First of all, you should probably UseCodeTags. It will make it more likely that people are going to help you out.
Now, in order to use your methods, you need an instance of Reihung. First you need to decide what Object you're going to use as a key. If it is a String, you use:
Where X is a subtype of Beschluesselt.
Take notice, Schluessel may be a nice descriptive name, but it is misleading, because it implies it is an existing class or interface. For generic parameters, we use single letter names. A better class declaration would be:
Joined: Mar 13, 2007
Then I get the error message: X cannot be resolved to a typ. What do I have to change?