Anything we do mindfully can cause a meditative experience. I am a software developer and sometimes when I am engrossed I don't feel the 'I', some kind of 'annatta' experience. Reaching somekind of Zen experience. I want to ask can we use this activity of programming computers as a kasina and achieve some jhana state? How do I go about it? Also, is my understanding of what kasina is correct?
What you are describing is colloquially known as being in the zone. It's a type of refined concentration where spectacular human feats can be achieved. There's a lack of ownership regarding the current task and often the concept of a body disappears. This should make you curious.
It's completely possible for jhana to occur under these circumstances, off the cushion and in the throes of daily life. In my personal understanding, experiencing jhana off the cushion has much more of an impact. In fact, this kind of exploration should be actively encouraged.
For dhamma practice, the object of concentration must not be generating continuity of thought, as with your task of programming code. There is something behind that primary task. This is the whole point: to bring meditative enquiry into the sensuous domain such that we become receptive to evermore subtle forms of awareness: hence, from coarse to fine, until we reach the bare subtleties of our awareness which flickers like a mirage in the desert. Like approaching a mirage in the desert, it vanishes; conditionality ceases. See the Lesser Discourse on Emptiness.
Look furthermore into these events you describe and see if you can replicate them in other settings. It's also important that, through study or teacher interaction, one is able to find a stable environment from where these experiences can be contextualized. See the Discourse on the Arousing of Mindfulness.
No. Its too active and mutable to be used to attain jhana. You need an object that is stable, unchanging, and that can essentially be "sat" on. Any activity that has discursive mental content, gross emotional feelings, or agitation is, pretty much by definition, antithetical to right concentration. In fact, the simple act of lifting your fingers to type is disruptive to the point where even access concentration will remain out of reach.