Illusory Joy: experienced for brief and fragile moments before you realize that the source of your joy, is in fact not. What follows ranges from casual acceptance, to unfashionable levels of grief, depending upon the level of non-joy actually present.
Unexpected Joy: you wake up, it is Monday morning. You are a working person with a job that has lost that lovin' feeling. You have a hangover so complete that it may be classifiable as a life form. Yet, you see your cat do that really weird thing it does occasionally (insert other pet/thing/person & behaviour here), and your grim lips crack in an idiot grin. A sub-category might be Much Needed Joy.
Understandable Joy: A really good thing has happened, or will soon. Doesn't have to be a big thing. Maybe it's a cute jumping in puddles joy, a sleazy lets pretend we're Hamsters joy, or a nerdy first box of Quake 12 of the shelf joy. Sources are obviously relative. Can become Illusory Joy quite quickly. I think of this as the default, no switches joy.
Fabricated Joy: Giving your neurotransmitters and activation states a shove in the joy direction with the help of your friends, the chemicals. Often ends up resulting in the extreme absence of joy. And regurgitation.
False Joy: Some people are so strange that they'll even fake being happy. Maybe they think they are being considerate.