While having a conversation with a Kashmiri muslim shop owner here in Mumbai, he mentioned vehemently that J&K was never a part of India and that India has taken over it and that it doesn't even have a mention in the Indian constitution led me to look on net on the facts about J&K..at that point I was trying to recollect history but could faintly remember on it..
Here is what I found an answer to the que.. was J&K taken over by India from wikipedia
"When India and Pakistan gained their independence on 15 and 14 August 1947, respectively; J&K chose to remain independent. There was an agreement by J&K with Pakistan and India that none of them will attack J&K. While India respected the agreement and exercised restraint, Pakistan attacked Kashmir in a bid to annex it by force. On 6 October 1947, Kashmir was attacked by “Azad Kashmir Forces” supported by Pakistan. To save J&K, Maharaja Hari Singh (the then ruler of J&K) chose to accede J&K to India.
In October 1947, the accession was made by the ruler in favour of India in consideration of certain commitments made by Pt. Jawahar Lal Nehru (the then Prime Minister of India). It was in the pursuance of those commitments that Article 370 was incorporated in the Constitution"
"Article 370 of the Indian constitution, which is of a temporary nature, grants special status to Jammu and Kashmir. Under Part XXI of the Constitution of India, which deals with “Temporary, Transitional and Special provisions”, the State of Jammu and Kashmir has been accorded special status under Article 370. Even though included in 1st Schedule as 15th state, all the provisions of the Constitution which are applicable to other states are not applicable to J&K. For example, till 1965, J&K had a Sadr-e-Riyasat for Governor and Prime Minister in place of Chief Minister."
It enjoys a separate Constitution J&K and is the only state in India which has a Constitution of its own. The Constitution of J&K was enacted by a separate Constituent Assembly set up by the State and it came into force on 26 January 1957.