_bananabread has the right idea. Follow the steps on this website:
https://www.adobe.io/apis/documentcloud/sign/docs/step-by-step-guide/get-the-access-token.html
up until you have your JSON response with your refresh_token, this is all you need.
Next, you going to need to make a refresh token request that refreshes your token everytime you need to use it and returns you a brand new OAuth token.
Here is a Java code snippet that will refresh your acquired token:
HttpResponse response = null;
String access_token = "";
HttpClient httpClient = HttpClientBuilder.create().build();
HttpPost request = new HttpPost("http://api.echosign.com/oauth/refresh?"+
"refresh_token=tokenYouJustGot&" +
"client_id=clientIdUsedInPreviousSteps&"+
"client_secret=clientSecretUsedInPreviousStep"+
"grant_type=refresh_token");
request.addHeader("content-type", "application/x-www-form-urlencoded");
response = httpClient.execute(request);
String json = EntityUtils.toString(response.getEntity());
JSONObject jobj = new JSONObject(json);
access_token = jobj.getString("access_token");
access_token String will now contain a brand new OAuth access token with you can use for any request ie POST or GET.