You must first add the service to your bluemix application.

After that, your code will get the credentials dynamically from the system environment variable called VCAP_SERVICES

 System.getenv("VCAP_SERVICES");

VCAP_SERVICES is a string in the JSON format that contains an entry for each service bound to your application, for example

   {
   "document_conversion": [
     {
       "name": "docConv",
       "label": "document_conversion",
       "plan": "experimental",
       "credentials": {
         "url": "https://gateway.watsonplatform.net/document-conversion-experimental/api",
         "username": "****",
         "password": "****"
       }
     }
   ]
 }

so you have to parse this JSON and extract your credentials from there.

In my case, for example, I use flexjson, but could be any other JSON library (GSON, Jackson, whatever)

 private static final JSONDeserializer<Map<String, List<Map<String, Map<String, String>>>>> json = new JSONDeserializer<>();
 
 String username = dto.setUsername(map.get("document_conversion").get(0).get("credentials")
                 .get("username"));

 String password = dto.setUsername(map.get("document_conversion").get(0).get("credentials")
                 .get("password"));

An then you can use the excellent java-wrappers library (as it seems you're already using)

 import com.ibm.watson.developer_cloud.document_conversion.v1.DocumentConversion;
 import com.ibm.watson.developer_cloud.document_conversion.v1.model.DocumentCollection;
 import java.util.HashMap;
 import java.util.Map;
 
 DocumentConversion service = new DocumentConversion();
 service.setUsernameAndPassword(username, password);
 
 Map<String, Object> docListParams = new HashMap<String, Object>();
 docListParams.put(DocumentConversion.LIMIT, 10);
 DocumentCollection documentCollection = service.getDocumentCollection(docListParams);
 System.out.println(documentCollection);

Notice that, if I am not wrong, password are periodicaly rotated in bluemix, so you must avoid using hardcoded ones.

The WDC docs contain a getting started section (Docs -> Getting Started) that we typically link to from service docs or excerpt from as the first step of our tutorials. It looks as though we didn't do that for the Doc Conversion tutorial and docs. Thanks for pointing that out - we will fix that. It contains sections on obtaining Bluemix credentials, discusses the VCAP_SERVICES environment variable, and also explains how to get and examine credentials within the Bluemix web interface. Comments welcome, and sorry that this information was hard to find in this case.