Setting ContentType on StringEntity or HTTP request?

Question

I am having a dilemma here: this is confusing to me.

Code

StringEntity stringEntityInstance = new StringEntity("some string", "UTF-8");
HttpPost req = new HttpPost("myurl");
req.setEntity(stringEntityInstance);

• If add code A then I face some JSON error from API
• If add code B then I don't face any issue.

Code A

stringEntityInstance.setContentEncoding(new BasicHeader(HTTP.CONTENT_TYPE,”application/json"));

Code B

req.setHeader(HTTP.CONTENT_TYPE,”application/json");

Any help is appreciated !