Java：我可以在外部 class 中创建可变类型的内部 class 吗？

Question

So We were covering the topic of LinkedList and noticed a variable of type Node (inner class name) was declared on the outer class SingleLinkedList. Why is this behavior allowed? I knew you could access members of your inner class in your outer class but not declare variables of the inner class

private class SingleLinkedList {

        private Node head; //This here is my doubt 
         //.
         //.
         //.
                
      private class Node {

        // reference to the next node in the chain, or null if there isn't one.
        Node next;
 
        // data carried by this node. could be of any type you need.
        Object data;
 
        // Node constructor
        public Node(Object dataValue) {
            next = null;
            data = dataValue;
        }
}

Thanks in advance.

Answer 1

nothing wrong with it, you define an inner class, then an instance of it. you can also declare instances of inner classes later defined in the outer class - the compiler is smart enough to make several passes through the code and find the references.

About the second question, you can do:

class Person {
    class Friend {
       Person p1,p2;
       Friend(Person a, Person b) { p1=a;p2=a}
    }
    List<Friend> friends;
}

But this will fail:

class Person {
    Person(String name) {}

    class Friend {
       

       Person p1=new Person("john"), p2=new Person("mike");

       Friend(Person a, Person b) { p1=a;p2=a}

    }
    List<Friend> friends;
}

Answer 2

Think about it: if not in the "SingleLinkedList" class scope, where else can "Node" be used?

Answer 3

Java allows you to define a class within another class, ie a nested class or an inner class. In Java, an inner/nested class is a member of its enclosing class.

class OuterClass {
   ...
   class InnerClass {
     ...
   }
}

Q1: Why is this behavior allowed?

A1: Here are three compelling reasons:

• More encapsulation: If InnerClass above was declared private, it can only be accessed by members of OuterClass . It would be invisible to the outside world.

• A class used only one place: If InnerClass is only used by OuterClass and no other class and is a component of OuterClass , it would make it more appropriate to define it inside OuterClass .

• More readability: Nesting classes in the location where they are relevant and used, makes the code more readable.

Q2: I knew you could access members of your inner class in your outer class but not declare variables of the inner class

A2: Yes, as mentioned before, inner classes are members of the enclosing class.

• So you can declare a variable of the inner class within the enclosing class ( OuterClass ).

• Also, an instance of InnerClass only exists within an instance of the OuterClass . The inner instance has direct access to the methods and fields of its enclosing instance (even if they are declared private).

To take this one step further, to create an object of inner class from a scope outside of both classes, 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();