public String addConnectionIdentifier(String identifier) {
        if (connectionIdentifiers.add(identifier))
            return identifier;
        return null;
    }
Add a connection identifier to this connection group, and
return the identifier if the add succeeds, otherwise
return null.

@param identifier
    The identifier of the connection to add to the group.

@return
    The String identifier of the connection if the add
    operation was successful; otherwise null.
extensions/guacamole-auth-quickconnect/src/main/java/org/apache/guacamole/auth/quickconnect/QuickConnectionGroup.java