0

If in a derived class you can override both a concrete method and an abstract method of an abstract parent class,then what's the point of having abstract methods in the first place?

If for example there in the parent class there is a concrete method called 'display' that just prints 'hi from the base class' which can be overridden in the derived class to print 'hi from the derived class',what would the difference be if the 'display' method in the parent would be abstract instead of veing concrete? It would be overridden too in any case.

microwth
  • 1,016
  • 1
  • 14
  • 27
  • 1
    C#: I don't understand your question. By concrete method, I assume you mean one without either the `abstract` or `virtual` keyword applied to it. Your assertion that you can override it without one of these keywords is wrong. You can _hide_ methods that aren't declared as `virtual` but you can't override them. – ProgrammingLlama Feb 19 '22 at 16:41
  • Related: https://stackoverflow.com/questions/6576206/what-is-the-difference-between-the-override-and-new-keywords-in-c – ProgrammingLlama Feb 19 '22 at 16:43
  • java != c# please only tag one – Charlieface Feb 19 '22 at 21:09

4 Answers4

4

The difference is simple: An abstract method must be overridden. When you have a non-abstract (virtual) method you can decide that you don't want to override it in a derived class. So the programmer of the base class can decide whether any derived class must override the method or can override the method.

C# and java differ in what is the default for "overridability". In java, methods are by default virtual and can be overridden, in C#, the default is that methods are not virtual, unless explicitly declared so. But both languages have all three concepts: abstract methods (these must be overridden), virtual methods (these can be overridden) and final/sealed methods (these cannot be overridden).

gunr2171
  • 16,104
  • 25
  • 61
  • 88
PMF
  • 14,535
  • 3
  • 23
  • 49
  • 1
    @Llama Since the question is tagged both C# and java, it gets complicated, because java doesn't have the virtual keyword, but needs to declare them `final` if they shall _not_ be virtual. – PMF Feb 19 '22 at 16:46
  • It' a Java question mostly,but I refer to C# to because the concept should be the same.True I'm not aware of the C# keywords used. – microwth Feb 19 '22 at 16:47
  • @microwth C# and Java differ in the way they handle virtual methods by default, see the updated answer. – PMF Feb 19 '22 at 16:53
4

If you mean the difference between abstract method and a virtual method in the base class: the difference is simply that the abstract one must be overridden, i.e. there is no meaningful base implementation, and a custom implementation must be provided for the type to be valid - whereas a virtual method can optionally be overridden if the inheritor wishes.

Marc Gravell
  • 1,026,079
  • 266
  • 2,566
  • 2,900
1

You use abstract class when not all methods can be implemented in a default way. For example Shape class as base class and derived Rectangle and Circle. Last two will implement Draw method but parent/base class what should do in a Draw method?

Use virtual if method has default implementation and abstract if it has not.

Mabakay
  • 328
  • 1
  • 8
1

As PMF mentioned, abstract methods must be overridden while concrete can be overridden.

The difference must also be understood in the context of hierarchy where one abstract class can be extended by (parent of) multiple other classes.

The concrete method definition in the abstract class provides a default implementation that is generally acceptable by many child class implementations but can also be overridden in case a different implementation is required.

But the abstract method must always be overridden by all the child classes.

gunr2171
  • 16,104
  • 25
  • 61
  • 88
Lalit Mehra
  • 1,183
  • 1
  • 13
  • 33