As noted in class, the Violet UML editor does not support communication/collaboration diagrams directly. However, you can use object diagrams to approximate them.

To model a collaboration between two objects, create the objects and link them with an association. Then, double click on the association and choose the arrow-head style from the "Properties" menu that pops up, with "Triangle" for a synchronous message and "V" for an asynchronous one.

The arrow will appear at the end of the line, instead of on top of the line as prescribed in actual collaboration diagrams, but it will do for us. You can add the sequence expression and the name of the message by filling in the "Middle label" field in the "Properties" menu of the association.