import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import org.json.JSONObject; //https://github.com/douglascrockford/JSON-java

public class Sample {

	public static void main(String[] args) {

		String key = "YOUR API KEY HERE";
		String secret = "YOUR API SECRET HERE";
		String url = "https://apicurrent-app.booker.ninja/WebService4/json/CustomerService.svc/access_token?client_id="+key+"&client_secret="+secret+"&grant_type=client_credentials";

		URL request = null;
		try {
			request = new URL(url);
		} catch (MalformedURLException e) {
			e.printStackTrace();
		}

		String line;
		StringBuilder response = new StringBuilder();
		URLConnection connection = null;
		try {
			connection = request.openConnection();
			connection.setDoOutput(true);
			BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
			while((line = reader.readLine()) != null) {
				response.append(line);
			}
		} catch (IOException e) {
			e.printStackTrace();
		}		

		JSONObject json = new JSONObject(response.toString());
		String token = (String)json.get("access_token");
		
		System.out.println(token);
	}
}