What is the difference between abstract methodand method overriding in java? Because same result can be found by using method overriding. so what is the necessity of abstract method.
An abstract method forces a programmer who inherits the method to define an implementation for it, i.e. the class says "I will need an implementation of this function, which the concrete implementation of this class must provide".
This is in contrast to overriden methods, which allow (rather than require) the inheriting class to change the implementation of the method for objects of that class.
In particular, overridable methods (or "virtual" methods) have a basic implementation (which can itself be empty) that the overriding method can call.
The abstract method needs to be implemented in order for the subclass to be utilized. You must also implement non-abstract methods, for instance, in the case of interface method signatures.
I think it is correct to say that you implement abstract methods in the same way that you implement method stubs of an interface. Overriding is not the same as implementing because you can override methods which are not stubs. Subclasses often override methods where the superclass is neither an interface nor an abstract class. So the term overriding has a broader meaning than implementation.
You may find this tutorial helpful as it has quite a clear definition.
http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html
Hope that helps!
The two have different usages which can't be compared.
An Abstract Class is created only to be sub classed by some other class. The implementing class necessarily has to implement all the abstract methods declared inside it. However Method Overriding is the ability of a subclass to override a method from an inherited superclass whose behavior is "close enough" and then to modify behavior as needed. An Abstract Method is one that is declared without an implementation and needs to be necessarily implemented in the subclass.
Related
In Java inheritance, can the expression "overriding an abstract method" be used interchangeably with the expression "implementing an abstract method"?
I came across the following question in the book "OCA Java SE 8 Programmer I Study Guide" in Chapter 5, page 296, question #15:
Q: Which of the following is true about a concrete subclass? (Choose all that apply)
A. A concrete subclass can be declared as abstract.
B. A concrete subclass must implement all inherited abstract methods.
C. A concrete subclass must implement all methods defined in an inherited interface.
D. A concrete subclass cannot be marked as final.
E. Abstract methods cannot be overridden by a concrete subclass.
My answer was B & E. But the book says, the correct answer is only B.
My question is specifically about the option E. The book says, option E is incorrect because abstract methods must be overridden by a concrete subclass. My initial thought was abstract methods must be implemented before being overridden by a subclass. So why is option be incorrect?
Let me explain each points one at a time.
A. A concrete subclass can be declared as abstract.
Ans: Incorrect.
By the definition, concrete classes are those which can be instantiated and this class can also extends the base abstract class.
B. A concrete subclass must implement all inherited abstract methods.
Ans: Correct
If the methods are marked as abstracts in the base class, then the child class extending base class should make sure that all the abstract methods are implemented. You will otherwise get a compile time error.
C. A concrete subclass must implement all methods defined in an inherited interface.
Ans: Incorrect
this is only the case when the concrete subclass has directly implemented the interface by this class itself.
But in this case, base class may have implemented an inherited interface .Hence, its not mandatory for concrete sub-class to implement all methods that are there in base class as a result of inheritance from the interface.
D. A concrete subclass cannot be marked as final.
Ans: Incorrect
If a subclass is marked as final, then this class can no longer be extended by any other available sub-classes. Marking a class with a final keyword is particularly used to achieve Immutability.
With Immutable classes, We can call accessor methods (e.g. getter methods), copy the objects, or pass the objects around — but no method should allow modifying the state of the object. Examples: String, wrapper classes like Float, Integer.etc.
E. Abstract methods cannot be overridden by a concrete subclass.
Ans: Incorrect
By the definition, Abstract methods are methods that are only declared but contains no body or implementation.And since we cannot instantiate abstract class, it is up-to sub classes to provide the implementations for these defined abstract methods.Hence abstract methods are not optional but mandatorily be overridden by a inheriting subclass.
An abstract method is a method that is declared, but contains no implementation.
you can override both abstract and normal methods inside an abstract class.
only methods declared as final cannot be overridden.
An abstract class can have both abstract and non abstract methods. What is the point of having non abstract methods in the abstract class if a new object of the abstract class can't be created?
I know you can override the non-abstract method in a child class and then use it through the object of the child class. But if you are doing that, what is the need of having the non-abstract method with an implementation in the first place?
Think more or google more.
If your child classes have the common functionality then why you will override the method in every class? you can use the base class(which is abstract in this case) method.There comes the need of non-abtract(concrete as they called mostly) methods.
while having abstract method there(as you know already i think), we can override according to our requirement.
If you need all methods should be override in every child classes according to their requirement, then you can go for Interface.
Simple answer: Reuse and maintainability.
Suppose there are 4 concrete classes extending your abstract class which are all going to share some behaviors.
In this case it is better to implement the method in abstract class rather than defining it separately in all your concrete classes.
Concrete subclasses can use methods in the abstract superclass. So all shared functionality between subclasses can go into the base abstract class.
Code reusage. If you don't override non-abstract methods in your inheriting class(es) you inherit them from the abstract class.
To instead have them in the subclass violates the principle of DRY (don't repeat yourself): if all the subclasses have the same function, why write it repeatedly in every class?
As I know the subclass constructor calls the super class constructor by using super();.
But since interface doesn't have any constructor, how can inheritance take place?
But since interface doesn't have any constructor how can inheritance take place??
Easy, an interface cannot have any instance fields so there is nothing to construct. You cannot place in code in an interface (up to Java 7 anyway) so there is nothing which needs to be called.
The interface is a contract, defining what methods mush be offered by the implementation. A class doesn't inherit an interface but implements it.
From the specification :
This type has no implementation, but otherwise unrelated classes can
implement it by providing implementations for its abstract methods.
Interfaces (also known as Service Contracts) are implemented, not constructed. They define a set of methods (Services) that a class provides, so a client knows what can he expect of the implementing class regardless of the actual type implementing the interface. The constructor is related to this particular instance of a given type, implementing the interface.
IYourObject yourObject = new YourObject();
On the other hand, interface inheritance is also by extension. It "adds" the methods of an interface to another one and allow the possibility of switching the interface type of an object amongst the different interfaces in the "hierarchy".
Interface is not inherited - it is rather implemented
Interface doesn't contain constructor because of the following reasons:
The data member of the interface are already initialized .
Constructors of the special defined methods which are not permitted to defined and moreover interface data member are static.
In java, we give body to an abstract method of parent class inside the child class and then call that function via child class object like:
//let the abstract function be fun then,
child c= new child();
c.fun();
then fun executes the body given by child but what's new in this as same being done in function overriding?
The difference is that subclasses that won't also be abstract must implement all inherited abstract methods. Also, the concrete implementation can't call super.method() as there's no implementation in the abstract class.
As to the why, or perhaps more explicitly, why choose the abstract superclass rather than an interface: often it's useful to provide implementations of only some methods of an interface - for instance for code-sharing - and require concrete subclasses to provide specific implementations for other parts of the interface.
The abstraction means that the developer has (explicit) to implement the method. Overriding means that the developer implement a method that's already implement. This is by definition of the languaje.
This is a very naiveish question, but here goes:
An overriden method from a base class will mean that calls to the sub class will call the derived, overriden method, correct?
Thus, if there is no override annotation, the method in the base class will be called. So the override method will serve purely to document the intent - call one version of a method over the other.
Is this the case?
This leads me to the following question:
What is the difference between an abstract class which 5-6 classes may derive from but the methods inherited in the derived classes are not overriden, and one class (Static or not being irrelevant), used by those 5-6 classes?
The #Override annotation is intended ONLY to catch errors at compilation time. It does not affect override behavior at runtime. The idea is that you give the compiler the chance to inform you that your method name or signature is wrong.
The override annotation is simply a marker to show the the method overrides a superclass method.
It being there has no effect at runtime. See here:
http://download.oracle.com/javase/1.5.0/docs/api/java/lang/Override.html
Using abstract is an architectural design to ensure that users either must implement an abstract method (declared but not implemented in the base class) or that they all have access to the same base methods and data (this is the same as the latter example you gave).
Thus, if there is no override
annotation, the method in the base
class will be called.
No. The overriding method in the derived class will always be called.
So the override
method will serve purely to document
the intent - call one version of a
method over the other.
Is this the case?
No. As per the other answers, it tells the compiler to insist that there was something to override.