For example, imagine something happens which invalidates a property which must be calculated (say, a bounding box for a graphical object).

Instead of immediately recomputing it, just set it to nil, and only recompute it the next time the property is actually accessed.

This way you can harmlessly invalidate it multiple times without doing a potentially expensive calculation (which just gets thrown out by subsequent invalidations).

In other words, "sort lst" gives you a sorted list, but "take 5 (sort lst)" gives you the first five elements, without sorting the rest of the list.

    if (logger.isDebugEnabled())
        logger.debug("..." + ...);

return theObject; }