在main方法中实例化内部类

Question

When we try to instantiate an Inner class (aka: non-static Nested class) in java, say we do that in two cases:

1.in a main method in the same file of the outer Class in which we have two options: (ex:)

public class Test1
{
   class InnerClass
   {

   }
  public static void main(String[] args)
  {
     InnerClass inner = new Test1().new InnerClass();
  }
}

or :

public class Test1
{
   class InnerClass
   {

   }
  public static void main(String[] args)
  {
     Test1.InnerClass inner = new Test1().new InnerClass();
  }
}

2. In a another class (say different file), then we have the second option only and using the first option requires us (of course) to import the InnerClass..,

Q: could you please explain why do we have the first option (without any import required) in the first case (the main method in the same file)?

Edit:

I guess the answer to the first question is some how related to the core idea of inner classes. but then +Q:

Q: Isn't an inner class a regular-member of an outer class, so if the inner class is not declared static (static nested class) then I suppose it is a non-static member and consequently its reference type, so why are we able to declare it within a static context (static method) ?

Answer 1

Simply, it is because when you instantiate the inner class from a main method outside the class in which the inner class is present, the Java compiler has no way on knowing inside which class lies that inner class. Hence you have to do

Test1.InnerClass innerClassObject = ...

instead of

InnerClass innerClassObject = ...

Answer 2

An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance.

To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:

OuterClass.InnerClass innerObject = outerObject.new InnerClass();

From Oracle Docs : https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html