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An Abstract class is a special kind of class that cannot be instantiated. It has one or more methods which are not implemented in the class. These methods are declared abstract and they do not contain any code inside them.
Ex:
abstract class Parent {
public abstract String getSon();
public abstract String getDaughter();
....
....
//More methods that contain specific behaviour/code in them
}
The above is an abstract class "Parent" that has a lot of functionality but it has declared two abstract methods which have no code inside them. Any class that has one or more abstract methods has to be abstract. This abstract class cannot be instantiated.
i.e., the below piece of code will not work. The code will not even compile.
Parent object = new Parent();
Purpose of Abstract Classes:
Abstract classes are generally used where you want an amount of behaviour to be used by the class that extends the abstract class while at the same time giving options to the child class to provide a certain amount of behaviour itself.
A Child Class extending the Abstract Class:
public class Child extends Parent {
public String getSon() {
return "Sons Name";
}
public String getDaughter(){
return "Daughters Name";
}
...
... //Code specific to the Child class
}
You can also have methods that have code implementation in the abstract class but those methods cannot have the abstract keyword in their method declaration.
What else can I help you with?
What can be declared in an abstract class?
Abstract classes are to be extended until to a concrete class.Can have both abstract & non abstract methods.An Abstract class can not be instantiated.A non abstract class can be extended to an abstract class.If At least one abstract method present in a class then that class must be abstract.abstract & final modifiers can never be together.abstract classes can have both abstract methods & non abstract methods.
When to use a non-abstract method in an abstract class?
In an abstract class some methods could be abstract meaning a sub-class must provide the actual implementation code or non-abstract in those cases the functionality is common to all or most of the sub-classes. With respect to the non-abstract classes, some sub-classes could override those as needed unless they're defined final.Example of such classes found in the core J2SE API include:java.awt.Component, java.awt.geom.Point2D, java.io.InputStream, java.io.Reader, java.util.AbstractCollection, and many others.Take java.util.AbstractCollection as a typical example. It provides placeholders for abstract methods iterator() and size(), and provides the concrete methods clear(), contains(), isEmpty(), etc. which use the former abstract methods to perform a generic function as defined in the specific implementation of a sub-class. The abstract class defers the details of the specific implementation to its sub-classes. The class java.util.HashSet extends AbstractCollection and defines the non-abstract implementation code for both iterator() and size() methods.
What is the difference between an interface and an abstract class?
We can't instantiate both interfaces and abstract classes.The only one difference between them is that an interface can't contain concrete(fully defined) methods where as an abstract class may contain them.An abstract class not necessarily contain abstract methods. we can make a class as abstract class even it does not has any abstract methods.When there is a need to write both abstract and concrete methods in a single unit we have to use an abstract class instead of an interface since an interface cant contain concrete methods.All the fields(or properties) of an interface are by default 'static final' even when you don't mention explicitly. And all methods are 'public abstract'.But in an abstract class we can have any type of fields and methods.
Subclasses of an abstract class that do not provide an implementation of an abtract method are also abstract?
Yes. Any class that does not provide implementation to all its methods as well as its parent class methods needs to be Abstract. The Java compiler would not successfully compile a class that does not do this.
What is abstract base class?
An abstract class is a class that cannot be directly instantiated. The purpose of such a class is to put some logic in a base class and force derived classes to implement the remaining functionality. Since the full functionality is only available in the derived class, the base class is declared as abstract so that it cannot be instantiated directly.
What can be declared in an abstract class?
Abstract classes are to be extended until to a concrete class.Can have both abstract & non abstract methods.An Abstract class can not be instantiated.A non abstract class can be extended to an abstract class.If At least one abstract method present in a class then that class must be abstract.abstract & final modifiers can never be together.abstract classes can have both abstract methods & non abstract methods.
When to use a non-abstract method in an abstract class?
In an abstract class some methods could be abstract meaning a sub-class must provide the actual implementation code or non-abstract in those cases the functionality is common to all or most of the sub-classes. With respect to the non-abstract classes, some sub-classes could override those as needed unless they're defined final.Example of such classes found in the core J2SE API include:java.awt.Component, java.awt.geom.Point2D, java.io.InputStream, java.io.Reader, java.util.AbstractCollection, and many others.Take java.util.AbstractCollection as a typical example. It provides placeholders for abstract methods iterator() and size(), and provides the concrete methods clear(), contains(), isEmpty(), etc. which use the former abstract methods to perform a generic function as defined in the specific implementation of a sub-class. The abstract class defers the details of the specific implementation to its sub-classes. The class java.util.HashSet extends AbstractCollection and defines the non-abstract implementation code for both iterator() and size() methods.
What is the difference between an interface and an abstract class?
We can't instantiate both interfaces and abstract classes.The only one difference between them is that an interface can't contain concrete(fully defined) methods where as an abstract class may contain them.An abstract class not necessarily contain abstract methods. we can make a class as abstract class even it does not has any abstract methods.When there is a need to write both abstract and concrete methods in a single unit we have to use an abstract class instead of an interface since an interface cant contain concrete methods.All the fields(or properties) of an interface are by default 'static final' even when you don't mention explicitly. And all methods are 'public abstract'.But in an abstract class we can have any type of fields and methods.
What is meant by function overriding in c plus plus?
Function overriding applies to class member functions (methods), where a derived class provides a more specialised implementation of its generic base class method. An override can still call the base class method and augment it with other instructions (before or after the call to the base class method), or it can provide a complete implementation of its own without calling the base class method. Classes that are intended to act as generic base classes will generally declare their methods to be virtual, meaning they are intended to be overridden, if required. Abstract base classes will contain one or more pure-virtual methods (which may or may not provide a generic implementation), meaning classes must be derived from the abstract base class and all the pure-virtual methods must be overridden in the derived class (otherwise they, too, become abstract). Only classes that fully implement all the pure-virtual methods they inherit from their base classes can actually be instantiated. That is, you cannot instantiate an instance of an abstract base class, even if it provides generic implementations for all its pure-virtual methods. They can only be instantiated by deriving classes from them.
Why an interface can not have anything but constants and abstract methods?
The idea is that an interface is not a class, it is just a specification of what classes that implement it must contain.
Implementation of queue?
Queues are commonly implemented for abstract routines and data access structures. In languages using object-orientation, they may be featured as classes. Some methods for implementation include circular buffers and linked tests.
What is abstract methods?
Simply, Abstract method is a method that is declared without or containing no implementation. It has a method signature
Subclasses of an abstract class that do not provide an implementation of an abtract method are also abstract?
Yes. Any class that does not provide implementation to all its methods as well as its parent class methods needs to be Abstract. The Java compiler would not successfully compile a class that does not do this.
What is interface in java?
Interface is collection of abstract methods which has only declaration and no implementation
What is abstract base class?
An abstract class is a class that cannot be directly instantiated. The purpose of such a class is to put some logic in a base class and force derived classes to implement the remaining functionality. Since the full functionality is only available in the derived class, the base class is declared as abstract so that it cannot be instantiated directly.
Can you declare abstract methods in normal classes in c?
yes
What is the abstract feature in c plus plus?
Abstraction generally refers to abstract classes. An abstract class is one that cannot itself be instantiated, but can be derived from. Abstract classes are conceptual classes rather than concrete classes, and are intended to provide a common interface to the concrete classes derived from them. A class becomes abstract when it contains one or more pure-virtual methods, which must be implemented in the derived classes otherwise they become abstract classes themselves. A concrete class is one that provides a complete implementation for is abstract base classes, or inherits implementations from base classes other than those that originally declared the function to be pure-virtual. An example of an abstract class is Shape. All shapes can be drawn, therefore Shape will have a Draw() method. However, it cannot implement this method unless it knows what type of shape it actually is. Therefore the Draw() method should be declared pure-virtual, thus rendering the Shape class to be abstract. You can then derive concrete shapes from the Shape class, such as Circle and Square classes, which will each provide the actual implementation for the Draw() method. Since Square and Circle are types of Shape, you can create collections of Shape objects and call their Draw() methods without regard to their actual type. If the object is really a Square, then calling Shape.Draw() will actually invoke Square.Draw(). There is no need to determine the actual type of the shape because every shape has a common interface. This is really no different to overriding virtual methods in non-abstract classes. The main difference is that one would not be expected to instantiate a Shape (because it is abstract), other than through derivation, and the pure-virtual methods MUST be implemented by the derived classes, whereas normal virtual methods are simply expected to be overridden but it is not a requirement to do so unless you require an enhancement to the base class method. Abstract classes typically have few if any member variables and little or no implementation. Ultimately, the class exists purely to provide a common interface to all its derivatives, and only functionality and member variables that are common to all its derivatives should be placed within it. Even if the abstract class provides a complete implementation for all its pure-virtual methods, you cannot instantiate an abstract class -- it must be derived from -- and all pure-virtual methods must still be implemented in the derivatives, even if only to call the base class methods explicitly.
