what is the code trying to say between inheritance and aggregation?

[sura watthana]

Hi,
I'm trying to figure out what the code below trying to say about the different between inheritance and aggregation. could somebody tell me, please? Thank you.

 
class Node {                                                   // (1)
    private Object data;    // Data
    private Node   next;    // Next node
 
    // Constructor for initializing data and reference to the next node.
    Node(Object data, Node next) {
        this.data = data;
        this.next = next;
    }
 
    // Methods
    public void   setData(Object obj) { data = obj; }
    public Object getData()           { return data; }
    public void   setNext(Node node)  { next = node; }
    public Node   getNext()           { return next; }
}
 
class LinkedList {                                             // (2)
    protected Node head = null;
    protected Node tail = null;
 
    // Methods
    public boolean isEmpty() { return head == null; }
    public void insertInFront(Object dataObj) {
        if (isEmpty()) head = tail = new Node(dataObj, null);
        else head = new Node(dataObj, head);
    }
    public void insertAtBack(Object dataObj) {
        if (isEmpty())
            head = tail = new Node(dataObj, null);
        else {
            tail.setNext(new Node(dataObj, null));
            tail = tail.getNext();
        }
    }
    public Object deleteFromFront() {
        if (isEmpty()) return null;
        Node removed = head;
        if (head == tail) head = tail = null;
        else head = head.getNext();
        return removed.getData();
    }
}
 
class QueueByAggregation {                                     // (3)
    private LinkedList qList;
 
    // Constructor
    QueueByAggregation() {
        qList = new LinkedList();
    }
 
    // Methods
    public boolean isEmpty() { return qList.isEmpty(); }
    public void enqueue(Object item) { qList.insertAtBack(item); }
    public Object dequeue() {
        if (qList.isEmpty()) return null;
        else return qList.deleteFromFront();
    }
    public Object peek() {
        return (qList.isEmpty() ? null : qList.head.getData());
    }
}
 
class StackByInheritance extends LinkedList {                  // (4)
    public void push(Object item) { insertInFront(item); }
    public Object pop() {
       if (isEmpty()) return null;
       else return deleteFromFront();
    }
    public Object peek() {
        return (isEmpty() ? null : head.getData());
    }
}
 
public class Client {                                           // (5)
    public static void main(String[] args) {
        String string1 = "Queues are boring to stand in!";
        int length1 = string1.length();
        QueueByAggregation queue = new QueueByAggregation();
        for (int i = 0; i<length1; i++)
            queue.enqueue(new Character(string1.charAt(i)));
        while (!queue.isEmpty())
            System.out.print((Character) queue.dequeue());
        System.out.println();
 
        String string2 = "!no tis ot nuf era skcatS";
        int length2 = string2.length();
        StackByInheritance stack = new StackByInheritance();
        for (int i = 0; i<length2; i++)
            stack.push(new Character(string2.charAt(i)));
        stack.insertAtBack(new Character('!'));                 // (6)
        while (!stack.isEmpty())
            System.out.print((Character) stack.pop());
        System.out.println();
    }
}

(added [C0DE][/C0DE] tags )
[ September 14, 2004: Message edited by: Barry Gaunt ]

[Fletcher Estes]

Aggregation means a collection of properties which defines a class. Your QueueByAggregation demonstrates this because it has a member variable (or property/attribute) which is the previously defined LinkedList. The nature of the QueueByAggregation is determined by what it is composed of, rather than what it is natively.

Inheritance means a class is a type of a superclass. In this case, your StackByInheritance is a type of LinkedList. It can do anything a LinkedList can do, and may be treated as a LinkedList by other classes. The StackByInheritance doesn't have a LinkedList, like the QueueByAggregation, it simply is one.

Hope this helps