how to convert ArrayList> to JSONArray

Question

I am storing my user cart in ArrayList<HashMap<String, String>> to send it server to store it through php script. I want to convert it to ArrayList<HashMap<String, String>> to JSONArray so that i can pass it to parameters.

protected String doInBackground(String... args) {
        // TODO Auto-generated method stub
        AddtoCart obj = (AddtoCart) getApplicationContext();
        //want to convert below arraylist to jsonarray
        ArrayList<HashMap<String, String>> cart= obj.getCart().toString())); //getting arraylist

        HashMap<String, String> params = new HashMap<String, String>();
        params.put("username", username);
        params.put("email", email);
        params.put("payment", payment);
        params.put("address", useraddress);
        params.put("contact", contact);
        params.put("city", usercity);
        //here i want to pass json array as parameter
        params.put("cartitems", );

        Log.d("params", params.toString());

        JSONObject json = jParser.makeHttpRequest(url_all_products, "POST", params);