How can I check for a key in json, which I am getting as an output from RESTful api
Try replacing :
if(json.has("DISP_NAME")) {with
if( json.DISP_NAME ) {Of course, that won't differentiate between a NULL or empty value and a missing value.
To check the field is in the JSON object, just do:
if( json.keySet().contains( 'DISP_NAME' ) ) {