You can instantiate an inner class without an enclosing instance.

gistfile1.java

import java.lang.reflect.*;

public class Outer {

    private String outer = "outer";

    public class Inner {
        public void hello() {
            System.out.println("Hello World" + outer); // NullPointerException at this line.
        }
    }

    public static void main(String[] args) throws Exception {
        Constructor<Inner> c = Inner.class.getConstructor(Outer.class);

        Inner instance = c.newInstance((Object) null); // Shouldn't be allowed ?
        instance.hello();
    }

}

You can never instantiate an inner class without an enclosing instance using the usual new operator. But if you use reflection API you can somehow instantiate an inner class without an enclosing instance.

Constructor class's Java doc says:
"If the constructor's declaring class is an inner class in a non-static context, the first argument to the constructor needs to be the enclosing instance; see The Java Language Specification, section 15.9.3."
, so this behavior seems to be a bug.