From the link Maurits gave, you can also have a look at the putExtras
methods.
See more info here and search for putExtra (the URL doesnt look good on SO)
*Edited:
From Donal Rafferty post, I think I understand now what you mean in the OP.
What you can do is the below:
String theClass = "Login";
StringBuilder myAction = new StringBuilder("com.yourpackage.");
myAction.append(theClass);
Intent i = new Intent(myAction.toString());
startActivity(i)
The myAction string (in this example com.yourpackage.Login
should be specified in the AndroidManifest.xml under an activity
From the doc:
public Intent (String action)
Since: API Level 1
Create an intent with a given action. All other fields (data, type, class) are null. Note that the action must be in a namespace because Intents are used globally in the system -- for example the system VIEW action is android.intent.action.VIEW; an application's custom action would be something like com.google.app.myapp.CUSTOM_ACTION.