Java current machine name and logged in user?

To get the currently logged in user:

System.getProperty("user.name"); //platform independent 

and the hostname of the machine:

java.net.InetAddress localMachine = java.net.InetAddress.getLocalHost();
System.out.println("Hostname of local machine: " + localMachine.getHostName());

To get the currently logged in user:

System.getProperty("user.name");

To get the host name of the machine:

InetAddress.getLocalHost().getHostName();

To answer the last part of your question, the Java API says that getHostName() will return

the host name for this IP address, or if the operation is not allowed by the security check, the textual representation of the IP address.