Inner/Outer class obj.new

Question

This is the example of member inner class that is invoked outside a class.

//Program of memeber inner class that is invoked outside a class

class Outer {
    private int data=10;

    class Inner {
        void msg() {
            System.out.println("Data: " + data);
        }
    }
}

class Test {
    public static void main(String args[]) {
          Outer obj=new Outer();
          Outer.Inner in = obj.new Inner();
          in.msg();
    }
}

Could someone explane this line of code: Outer.Inner in = obj.new Iner(); what does obj.new Inner() mean?

Answer 1

It's important to understand that Outer and Inner are related. More specifically, you need an Outer instance in order to create an Inner instance.

Outer.Inner in = obj.new Inner();

creates an Inner instance from obj, an Outer instance. You can see that these two are related in that the msg() method of in will use obj's data field: in makes use of obj's state. If Inner was static it wouldn't have any relationship with Outer, so you could just use

Outer.Inner in = new Outer.Inner();  // no Outer instance needed

Of course in your case you can't simply make Inner static because it uses the data field.

Answer 2

Outer.Inner in = obj.new Iner();

Using reference of outer object you are creating object for Iner class, because Iner is part of Outer class. Here is more information.