Understanding OO class definition

Question

This is an extract from IB Computer Science Higher Level Paper 2 November 2010, Q1b

I am trying to understand what are the “roles” of: “Node” and “item” in the following class definition << from question >>

class Node (1)
{
public int item; (2)
public Node next: (3)

public Node(int d) (4)
{
item = d; (5)
next = null; (5)
}
public void displaynode()
{
output(Item + “ “);
}

<< end of question extract >>

The numbers in parentheses after certain lines in the class definition are my references used in the questions below.

Is the following correct?

(1) Node is the name of the class which is used when I want to create a new, single Node by issuing Node x = new Node(5) which results in a new Node containing the value 5, stored in (4,5).

(2,3) These are data items with the labels item and next, of type integer (2) and Node (3) respectively (I don’t understand what it means to have type Node) ???? Being public can I access and alter the contents by using following references x.item, x.Node ???

(4) This is the method Node which accepts a single, integer parameter named d.

(5) The input parameter “d “ gets put in the object variable “item”, and “next” is set to the value Null when a new Node is created.

Answer 1

(1) Node is the name of the class which is used when I want to create a new, single Node by issuing Node x = new Node(5) which results in a new Node containing the value 5, stored in (4,5).

This is a reasonable explanation.

(2,3) These are data items with the labels item and next, of type integer (2) and Node (3) respectively (I don’t understand what it means to have type Node) ???? Being public can I access and alter the contents by using following references x.item, x.Node ???

Please call them variables rather than "data items". item and next are instance variables, and since next is of type Node, we can say that class Node is recursively defined. This an implementation of a linked list in which each element contains an integer value and a reference to the next element.

You're correct about the meaning of public.

(4) This is the method Node which accepts a single, integer parameter named d.

It's important that you recognize that this is a constructor method which is automatically called to initialize the state of a Node when it's instantiated.

(5) The input parameter “d “ gets put in the object variable “item”, and “next” is set to the value Null when a new Node is created.

I'd rather say the value of input parameter d gets assigned to the variable item. I wouldn't call it an object variable - in some languages, ints are considered objects, in others not. In some perspectives, variables themselves are seen as primitive objects, distinct from their values. Regardless, I think it detracts from the clarity to say "object variable".

For further study, look into the following distinctions:

• Variable vs value: A variable is an element that holds a value. Variables allow us to write code that operate over different values. Values can be anything - numbers, text, arrays, records, objects, functions, and more, depending on the language.

• Class vs object: A class is a blueprint for a state machine, while an object is a specific instance of one. Objects have state and methods which operate on that state.

• Class vs type: A class declares what goes on inside a state machine. A type declares (what is known for) a set of values. Value types are associated with the operators that can be called on those types of values. Object types declare the methods that can be called on those types of objects (but not what those methods do).

In many OOP languages and discussions, class and type are conflated. Even variable and value are commonly conflated. As a computer scientist, you should know and understand the differences.