The laws for fixed blades are different than the laws for folders. Folding knives (including assisted folders) may be legally carried concealed, with some minor restrictions. I provided the links for people to inform themselves of the law directly, rather than swaying with the forever changing Internet rumors.

By the way, I have been carrying a folder for about 25 years in California, have been stopped by cops and have informed them of my little knife. One or two times, the cops seemed a little annoyed as to why I even brought it up. You should also notice that Walmart, Big5 and many other sporting goods stores prominently sell a large variety of folding knifes, including assisted folders. They would have stopped doing so long ago if people were getting arrested merely for carrying folders in public.

For self-defense in California, it would actually be easier for you in the legal aftermath if you use a knife instead of a blunt object (e.g., baseball bat, golf club, hammer, kubaton, etc.). No, I'm not exaggerating. The laws are that strange in California and are too complex to discuss coherently here.

What others carry for self-defense is their own business, but I will always carry something besides my bare hands. Do yourself a favor by considering all the viable candidates for self-defense tools. Then, go to Calguns.net to discuss their legalities. Calguns.net has many threads discussing knife carry in California, and many knowledgeable people are there.