ShapeCollection.java

////////////////////////////////////////////////////////////
// class: ShapeCollection
//
// description: a class for storing an ordered collection
// of Shapes. Supports the operations of appending and
// removing shapes from the collection. Sequential access to
// the shapes stored in the collection is given through
// the getFirst() and getNext() methods.
////////////////////////////////////////////////////////////


public class ShapeCollection {
    private ShapeNode first; 
    private ShapeNode next;
    private ShapeNode last;

    public ShapeCollection () {
		first = null;
		next = null;
		last = null;
    }


    ////////////////////////////////////////////////////////////
    // method: Shape getFirst()
    //
    // description: returns the first Shape stored in the
    // collection, and resets the "next" Shape to point to the 
    // following (i.e., second) Shape in the collection (if any)
    ////////////////////////////////////////////////////////////
    
    public Shape getFirst () {
		next = first;
		return getNext();
    }

    ////////////////////////////////////////////////////////////
    // method: Shape getNext()
    //
    // description: returns the Shape currently pointed to by
    // Next and updates Next to point to the following Shape
    // (if any). In the case where next is null, getNext()
    // returns null.
    ////////////////////////////////////////////////////////////

    public Shape getNext () {
		if (next == null)
			return null;

		Shape s = next.getShape();
		next = next.getNext();
		return s;
    }

    ////////////////////////////////////////////////////////////
    // method: void append (Shape s)
    //
    // description: Appends a Shape s to the end of the
    // collection. After this operation, s will be the last
    // shape stored in the collection.
    ////////////////////////////////////////////////////////////
    
    public void append (Shape s) {
		ShapeNode node = new ShapeNode(s);

		if (last == null) {
			first = node;
			next = node;
			last = node;
		} else {
			node.setPrev(last);
			last.setNext(node);
			last = node;
		}
    }

    ////////////////////////////////////////////////////////////
    // method: void remove (Shape s)
    //
    // description: Searches the collection for a node
    // containing s, and if found, s is removed from the
    // collection. This method has no effect if s is not found
    // the the collection.
    ////////////////////////////////////////////////////////////

    public void remove (Shape s) {
		if (first == null)
			return;

		ShapeNode cur = first;

		while (cur != null && cur.getShape() != s)
			cur = cur.getNext();

		if (cur == null)
			return;

		if (cur.getNext() != null) {
			cur.getNext().setPrev(cur.getPrev());
		} else {
			// cur is last
			last = cur.getPrev();
		}

		if (cur.getPrev() != null) {
			cur.getPrev().setNext(cur.getNext());
		} else {
			// cur is the first
			first = cur.getNext();
		}

		if (next == cur) {
			next = cur.getNext();
		}
		}
}


////////////////////////////////////////////////////////////
// class: ShapeNode
//
// description: a class for storing a Shape. Used by
// ShapeCollection.
////////////////////////////////////////////////////////////


class ShapeNode {
    private Shape shape;
    private ShapeNode next;
    private ShapeNode prev;

    public ShapeNode (Shape shape) {
		this.shape = shape;
		next = null;
		prev = null;
    }

    public Shape getShape() { return shape; }
    public ShapeNode getNext() { return next; }
    public ShapeNode getPrev() { return prev; }
    public void setNext (ShapeNode next) { this.next = next; }
    public void setPrev (ShapeNode prev) { this.prev = prev; }
}