Confused about anonymous classes vs anonymous inner class

Question

I went searching to learn how to do lambda expressions in Java, but instead a confusion came up for me. So my understanding of an anonymous class is this:

public class SomeObject {
   public static void main(String[] args) {
    ArrayList list = new ArrayList();
    list.add(new SomeObject());
   }

}

I saw the term anonymous inner class before, but at that time, I didn't know what a regular anonymous class was. Lot of threads and videos I'm seeing seem to call anonymous inner classes just "anonymous classes." Are they synonymous? My understanding of anonymous inner class is:

 public class Rectangle {
 private double length;
 private double width;
 private double perimeter;

    public void calculatePerimeter() {
    perimeter = (2*length) +(2*width);
   }

     public static void main(String[] args) {
       Rectangle square = new Rectangle() {
        public void calculatePerimeter() {
            perimeter = 4*length;
        }
    };
   }

  }

So essentially, instead of having to write a subclass for Square, and then override the calculatePerimeter() method, I can just make a one-time square class, and override the method in their. Is this correct?

So, anonymous inner classes have to do with inheritance. I'm not understanding the use of it though. Perhaps, it's because I've never used them before, or because I don't have much programming experience. Can you can give me examples or explain when it's useful?

UPDATE: When I moved my code for the anonymous inner class to an IDE, I learned that there are errors; So apparently, the "square" doesn't even inherit the fields of the rectangle. Doesn't this make it even more useless?

Would the equivalent be:

public class Rectangle {
 private double length;
 private double width;
 private double perimeter;

    public void calculatePerimeter() {
    perimeter = (2*length) +(2*width);
   }
 }


public class Square extends Rectangle {
   @Override   
   public void calculatePerimeter() {
      perimeter = 4*getLength();
   }

  public double getLength() {
    return length;
  }



    }

Answer 1

So my understanding of an anonymous class is this:

public class SomeObject {
   public static void main(String[] args) {
    ArrayList list = new ArrayList();
    list.add(new SomeObject());
   }
}

There is no anonymous class there. The class SomeObject has a name ... therefore it is not anonymous. In fact, it is just a normal (non-nested, non-inner, non-anonymous) Java class.

---

I saw the term anonymous inner class before, but at that time, I didn't know what a regular anonymous class was.

There is no such thing as a "regular anonymous class". All Java anonymous classes are "inner".

As the JLS says:

"An inner class is a nested class that is not explicitly or implicitly declared static.

Inner classes include local (§14.3), anonymous (§15.9.5) and non-static member classes (§8.5)."

---

So, anonymous inner classes have to do with inheritance.

Anonymous inner classes do involve inheritance, but that's not what makes them "inner". See above.

---

I meant the "list.add(I meant the "list.add(new SomeObject());". All this time, I thought the object you added to the ArrayList, was called an anonymous class since we didn't name it.);". All this time, I thought the object you added to the ArrayList, was called an anonymous class since we didn't name it.

You are incorrect. An object is not a class¹.

The new SomeObject() is creating an object, not a class. But that's just normal. Objects / instances don't have names ... as far as the JLS is concerned.

Now variables and fields have names ... but variables are not objects / instances or classes. They are bindings between a name and a slot that can hold a reference to an object (if that's what the type declaration allows).

^{1 - except in the case of instances of java.lang.Class ... and even then the object is not actually the class / type from a theoretical standpoint.}

---

Or is it called simply an anonymous object and I had two mixed up?

Nope. Objects don't have names. All Java objects are "anonymous". It is not a useful distinction to make. (And see above where I talk about variables ...)

---

As for your Rectangle / Square examples, they have nothing to do with anonymous classes, inner classes, nested classes or anything like that. They are just top-level classes, using ordinary Java inheritance. (Not that I'm suggesting there is another "non-ordinary" kind of inheritance ...)