These methods control the state of the thread. Any interface that will use the method names start and stop can never be implemented by a subclass of the Thread class. The final problem is the lack of default implementation. When a class extends another class, this other class can provide default implementations for all the methods. Sometimes a subclass can be created simply by extending a superclass and using the entire implementation of the superclass.
Some interfaces are quite extensive, and implementing all methods can be a lot of work.
This is especially irritating when only one or two methods are useful to implement in a certain situation. The interface concept forces the user of the interface to implement every method in the definition. Quite often, an implementation wants to implement only a part of this interface, and it should be possible to do this. Any operation in this area should be implemented with care. Doing the mental exercise to formulate a solution to the sketched problems, however, can't do any harm.