code: public void getExchangeInfo(Exchange.Type currency, long quantity, Callback<Exchange> callback) throws GuildWars2Exception, NullPointerException {
  		isValueValid(quantity);
  		gw2API.getExchangeInfo(currency.name(), Long.toString(quantity)).enqueue(callback);
  	}
docs: For more info on exchange coins API go <a href="https://wiki.guildwars2.com/wiki/API:2/commerce/exchange/coins">here</a><br/>
  Give user the access to {@link Callback#onResponse(Call, Response)} and {@link Callback#onFailure(Call, Throwable)} methods for custom interactions
  
  @param currency exchange currency type
  @param quantity The amount to exchange
  @param callback callback that is going to be used for {@link Call#enqueue(Callback)}
  @throws GuildWars2Exception  invalid value
  @throws NullPointerException if given {@link Callback} is empty
  @see Exchange Exchange info