The TieWeavable Interface and Tie Class

  The basic link/edge interface is defined in TieWeavable, and implemented in the haystack.object.Tie class. The TieWeavable interface extends the Weavable interface, and similarly the Tie class extends the Straw class. This means that some functions are overridden and functions unique to links are implemented.

TieWeavable provides the following methods: setForward, setBack, getForward, and getBack. Ties are different than standard Straws in one major way: in their jobs as links they connect two, and only two nodes. While it is possible to add fields to Ties (such as needle.Creator, or needle.Comment), Ties maintain a single back pointer and forward pointer that is seperate from the StrawTie objects. This idea is represented in Figure . Ties override the get...Links() functions of Straw so that they will not only include the links held within their StrawTies but also the link held within their ``specialty'' pointer field. In the sample figure, a Straw (1) points another Straw (3) by means of a Tie (2). The label used within the two Straws matches the type of the Tie. Similarly, the Tie (2) is connected to another Straw (5) by means of yet another Tie (4).

  
Figure: A sample Tie cluster.

The benefit of the single back/forward pointer property is that it disambiguates the intention of the Tie. It is easy to determine what nodes the link connects and what nodes are associated with the link in some other way (as before, the example is a comment on the link).

• The tie.Causal Class