What is the difference between an abstract method and a virtual method? In which cases is it recommended to use abstract or virtual methods? Which one is the best approach?

What is the difference between an abstract method and a virtual method?

During one of my interview, I was asked &quot;If we can instantiate an abstract class?&quot;

My reply was &quot;No. we can&#39;t&quot;. But, interviewer told me &quot;Wrong, we can.&quot; 

I argued a bit on this. Then he told me to try this myself at home. 

    abstract class my {
        public void mymethod() {
            System.out.print(&quot;Abstract&quot;);
        }
    }
    
    class poly {
        public static void main(String a[]) {
            my m = new my() {};
            m.mymethod();
        }
    }

Here, I&#39;m creating instance of my class and calling method of abstract class. Can anyone please explain this to me? Was I really wrong during my interview?

Can we instantiate an abstract class?

How can I make a class or method abstract in Python?

I tried redefining `__new__()` like so:

    class F:
        def __new__(cls):
            raise Exception(&quot;Unable to create an instance of abstract class %s&quot; %cls)

but now if I create a class `G` that inherits from `F` like so:

    class G(F):
        pass

then I can&#39;t instantiate `G` either, since it calls its super class&#39;s `__new__` method.

Is there a better way to define an abstract class?

Is it possible to make abstract classes in Python?

Data classes seem to be the replacement to the old-fashioned POJOs in Java. It is quite expectable that these classes would allow for inheritance, but I can see no convenient way to extend a data class. What I need is something like this:

    open data class Resource (var id: Long = 0, var location: String = &quot;&quot;)
    data class Book (var isbn: String) : Resource()

The code above fails because of clash of `component1()` methods. Leaving `data` annotation in only one of classes does not do the work, too.

Perhaps there is another idiom to extend data classes?

UPD: I might annotate only child child class, but `data` annotation only handles properties declared in the constructor. That is, I would have to declare all parent&#39;s properties `open` and override them, which is ugly:

    open class Resource (open var id: Long = 0, open var location: String = &quot;&quot;)
    data class Book (
        override var id: Long = 0,
        override var location: String = &quot;&quot;,
        var isbn: String
    ) : Resource()

Extend data class in Kotlin

I am having trouble in using inheritance with Python. While the concept seems too easy for me in Java yet up till now I have been unable to understand in Python which is surprising to me at least.

I have a prototype which follow:

    class Shape():
       def __init__(self, shape_name):
           self.shape = shape_name

    class Rectangle(Shape):
       def __init__(self, name):
           self.shape = name

In the above code how can I make an abstract method that would need to be implemented for all the subclasses?

Abstract methods in Python

Consider an example (which compiles in java)

    public abstract interface Interface {
    	public void interfacing();
    	public abstract boolean interfacing(boolean really);
    }

Why is it necessary for an interface to be &quot;declared&quot; abstract? Is there other rules that applies with an abstract interface?


----------
Finally: If `abstract` is obsolete, why is it included in Java? Is there a history for abstract interface?

Java abstract interface

I am interating through classes in a Jar file and wish to find those which are not abstract. I can solve this by instantiating the classes and trapping InstantiationException but that has a performance hit as some classes have heavy startup. I can&#39;t find anything obviously like isAbstract() in the Class.java docs.


How can I determine whether a Java class is abstract by reflection

I used the &quot;pull interface&quot; refactoring feature of Eclipse today to create an interface based on an existing class. The dialog box offered to create all the new methods of the new interface as &quot;abstract&quot; methods.

What would be the benefit of that?

I thought that the fact that you were allowed to declare interface methods as abstract was a superfluous and harmless feature of the language that is not particularly encouraged.

Why would Eclipse support such a style, or why would someone voluntarily choose to do so?

Clarification: I am not asking why interface methods are abstract, that is obvious. I am asking why one would explicitly choose to mark them as abstract since if they&#39;re in an interface they are abstract anyway. 

Why would one declare a Java interface method as abstract?

Is it possible to simulate abstract base class in JavaScript? What is the most elegant way to do it?

Say, I want to do something like: -

    var cat = new Animal(&#39;cat&#39;);
    var dog = new Animal(&#39;dog&#39;);
    
    cat.say();
    dog.say();

It should output: &#39;bark&#39;, &#39;meow&#39;




How do I create an abstract base class in JavaScript?

Can I define an abstract class without adding an abstract method?

Defining an abstract class without any abstract methods

After enabling strict warnings in PHP 5.2, I saw a load of strict standards warnings from a project that was originally written without strict warnings:

&gt; **Strict Standards**: **Static function** Program::getSelectSQL() **should not be abstract** in Program.class.inc

The function in question belongs to an abstract parent class Program and is declared abstract static because it should be implemented in its child classes, such as TVProgram.

I did find references to this change [here][1]:

&gt; Dropped abstract static class functions. Due to an oversight, PHP 5.0.x and 5.1.x allowed abstract static functions in classes. As of PHP 5.2.x, only interfaces can have them. 

My question is: can someone explain in a clear way why there shouldn&#39;t be an abstract static function in PHP?


  [1]: http://php.net/manual/migration52.incompatible.php

Why does PHP 5.2+ disallow abstract static class methods?

Why can&#39;t Java classes have abstract fields like they can with abstract methods?

For example: I have two classes that extend the same abstract base class.  These two classes each have a method that is identical except for a String constant, which happens to be an error message, within them. If fields could be abstract, I could make this constant abstract and pull the method up into the base class. Instead, I have to create an abstract method, called `getErrMsg()` in this case, that returns the String, override this method in the two derived classes, and then I can pull up the method (which now calls the abstract method).

Why couldn&#39;t I just make the field abstract to begin with? Could Java have been designed to allow this?

Why not abstract fields?

C# methods in interfaces are declared without using the `virtual` keyword, and overridden in the derived class without using the `override` keyword.

Is there a reason for this?  I assume that it is just a language convenience, and obviously the CLR knows how to handle this under the covers (methods are not virtual by default), but are there other technical reasons?

Here is the IL that a derived class generates:

    class Example : IDisposable {
        public void Dispose() { }
    }

    .method public hidebysig newslot virtual final 
            instance void  Dispose() cil managed
    {
      // Code size       2 (0x2)
      .maxstack  8
      IL_0000:  nop
      IL_0001:  ret
    } // end of method Example::Dispose

Notice that the method is declared `virtual` `final` in the IL. 

Why are C# interface methods not declared abstract or virtual?

Is there a standard way to make a &quot;pure virtual function&quot; in Swift, ie. one that **must** be overridden by every subclass, and which, if it is not, causes a compile time error?

Swift - class method which must be overridden by subclass

What&#39;s the best practice to define an abstract instance attribute, but not as a property?

I would like to write something like:

    class AbstractFoo(metaclass=ABCMeta):

        @property
        @abstractmethod
        def bar(self):
            pass

    class Foo(AbstractFoo):

        def __init__(self):
            self.bar = 3

Instead of:

    class Foo(AbstractFoo):

        def __init__(self):
            self._bar = 3
        
        @property
        def bar(self):
            return self._bar

        @bar.setter
        def setbar(self, bar):
            self._bar = bar

        @bar.deleter
        def delbar(self):
            del self._bar

Properties are handy, but for simple attribute requiring no computation they are an overkill. This is especially important for abstract classes which will be subclassed and implemented by the user (I don&#39;t want to force someone to use `@property` when he just could have written `self.foo = foo` in the `__init__`).

https://stackoverflow.com/questions/2736255/abstract-attributes-in-python question proposes as only answer to use `@property` and `@abstractmethod`: it doesn&#39;t answer my question.

The ActiveState recipe for an abstract class attribute via [`AbstractAttribute`](http://code.activestate.com/recipes/577761-simple-abstract-constants-to-use-when-abstractprop) may be the right way, but I am not sure. It also only works with class attributes and not instance attributes.


Abstract attribute (not property)?

A class with one (or more) virtual pure functions is abstract, and it can&#39;t be used to create a new object, so it doesn&#39;t have a constructor.

I&#39;m reading a book that provides the following example:

    class Employee {
       public:
           Employee(const char*, const char*);
           ~Employee();
           const char* getFirstName() const;
           const char* getLastName() const;


           virtual double earnings() const=0  // pure virtual =&gt; abstract class
           virtual void print() const

      private:
           char* firstName, lastName;
    };


If the class is abstract why we have a constructor? It uses this class later (`Boss` is public derived from `Employee`):


    void Boss::Boss (const char* first, const char* last, double s)
         : Employee (first, last)
 



C++ Abstract Class: constructor yes or no?

Recently I started exploring Java 8 and I can&#39;t quite understand the concept of &quot;functional interface&quot; that is essential to Java&#39;s implementation of lambda expressions. There is a [pretty comprehensive guide][1] to lambda functions in Java, but I got stuck on [the chapter that gives definition to the concept of functional interfaces][2]. The definition reads:

&gt; More precisely, a functional interface is defined as any interface that has exactly one abstract method.

An then he proceeds to examples, one of which is `Comparator` interface:

&gt;     public interface Comparator&lt;T&gt; {
&gt;         int compare(T o1, T o2);
&gt;         boolean equals(Object obj);
&gt;     } 

I was able to test that I can use a lambda function in place of Comparator argument and it works(i.e. `Collections.sort(list, (a, b) -&gt; a-b)`). 

But in the Comparator interface both `compare` and `equals` methods are abstract, which means it has **two abstract methods**. So how can this be working, if the definition requires an interface to have **exactly one abstract method**? What am I missing here?


  [1]: http://www.lambdafaq.org/
  [2]: http://www.lambdafaq.org/what-is-a-functional-interface/

Precise definition of &quot;functional interface&quot; in Java 8

I&#39;ve be fiddling around with ui-router today in trying to better understand the scaffolding in Ionic and one thing that I noticed was that they give the abstracted state of &quot;tabs&quot; a url. 

The only times I&#39;ve ever used abstract states, I used an empty string as the url and I notice that if I&#39;ve ever accidentally attempted to navigate to an abstracted state (as opposed to the child state) I get the error: 

&gt; *Cannot transition to abstract state &#39;[insertAbstractStateHere]&#39;*

**edit:** 

&quot;Moreover, in experimenting, when I try to assign a url to my abstract state (outside of Ionic) and still render the nested state views, I get a big goose egg. Nothing shows up at all.&quot;

the above quoted statement is false! I tried it again in [Plunker][1] and the nested states did show up. 

   
     angular.module(&#39;routingExperiments&#39;, [&#39;ui.router&#39;])
          .config(function($urlRouterProvider, $stateProvider) {
    
        $stateProvider
    
          .state(&#39;abstractExperiment&#39;, {
            abstract: true,
            url: &#39;&#39;, //&lt;--- seems as if any string can go here.
            templateUrl: &#39;abstractExperiment.html&#39;
          })
          .state(&#39;abstractExperiment.test1&#39;, {
            url: &#39;/test1&#39;,
            templateUrl: &#39;abstractTest1.html&#39;
          });
      });

Apparently I was indeed doing it wrong. So my new question is:

Is there any reason why one would use a named state as opposed to an empty string in employing abstract states, or is it just a style choice? 


  [1]: http://plnkr.co/edit/KnMuiEsVAjviz4n6xyuj?p=preview

Why give an &quot;abstract: true&quot; state a url?

What is the difference between `sealed abstract` and `abstract` Scala class?

Scala&#39;s sealed abstract vs abstract class

I have an abstract class A that define abstract methods. This means that, for a class to be instanciable, all the abstract method have to be implemented.

I&#39;d like all my subclasses to implement a constructor with 2 ints as parameters. 

Declaring a constructor defeats my purpose, as I want the constructor defined in subclasses and I don&#39;t know anything about the implementation. Moreover I cannot declare a constructor as being abstract;

Is there a way to do this ?

__Example of what I want:__

Lets say that I am defining the API of a Matrix class. In my problem, Matrix cannot change their dimensions.

For a Matrix to be created, I need to provide its size.

Hence, I want all my implementors to provide the constructor with the size as a parameter. This constructor is motivated by the problem, not by an implementation concern. The implementation can do whatever it wants with these, provided that all the semantic of the methods are kept.

Let&#39;s say I want to provide a basic implementation of the `invert()` method in my abstract class. This method will create a new matrix with `this` inverted dimensions. More specifically, as it is defined in the abstract class, it will create a new instance of the same class as `this`, using a constructor that takes two ints. As it does not know the instance it will use reflection (getDefinedConstructor) and I want a way to waranty that I&#39;ll get it and that it will be meaningfull for the implementation.


How can I force a Constructor to be defined in all subclass of my abstract class

I am coming from c# where this was easy, and possible.

I have this code:

    public abstract class clsAbstractTable {
    	
    	public abstract String TAG;
        public abstract void init();
    
    }

but Eclipse tells me I use illegal modifier. 

I have this class:

    public class clsContactGroups extends clsAbstractTable {
    
    	
    }

I want the variable and method defined in such way, that Eclipse to **prompt me**, I have unimplemented abstract **variables** and methods.

How do I need to define my abstract class so I should be prompted to implement the abstracts?

**EDIT 1**

I will create different classes for different db tables. Each class should have it&#39;s own TABLENAME variable, no exception. I have to make sure this variable is static each time when I create a new class that extends the abstract class.

Then in the abstract class I will have a method eg: init();

If in this init() method I call TABLENAME, it should take the value from the sub-class.

something like this should also work out

    String tablename=(clsAbstract)objItem.TABLENAME;
    // where objItem can be any class that extended clsAbstract;

**EDIT 2**

I want a constant(static) defined in each class having it&#39;s name defined in abstract. 

 - I define variable TABLENAME in abstract, but no value given.
 - I create a clsContactGroups, I should be prompted to implement TABLENAME, this is where gets some data. eg: TABLENAME=&quot;contactgroups&quot;;
 - I create a second class clsContacts, I should be prompted to implement TABLENAME, this is where gets some data. eg: TABLENAME=&quot;contacts&quot;;  
etc...

Abstract variables in Java?

What are the advantages and disadvantages of each of these?

 - interface methods
 - virtual methods
 - abstract methods

When one should choose what? What are the points one should keep in mind when making this decision?

Comparison : interface methods vs virtual methods vs abstract methods

I noticed that you can&#39;t have abstract constants in PHP.

Is there a way I can force a child class to define a constant (which I need to use in one of the abstract class internal methods) ?

Abstract constants in PHP - Force a child class to define a constant

I&#39;m not really sure what looks better or when do I really use in abstract classes and properties, or when to use non abstract properties. I&#39;ll try to make a simple example. Let&#39;s say I have this:

    abstract class Human
    {
      public GenderType Gender { get; set; }
      public string Name { get; set; }
      public Date Born { get; set; }
      public bool IsNerd { get; set; }
    
      abstract public void Speak();
      abstract public void Sleep();
      abstract public void AnoyingPeopleOnStackOverflow();
      //... so on
    }
    
    class Peter : Human
    {
      //Peter is special, he got a second name
      //But thats all, everything else is the same as like on other humans
      public string SecondName { get; set; }
    
      //...override abstract stuff
    }

Is this alright? As I understood, I don&#39;t have to use an abstract property if I dont want to override it. And in this situation it would be ok, just the methods like `Speak`, `Sleep` and so on should be abstract.

Now, if this is ok, when would or should I use an abstract property?

Not sure when to use an abstract property and when not

When overriding a non-virtual method in Java, use of the `@Override` annotation is recommended, but what if I implement an abstract method? Should I use `@Override` then as well?

Should I add an @Override annotation when implementing abstract methods in Java?

Can an enum have abstract methods? If so, what is the use, and give a scenario which will illustrate this usage.

Can an enum have abstract methods?

Just out of curiosity I tried overriding a abstract method in base class, and method the implementation abstract. As below:



    public abstract class FirstAbstract
    {
        public abstract void SomeMethod();
    }
    
    public abstract class SecondAbstract : FirstAbstract
    {
        public abstract override void SomeMethod();
        //?? what sense does this make? no implementaion would anyway force the derived classes to implement abstract method?
    }


Curious to know why C# compiler allows writing &#39;abstract override&#39;. Isn&#39;t it redundant? Should be a compile time error to do something like this. Does it serve to some use-case?

Thanks for your interest.


What is the use of &#39;abstract override&#39; in C#?

I&#39;ve set up an abstract parent class, and a concrete class which extends it. Why can the parent class not call the abstract function?

    //foo.php
    &lt;?php
        abstract class AbstractFoo{
            abstract public static function foo();
            public static function getFoo(){
                return self::foo();//line 5
            }
        }
        
        class ConcreteFoo extends AbstractFoo{
            public static function foo(){
                return &quot;bar&quot;;
            }
        }
        
        echo ConcreteFoo::getFoo();
    ?&gt;

Error: 

&gt; Fatal error: Cannot call abstract method AbstractFoo::foo() in **foo.php**  on line 5

Why can&#39;t you call abstract functions from abstract classes in PHP?

In Java Programming, Can we call a static method of an abstract class?&lt;br&gt;
Yes I know we can&#39;t use static with a method of an abstract class. but I want to know why.. ?




Can we use static method in an abstract class?

I am not asking this -&gt; [Why is there no multiple inheritance in Java, but implementing multiple interfaces is allowed?][1]

In Java, multiple inheritance isn&#39;t allowed, but, after Java 8, Interfaces can have *default methods* (can implement methods itself), just like abstract classes. Within this context, it multiple inheritance should also be allowed.

    interface TestInterface 
    { 
        // abstract method 
        public void square(int a); 
      
        // default method 
        default void show() 
        { 
          System.out.println(&quot;Default Method Executed&quot;); 
        } 
    } 


  [1]: https://stackoverflow.com/questions/2515477/why-is-there-no-multiple-inheritance-in-java-but-implementing-multiple-interfac

Why does Java not allow multiple inheritance but does allow conforming to multiple interfaces with default implementations

I just start out with an example, that explains it best:

    public abstract class A{
    	static String str;
    }

    public class B extends A{
    	public B(){
    		str = &quot;123&quot;;
    	}
    }

    public class C extends A{
    	public C(){
    		str = &quot;abc&quot;;
    	}
    }

    public class Main{
    
    	public static void main(String[] args){
    		A b = new B();
    		A c = new C();
    		System.out.println(&quot;b.str = &quot; + b.str);
    		System.out.println(&quot;c.str = &quot; + c.str);
    	}
    }

This will print out:

b.str = abc

c.str = abc


But I would like a solution where each subclass that instantiate the super class, has their **own** class variable, at the same time I want to be able to reference that class variable through the identifier, or a method call, defined in the abstract super class.

So I would like the output to be: 

b.str = 123 

c.str = abc

Is that doable? 



Java: static field in abstract class

How can I force a subclass to implement a method in Ruby. There doesn&#39;t seem to be an abstract keyword in Ruby, which is the approach I would take in Java. Is there another more Ruby-like way to enforce abstract? 

Abstract Method in Ruby

As we all know, some languages have the notion of interfaces. This is Java:

    public interface Testable {
      void test();
    }

How can I achieve this in C++ (or C++11) in most compact way and with little code noise? I&#39;d appreciate a solution that wouldn&#39;t need a separate definition (let the header be sufficient). This is a very simple approach that even I find buggy ;-) 

    class Testable {
    public:
      virtual void test() = 0;
    protected:
      Testable();
      Testable(const Testable&amp; that);
      Testable&amp; operator= (const Testable&amp; that);
      virtual ~Testable();
    }

This is only the beginning.. and already longer that I&#39;d want. How to improve it? Perhaps there is a base class somewhere in the std namespace made just for this?

All Abstract Solutions on Gang of Coders

Total of 33 Abstract Solutions