A sandbox system is available for testing. The sandbox will not modify your account in any way, but in almost all other respects it will respond normally to your requests.
In other words if you use the sandbox to get the number of credits on your account, it will provide the correct number. However if you use it to transfer credits between accounts, it will simulate a successful transfer (assuming you have enough credits on the account, and provide valid target account details, etc.) but it will not actually modify the number of credits on either account.
Similarly, the sandbox system will not actually send SMS messages and will not deduct credits for simulated sends.

To use the sandbox simply specify ENV_SANDBOX instead of ENV_PRODUCTION when you create the wrapper object:

RestClient tmClient;
tmClient = new RestClient("myAPIusername", "myAPIpassword", RestClient.ENV_SANDBOX);

the values returned when using the sandbox may differ slightly from the values returned when using the production system.