interface DBObject { DBObject initializeIdAndName(long id, String name); } static class User implements DBObject { long id; String name; public User initializeIdAndName(long id, String name){ this.name = name; this.id = id; return this; //<--- warum return this? } @Override public String toString() { return String.format("Der Name des Benutzer ist %s, ID = %d", name, id); } }