can definitely forget about instance of inner class for now) are not shared with all instance of the outer/enclosing class (at least conceptually), then how can we expect that some variable of inner class will be shared among all the instance of the inner class.

So if Java allow us to use static variable inside not static nested class. there will be two cases.

• If it is shared with all the instance of inner class it will violate the concept of context of instance(instance variable). It's a NO then.
• If it is not shared with all instance it will violate the the concept of being static. Again NO.

In simple words, non-static inner classes are instance variable for outer class, and they are created only when an outer class is created and an outer class object is created at run-time while static variables are created at class loading time. So non-static inner class is runtime thing that's why static not the part of a non-static inner class.

NOTE: treat inner classes always like a variable for an outer class they may be static or non-static like any other variables.

Because it would cause ambiguity in the meaning of "static".

Inner classes cannot declare static members other than compile-time constants. There would be an ambiguity about the meaning of “static.” Does it mean there is only one instance in the virtual machine? Or only one instance per outer object? The language designers decided not to tackle this issue.

Taken from "Core Java SE 9 for the Impatient" by Cay S. Horstmann. Pg 90 Chapter 2.6.3

From Java 16 onwards, this is no longer the case. Quoting from JEP 395 (on finalizing records):

Relax the longstanding restriction whereby an inner class cannot declare a member that is explicitly or implicitly static. This will become legal and, in particular, will allow an inner class to declare a member that is a record class.

Indeed, the following code can be compiled with Java 16 (tried with 16.ea.27):

public class NestingClasses {


public class NestedClass {


static final String CONSTANT = new String(
"DOES NOT COMPILE WITH JAVA <16");


static String constant() {
return CONSTANT;
}


}


}

In the Java language designers' own words:

Since nested classes were first introduced to Java, nested class declarations that are inner have been prohibited from declaring static members... It simplifies the language's task of resolving and validating references to in-scope variables, methods, etc.

There was never any particularly grand conceptual or philosophical reason to prohibit this.

Simplifying things for the language was deemed an insufficient reason to continue to maintain this restriction. Along with the introduction of records in Java 16, they made the decision to relax the restriction.

Class Inner will be initialize if a static field declared by Inner is used and the field is not a constant variable.

class Outer{


class Inner{


static Inner obj = new Inner();
}


public static void main(String[] args){


Inner i = Inner.obj; // It woulds violate the basic rule: without existing Outer class Object there is no chance of existing Inner class Object.
}
}