Why does Java prohibit static fields in inner classes?

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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.

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