Are there any reasons to have an abstract class with every method in the class defined?

Question

It seems that an abstract class means the definition of the class is not complete and hence cannot be instantiated.

And I saw some simple Java code which has an abstract

Answer 1

Yes, there is. Sometimes you know you're creating an abstract class - one that will have to be derived from to make any actual sense, but you want to provide a default implementation to all the methods. This doesn't happen a lot, but it does happen.

UPDATE: I just had a case like this. I'm recording various user generated events. Each event has a TimeSpan and a Description, and they all have other things as well. I've created a base event class:

public abstract class BaseEvent
{
    public TimeSpan EventTime {get; private set;}
    public string Description {get; protected set;}

    public BaseEvent(TimeSpan time, string desc) ...
}

Granted, this is C# and not Java, but had I written this in Java, I would have done exactly the same thing)