Adicionei a biblioteca apache httpclient, mas ocorrendo erro na importação do StringEntity.
public String post(String json){
        try{
            HttpPost post = new HttpPost(url);
            post.setEntity(new **StringEntity**(json));
            post.setHeader("Content-type", "application/json");
            post.setHeader("Accept", "application/json");
            HttpClient client = new DefaultHttpClient();
            HttpResponse resposta = client.execute(post);
            return EntityUtils.toString(resposta.getEntity());
        } catch(IOException e){
        } 
            