My design problem is as follows.
I have two classes, each with a number of subclasses. I have a factory, which needs to create an object based on the subclass of each of these objects.
This is an authentication problem. The factory generates a rule object based on the type of person and the type of resource they wish to access. The rule has alwaysAllow, NeverAllow and timeBasedAllow subclasses. With the potential for more if a more complex access system is needed in the future.
So in future ideally a new person could be created with a new subclass, a new resource with a new subclass. The parameters on which access is determined could be changed with a new rule subclass, and the specific access of each person type and room type could be changed within the rule factory.
So far the only way I can think to do this would be to have an enumeration inside the subclasses, which defeats the point because then adding a new person or room requires a new class and a change in the enum class which seems messy.
I also am very keen to keep the data and the logic separate so I can’t just move authentication methods into the person class because this would require the person class to know how many room types there were, which is definitely not ideal.
I may be after something that isn’t realistically achievable but I can’t help the feeling that there is a nice clean solution just out of my grasp.
Any help would be greatly appreciated.
Your question title makes it sound as if you are searching for multiple-inheiritance, which is not allowed in Java. Unlike in C++, a class may extend one and only one class. However, Java also has interface, which I suspect may be what you seek.
An interface class cannot be instantiated, and may have abstract methods. A
concrete class may implement as many interfaces as desired, and the concrete class must implement each abstract method the interface declares. Abstract classes may also implement interfaces, and abstract methods they do not implement must be implemented by concrete classes extending them.
I suggest extracting your authentication methods into aninterface, perhaps called AuthRule or somesuch. AuthRule can have abstract methods with represent authenticating, without exposing the exact style used to authenticate. So, you would implement AlwaysAllow implements AuthRule and then the authenticate methods on AlwaysAllow would always return true.
The second thing, however, is that you appear to be attempting to use inheritance when composition would better suit your needs. Now instread of having a Person inherit his authentication-rule, the rule should instead be a member field inside Person. So, for example:
class Person extends User {
AuthRule rule;
Person(AuthRule myrule) {
rule = myrule;
}
bool authenticate(...) {
return rule.authenticate(...);
}
}
If you follow a design pattern based on injecting objects into other objects to mix in the functionality you desire, your code will become far more usable and extensible. I hope this helps your problem.
I'm new at programming and I'm learning Java.
I was just wondering why I should use an interface when there is only one implementation class?
You do this to prevent others from accessing your implementing type. For example, you could hide your implementing type inside a library, give the type package access, and return an instance of your interface to the users of your library:
// This is what the users of your library know about the class
// that does the work:
public interface SomeInterface {
void doSomethingUseful();
void doSomethingElse();
}
// This is the class itself, which is hidden from your clients
class MyImplementation implements SomeInterface {
private SomeDependency dependency = new SomeDependency();
public void doSomethingUseful() {
...
}
public void doSomethingElse() {
...
}
}
Your clients obtain objects like this:
public class MyFactory {
static SomeInterface make() {
// MyFactory can see MyImplementation
return new MyImplementation();
}
}
This trick becomes useful when the implementation uses lots of libraries. You efficiently decouple the interface of your library from its implementation, so that the user wouldn't have to know about the dependencies internal to your library.
One reason is to maintain the open/closed principle, which states that your code should be open for extension, but closed for modification. Although you only have one implementing class now, chance is that you will need another differing implementation class with the passing of time. If you extract the implementation into an interface beforehand, you just have to write another implementing class ie. You don't have to modify a perfectly working piece of code, eliminating the risks of introducing bugs.
You shoulnt do anything without thinking and reasoning.
There might be cases where you might want to add an interface even for a single implementation ... but IMO that's an OBSOLETE PRACTICE coming from old EJB times, that people use and enforce without the proper reasoning and reflection.
... and both Martin Fowler, Adan Bien, and others has been saying it for years.
https://martinfowler.com/bliki/InterfaceImplementationPair.html
https://www.adam-bien.com/roller/abien/entry/service_s_new_serviceimpl_why
To respect the Interface Segregation Principle.
The decision to create an interface should not be based on the number of implementing classes, but rather on the number of different ways that object is used. Each ways the object is used is represented by an interface, defined with the code that uses it. Say your object needs to be stored in memory, in collections that keep objects in order. That same object and also needs to be stored in some persistent storage.
Say you implement persistence first. What is needed by the storage system is a unique identifier for the persisted objects. You create an interface, say Storable, with a method getUniqueId. You then implement the storage.
Then, you implement the collection. You define what the collection needs from stored objects in an interface, like Comparable, with a method compareTo. You can then implement the collection with dependency on Comparable.
The class you want to define would implement both interfaces.
If the class you are defining implement a single interface, that interface would have to represent the needs of the collection and storage system. That would cause, for example:
unit tests for the collection would have to be written with objects that implement Storable, adding a level of complexity.
if the need arise later to display the object, you would have to add methods needed by the display code to the single interface, and modify the tests for collection and storage to also implement the methods needed for display.
I talk about impact on test code here. The problem is larger if other production level objects need storage and not display. The larger the project, the larger the issue created by not respecting the interface segregation principle will become.
It can give you the flexibility to add more implementations in the future without changing the client code which references the interface.
Another example of when it can be useful is to simulate multiple inheritance in Java when it is needed. For example, suppose you have an interface MyInterface and an implementation:
public interface MyInterface {
void aMethod1();
void aMethod2();
}
class MyInterfaceImpl implements MyInterface {
public void aMethod1() {...}
public void aMethod2() {...}
}
You also have an unrelated class with its own hierarchy:
public class SomeClass extends SomeOtherClass {
...
}
Now you want to make SomeClass be of type MyInterface but you also want to inherit all the code that is already existing in MyInterfaceImpl. Since you cannot extend both SomeOtherClass and MyInterfaceImpl, you can implement the interface and use delegation:
public class SomeClass extends SomeOtherClass implements MyInterface {
private MyInterface myInterface = new MyInterfaceImpl();
public void aMethod1() {
myInterface.aMethod1();
}
public void aMethod2() {
myInterface.aMethod2();
}
...
}
I see a lot of good points being made in this post. Also wanted to add my 2 cents to this collection of knowledge.
Interfaces Encourage parallel development in a team environment. There can be 2 classes A and B, with A calling B's API. There can be 2 developers simultaneously working on A and B. while B is not ready, A can totally go about it's own implementation by integrating with B's interfaces.
Interfaces serve as a good ground for establishing API Contracts between different layers of code.
It's good to have a separation of concerns with Interface handling implicit API documentation. it's super easy to refer to one and figure which APIs are accessible for the clients to call.
Lastly, it's better to practice using interfaces as a standard in a project than having to use it on a case by case bases (where you need multiple implementations). This ensures consistency in your project.
For the Art that Java Code is, interfaces make thmem even more beautiful :)
Interfaces can be implemented by multiple classes. There is no rule that only one class can implement these. Interfaces provide abstraction to the java.
http://www.tutorialspoint.com/java/java_interfaces.htm
You can get more information about interfaces from this link
How is loose coupling associated with interfaces when we are bound to create an implementation class regardless? The implementation class is forced to implement all those methods defined in the interface. I don't understand how this allows for lose coupling? I'm new to object oriented programming and software design so if you could shed some light on this topic it would super helpful. An example would totally be icing on the cake.
The key point is that an interface doesn't just allow you to write one class which implements it, it allows you to write several.
When you have code which interacts with a class by using an interface, that code is able to work together with any class which implements said interface, regardless of how it implements it. That allows you to feed different classes to the same code without having to modify it.
Please note that interfaces are not the only way to reach a loose coupling of components. Loose coupling just means that components are able to work together without assuming anything about the internal workings of each other. You do that because the more your components treat each other as black boxes, the easier it becomes to do changes at one component without affecting any others. Interfaces can be one tool to work towards this goal, but neither are they required, nor are they the only tool which is worth mentioning in this regard.
The implementing class is able to choose HOW to implement the functionality.
public interface PersonRepository {
Person getPerson(String name);
}
Could be implemented by reading through a CSV file or by querying a database. The object which needs the person does not care how the person is found or loaded just that it is.
Hence it is deemed to be loosely coupled.
If it was tightly coupled it would need to know how to construct a SQL query or read a CSV file.
the client code is coupled to the interface. it is not coupled to the implementation. you can change t he implementation without compiling the client code or the interface.
see http://en.wikipedia.org/wiki/Dependency_inversion_principle and http://en.wikipedia.org/wiki/Open/closed_principle
IMHO, interfaces dont solve the coupling problem you describe. They solve implementation coupling problem. As long as you use the interface you are unaware of the implementation you choose. But you are sill bound to that interface. And what you describe, call coupling remains. In order to reduce call coupling you had to use another approach, i.e. Command Pattern
Might below explanation can answer this :
Into class A we need to have Object of class B. If directly exposure of B into A is there means there is Tight coupling. ex: one can add more methods into B or anything.
Which means Behavior of A can be changed based on more exposure of B. But if B class is implementing any Interface and we are passing that ref of Interface into A. means whatever changes in B class further user A class is not bother because we use ref of Interface for accessing B and got only required access.
Ex :
class A {
public void add( B b){
// Implementation
}
}
class B{
}
this is Tight coupling. Because user can make any changes to class B which are directly exposed to class A and this defines Tight Coupling(Greater the exposure of depending Object , more Tight Coupling). To resolve this one refer :
Interface L{
}
class B implements L{}
class A{
public void add(L b){
// Implementation
}
}
Since we are passing the ref of Interface L, adding changes into Implementation of B will not make any difference to class A.
I still have some confusion about this thing. What I have found till now is
(Similar questions have already been asked here but I was having some other points.)
Interface is collection of ONLY abstract methods and final fields.
There is no multiple inheritance in Java.
Interfaces can be used to achieve multiple inheritance in Java.
One Strong point of Inheritance is that We can use the code of base class in derived class without writing it again. May be this is the most important thing for inheritance to be there.
Now..
Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing any interface then it is inheritance ? We are not using its code.
Q2. If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritance ?
Q3. Anyhow what is the benefit of using Interfaces ? They are not having any code. We need to write code again and again in all classes we implement it.
Then why to make interfaces ?
NOTE : I have found one case in which interfaces are helpful. One example of it is like in Runnable interface we have public void run() method in which we define functionality of thread and there is built in coding that this method will be run as a separate thread. So we just need to code what to do in thread, Rest is pre-defined. But this thing also can be achieved using abstract classes and all.
Then what are the exact benefits of using interfaces? Is it really Multiple-Inheritance that we achieve using Interfaces?
Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing any interface then it is inheritance ? We are not using its code.
We can't. Interfaces aren't used to achieve multiple inheritance. They replace it with safer, although slightly less powerful construct. Note the keyword implements rather than extends.
Q2. If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritance ?
They are not. With interfaces a single class can have several "views", different APIs or capabilities. E.g. A class can be Runnable and Callable at the same time, while both methods are effectively doing the same thing.
Q3. Anyhow what is the benefit of using Interfaces ? They are not having any code. We need to write code again and again in all classes we implement it.
Interfaces are kind-of multiple inheritance with no problems that the latter introduces (like the Diamond problem).
There are few use-cases for interfaces:
Object effectively has two identities: a Tank is both a Vehicle and a Weapon. You can use an instance of Tank where either the former or the latter is expected (polymorphism). This is rarely a case in real-life and is actually a valid example where multiple inheritance would be better (or traits).
Simple responsibilities: an instance of Tank object in a game is also Runnable to let you execute it in a thread and an ActionListener to respond to mouse events.
Callback interfaces: if object implements given callback interface, it is being notified about its life-cycle or other events.
Marker interfaces: not adding any methods, but easily accessible via instanceof to discover object capabilities or wishes. Serializable and Cloneable are examples of this.
What you are looking for are trait (like in Scala), unfortunately unavailable in Java.
Interfaces are collection of final static fields and abstract methods (Newly Java 8 added support of having static methods in an interface).
Interfaces are made in situations when we know that some task must be done, but how it should be done can vary. In other words we can say we implement interfaces so that our class starts behaving in a particular way.
Let me explain with an example, we all know what animals are. Like Lion is an animal, monkey is an animal, elephant is an animal, cow is an animal and so on. Now we know all animals do eat something and sleep. But the way each animal can eat something or sleep may differ. Like Lion eats by hunting other animals where as cow eats grass. But both eat. So we can have some pseudo code like this,
interface Animal {
public void eat();
public void sleep();
}
class Lion implements Animal {
public void eat() {
// Lion's way to eat
}
public void sleep(){
// Lion's way to sleep
}
}
class Monkey implements Animal {
public void eat() {
// Monkey's way to eat
}
public void sleep() {
// Monkey's way to sleep
}
}
As per the pseudo code mentioned above, anything that is capable of eating or sleeping will be called an animal or we can say it is must for all animals to eat and sleep but the way to eat and sleep depends on the animal.
In case of interfaces we inherit only the behaviour, not the actual code as in case of classes' inheritance.
Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing any interface then it is inheritance ? We are not using its code.
Implementing interfaces is other kind of inheritance. It is not similar to the inheritance of classes as in that inheritance child class gets the real code to reuse from the base class.
Q2. If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritance ?
It is said because one class can implement more than one interfaces. But we need to understand that this inheritance is different than classes' inheritance.
Q3. Anyhow what is the benefit of using Interfaces ? They are not having any code. We need to write code again and again in all classes we implement it.
Implementing an interface puts compulsion on the class that it must override its all abstract methods.
Read more in my book here and here
Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing any interface then it is inheritance ? We are not using its code.
Unfortunately, in colloquial usage, the word inheritance is still frequently used when a class implements an interface, although interface implementation would be a preferable term - IMO, the term inheritance should strictly be used with inheritance of a concrete or abstract class. In languages like C++ and C#, the same syntax (i.e. Subclass : Superclass and Class : Interface) is used for both class inheritance and interface implementation, which may have contributed to the spread of the misuse of the word inheritance with interfaces. Java has different syntax for extending a class as opposed to implementing an interface, which is a good thing.
Q2 If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritance ?
You can achieve the 'effect' of multiple inheritance through composition - by implementing multiple interfaces on a class, and then providing implementations for all methods, properties and events required of all the interfaces on the class. One common technique of doing this with concrete classes is by doing 'has-a' (composition) relationships with classes which implement the external interfaces by 'wiring up' the implementation to each of the internal class implementations. (Languages such as C++ do support multiple concrete inheritance directly, but which creates other potential issues like the diamond problem).
Q3 Anyhow what is the benefit of using Interfaces ? They are not having any code. We need to write code again and again in all classes we implement it.
Interfaces allow existing classes (e.g. frameworks) to interact with your new classes without having ever 'seen' them before, because of the ability to communicate through a known interface. Think of an interface as a contract. By implementing this interface on a class, you are contractually bound to meet the obligations required of it, and once this contract is implemented, then your class should be able to be used interchangeably with any other code which consumes the interface.
Real World Example
A 'real world' example would be the legislation and convention (interface) surrounding an electrical wall socket in a particular country. Each electrical appliance plugged into the socket needs to meet the specifications (contract) that the authorities have defined for the socket, e.g. the positioning of the line, neutral and earth wires, the position and colouring of the on / off switch, and the conformance the the electrical voltage, frequency and maximum current that will be supplied through the interface when it is switched on.
The benefit of decoupling the interface (i.e. a standard wall socket) rather than just soldering wires together is that you can plug (and unplug) a fan, a kettle, a double-adapter, or some new appliance to be invented next year into it, even though this appliance didn't exist when the interface was designed. Why? Because it will conform to the requirements of the interface.
Why use interfaces?
Interfaces are great for loose coupling of classes, and are one of the mainstay's of Uncle Bob's SOLID paradigm, especially the Dependency Inversion Principle and Interface Segregation Principles.
Simply put, by ensuring that dependencies between classes are coupled only on interfaces (abstractions), and not on other concrete classes, it allows the dependency to be substituted with any other class implementation which meets the requirements of the interface.
In testing, stubs and mocks of dependencies can be used to unit test each class, and the interaction the class has with the dependency can be 'spyed' upon.
KISS
I have searched for days, nay weeks trying to understand interfaces and seem to read the same generic help; I'm not trying to disparage the contributions, but i think the light-bulb just clicked so I'm chuffed :))
I prefer to Keep It Simple Stupid, so will proffer my new found view of interfaces.
I'm a casual coder but i want to post this code i wrote in VB.NET (the principle is the same for other languages), to help others understand interfaces.
If i have it wrong, then please let others know in follow up comments.
Explanation
Three buttons on a form, clicking each one saves a different class reference to the interface variable (_data). The whole point of different class references into an interface variable, is what i didn't understand as it seemed redundant, then its power becomes evident with the msgbox, i only need to call the SAME method to perform the task i need, in this case 'GetData()', which uses the method in the class that's currently held by the interface reference variable (_data).
So however i wish to get my data (from a database, the web or a text file), it's only ever done using the same method name; the code behind that implementation...i don't care about.
It's then easy to change each class code using the interface without any dependency...this is a key goal in OO and encapsulation.
When to use
Code classes and if you notice the same verb used for methods, like 'GetData()', then it's a good candidate to implement an interface on that class and use that method name as an abstraction / interface.
I sincerely hope this helps a fellow noob with this difficult principle.
Public Class Form1
Private _data As IData = Nothing
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
_data = New DataText()
MsgBox(_data.GetData())
End Sub
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
_data = New DataDB()
MsgBox(_data.GetData())
End Sub
Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
_data = New DataWeb()
MsgBox(_data.GetData())
End Sub
End Class
Public Interface IData
Function GetData() As String
End Interface
Friend Class DataText : Implements IData
Friend Function GetData() As String Implements IData.GetData
Return "DataText"
End Function
End Class
Friend Class DataDB : Implements IData
Friend Function GetData() As String Implements IData.GetData
Return "DataDB"
End Function
End Class
Friend Class DataWeb : Implements IData
Friend Function GetData() As String Implements IData.GetData
Return "DataWeb"
End Function
End Class
Old question. I'm suprised that nobody quoted the canonical sources: Java: an Overview by James Gosling, Design Patterns: Elements of Reusable Object-Oriented Software by the Gang of Four or Effective Java by Joshua Bloch (among other sources).
I will start with a quote:
An interface is simply a specification of a set of methods that an object responds to. It does not include any instance variables or implementation. Interfaces can be multiply-inherited (unlike classes) and they can be used in a more flexible way than the usual rigid class
inheritance structure. (Gosling, p.8)
Now, let's take your assumptions and questions one by one (I'll voluntarily ignore the Java 8 features).
Assumptions
Interface is collection of ONLY abstract methods and final fields.
Did you see the keyword abstract in Java interfaces? No. Then you should not consider an interface as a collection of abstract methods. Maybe you are misleaded by the C++ so-called interfaces, which are classes with only pure virtual methods. C++, by design, does not have (and does not need to have) interfaces, because it has mutliple inheritance.
As explained by Gosling, you should rather consider an interface as "a set of methods that an object responds to". I like to see an interface and the associated documentation as a service contract. It describes what you can expect from an object that implements that interface. The documentation should specify the pre and post-conditions (e.g. the parameters should be not null, the output is always positive, ...) and the invariants (a method that does not modify the object internal state). This contract is the heart, I think, of OOP.
There is no multiple inheritance in Java.
Indeed.
JAVA omits many rarely used, poorly understood, confusing features of C++ that in our experience bring more grief than benefit. This primarily consists of operator overloading (although it does have method overloading), multiple inheritance, and extensive automatic coercions. (Gosling, p.2)
Nothing to add.
Interfaces can be used to achieve multiple inheritance in Java.
No, simlpy because there is no multiple inheritance in Java. See above.
One Strong point of Inheritance is that We can use the code of base class in derived class without writing it again. May be this is the most important thing for inheritance to be there.
That's called "implementation inheritance". As you wrote, it's a convenient way to reuse code.
But it has an important counterpart:
parent classes often define at least part of their subclasses' physical representation. Because inheritance exposes a subclass to details of its parent's implementation, it's often said that "inheritance breaks encapsulation" [Sny86]. The implementation of a subclass becomes so bound up with the implementation of its parent class that any change in the parent's implementation will force the subclass to change. (GOF, 1.6)
(There is a similar quote in Bloch, item 16.)
Actually, inheritance serves also another purpose:
Class inheritance combines interface inheritance and implementation inheritance. Interface inheritance defines a new interface in terms of one
or more existing interfaces. Implementation inheritance defines a new implementation in terms of one or more existing implementations. (GOF, Appendix A)
Both use the keyword extends in Java. You may have hierarchies of classes and hierarchies of interfaces. The first ones share implementation, the second ones share obligation.
Questions
Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing any interface then it is inheritance ? We are not using its code.**
Implementation of an interface is not inheritance. It's implementation. Thus the keyword implements.
Q2. If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritance ?**
No multiple inheritance in Java. See above.
Q3. Anyhow what is the benefit of using Interfaces ? They are not having any code. We need to write code again and again in all classes we implement it./Then why to make interfaces ?/What are the exact benefits of using interfaces? Is it really Multiple-Inheritance that we achieve using Interfaces?
The most important question is: why would you like to have multiple-inheritance? I can think of two answers: 1. to give mutliple types to an object; 2. to reuse code.
Give mutliple types to an object
In OOP, one object may have different types. For instance in Java, an ArrayList<E> has the following types: Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess, AbstractList<E>, AbstractCollection<E> and Object (I hope I have not forgotten anyone). If an object has different types, various consumers will be able use it without be aware of its specificities. I need an Iterable<E> and you give me a ArrayList<E>? It's ok. But if I need now a List<E> and you give me a ArrayList<E>, it's ok too. Etc.
How do you type an object in OOP? You took the Runnable interface as an example, and this example is perfect to illustrate the answer to this question. I quote the official Java doc:
In addition, Runnable provides the means for a class to be active while not subclassing Thread.
Here's the point: Inheritance is a convenient way of typing objects. You want to create a thread? Let's subclass the Thread class. You want an object to have different types, let's use mutliple-inheritance. Argh. It doesn't exist in Java. (In C++, if you want an object to have different types, multiple-inheritance is the way to go.)
How to give mutliple types to an object then? In Java, you can type your object directly. That's what you do when your class implements the Runnable interface. Why use Runnable if your a fan of inheritance? Maybe because your class is already a subclass of another class, let's say A. Now your class has two types: A and Runnable.
With multiple interfaces, you can give multiple types to an object. You just have to create a class that implements multiple interfaces. As long as you are compliant with the contracts, it's ok.
Reuse code
This is a difficult subject; I've already quoted the GOF on breaking the encapsulation. Other answer mentionned the diamond problem. You could also think of the Single Responsibility Principle:
A class should have only one reason to change. (Robert C. Martin, Agile Software Development, Principles, Patterns, and Practices)
Having a parent class may give a class a reason to change, besides its own responsibilities:
The superclass’s implementation may change from release to release, and if it does, the subclass may break, even though its code has not been touched. As a consequence, a subclass must evolve in tandem with its superclass (Bloch, item 16).
I would add a more prosaic issue: I always have a weird feeling when I try to find the source code of a method in a class and I can't find it. Then I remember: it must be defined somewhere in the parent class. Or in the grandparent class. Or maybe even higher. A good IDE is a valuable asset in this case, but it remains, in my mind, something magical. Nothing similar with hierarchies of interfaces, since the javadoc is the only thing I need: one keyboard shortcut in the IDE and I get it.
Inheritance howewer has advantages:
It is safe to use inheritance within a package, where the subclass and the superclass implementations are under the control of the same programmers. It is also safe to use inheritance when extending classes specifically designed and documented for extension (Item 17: Design and document for inheritance or else prohibit it). (Bloch, item 16)
An example of a class "specifically designed and documented for extension" in Java is AbstractList.
But Bloch and GOF insist on this: "Favor composition over inheritance":
Delegation is a way of making composition as powerful for reuse as inheritance [Lie86, JZ91]. In delegation, two objects are involved in handling a request: a receiving object delegates operations to its delegate. This is analogous to subclasses deferring requests to parent classes. (GOF p.32)
If you use composition, you won't have to write the same code again and again. You just create a class that handles the duplications, and you pass an instance of this class to the classes that implements the interface. It's a very simple way to reuse code. And this helps you to follow the Single Responsibility Principle and make the code more testable. Rust and Go don't have inheritance (they don't have classes either), but I don't think that the code is more redundant than in other OOP languages.
Furthermore, if you use composition, you will find yourself naturally using interfaces to give your code the structure and the flexibility it needs (see other answers on use cases of interfaces).
Note: you can share code with Java 8 interfaces
And finally, one last quote:
During the memorable Q&A session, someone asked him [James Gosling]: "If you could do Java over again, what would you change?" "I'd leave out classes" (anywhere on the net, don't know if this is true)
This is very old question and java-8 release have added more features & power to interface.
An interface declaration can contain
method signatures
default methods
static methods
constant definitions.
The only methods that have implementations in interface are default and static methods.
Uses of interface:
To define a contract
To link unrelated classes with has a capabilities (e.g. classes implementing Serializable interface may or may not have any relation between them except implementing that interface
To provide interchangeable implementation e.g. Strategy_pattern
default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces
Organize helper methods in your libraries with static methods ( you can keep static methods specific to an interface in the same interface rather than in a separate class)
Have a look at this related SE question for code example to understanding the concepts better:
How should I have explained the difference between an Interface and an Abstract class?
Coming back to your queries:
Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing any interface then it is inheritance ? We are not using its code.
Q2. If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritance ?
Interface can contain code for static and default methods. These default methods provides backward compatibility & static methods provides helper/utility functions.
You can't have true multiple inheritance in java and interface is not the way to get it. Interface can contain only constants. So you can't inherit state but you can implement behaviour.
You can replace inheritance with capability. Interface provides multiple capabilities to implementing classes.
Q3. Anyhow what is the benefit of using Interfaces ? They are not having any code. We need to write code again and again in all classes we implement it.
Refer to "uses of interface" section in my answer.
Inheritance is when one class derives from another class (which can be abstract) or an Interface. The strongest point of object oriented (inheritance) is not reuse of code (there are many ways to do it), but polymorphism.
Polymorphism is when you have code that uses the interface, which it's instance object can be of any class derived from that interface. For example I can have such a method:
public void Pet(IAnimal animal) and this method will get an object which is an instance of Dog or Cat which inherit from IAnimal. or I can have such a code:
IAnimal animal
and then I can call a method of this interface:
animal.Eat() which Dog or Cat can implement in a different way.
The main advantage of interfaces is that you can inherit from some of them, but if you need to inherit from only one you can use an abstract class as well. Here is an article which explains more about the differences between an abstract class and an interface:
http://www.codeproject.com/KB/cs/abstractsvsinterfaces.aspx
Both Methods Work (Interfaces and Multiple Inheritance).
Quick Practical Short Answer
Interfaces are better when you have several years of experience using Multiple Inheritance that have Super Classes with only method definition, and no code at all.
A complementary question may be: "How and Why to to migrate code from Abstract Classes to Interfaces".
If you are not using many abstract classes, in your application, or you don't have many experience with it, you may prefer to skip interfaces.
Dont rush to use interfaces.
Long Boring Answer
Interfaces are very similar, or even equivalent to abstract Classes.
If your code has many Abstract classes, then its time you start thinking in terms of Interfaces.
The following code with abstract classes:
MyStreamsClasses.java
/* File name : MyStreamsClasses.java */
import java.lang.*;
// Any number of import statements
public abstract class InputStream {
public void ReadObject(Object MyObject);
}
public abstract class OutputStream {
public void WriteObject(Object MyObject);
}
public abstract class InputOutputStream
imnplements InputStream, OutputStream {
public void DoSomethingElse();
}
Can be replaced with:
MyStreamsInterfaces.java
/* File name : MyStreamsInterfaces.java */
import java.lang.*;
// Any number of import statements
public interface InputStream {
public void ReadObject(Object MyObject);
}
public interface OutputStream {
public void WriteObject(Object MyObject);
}
public interface InputOutputStream
extends InputStream, OutputStream {
public void DoSomethingElse();
}
Cheers.
So. There are a lot of excellent answers here explaining in detail what an interface is. Yet, this is an example of its use, in the way one of my best colleagues ever explained it to me years ago, with what I have learned at university in the last couple of years mixed in.
An interface is a kind of 'contract'. It exposes some methods, fields and so on, that are available. It does not reveal any of its implementation details, only what it returns, and which parameters it takes. And in here lies the answer to question three, and what I feel is one of the greatest strengths of modern OOP:
"Code by addition, Not by modification" - Magnus Madsen, AAU
That's what he called it at least, and he may have it from some other place. The sample code below is written in C#, but everything shown can be done just about the same way in Java.
What we see is a class called SampleApp, that has a single field, the IOContext. IOContext is an interface.
SampleApp does not care one wit about how it saves its data, it just needs to do so, in its "doSomething()" method.
We can imagine that saving the data may have been more important than HOW it was saved at the beginning of the development process, so the developer chose to simply write the FileContext class. Later on, however, he needed to support JSON for whatever reason. So he wrote the JSONFileContext class, which inherits FileContext. This means that it is effectively an IOContext, which has the functionality of FileContext, save the replacement of FileContexts SaveData and LoadData, it still uses its 'write/read' methods.
Implementing the JSON class has been a small amount of work, comparing to writing the class, and having it just inherit IOContext.
The field of SampleApp could have been just of type 'FileContext', but that way, it would have been restricted to ever only using children of that class. By making the interface, we can even do the SQLiteContext implementation, and write to a database, SampleApp will never know or care, and when we have written the SQL lite class, we need only make one change to our code: new JSONFileContext(); instead becomes new SQLiteContext();
We still have our old implementations and can switch back to those if the need arises. We have broken nothing, and all the changes to our code are half a line, that can be changed back within the blink of an eye.
so: Code by addition, NOT by modification.
namespace Sample
{
class SampleApp
{
private IOContext context;
public SampleApp()
{
this.context = new JSONFileContext(); //or any of the other implementations
}
public void doSomething()
{
//This app can now use the context, completely agnostic of the actual implementation details.
object data = context.LoadData();
//manipulate data
context.SaveData(data);
}
}
interface IOContext
{
void SaveData(object data);
object LoadData();
}
class FileContext : IOContext
{
public object LoadData()
{
object data = null;
var fileContents = loadFileContents();
//Logic to turn fileContents into a data object
return data;
}
public void SaveData(object data)
{
//logic to create filecontents from 'data'
writeFileContents(string.Empty);
}
protected void writeFileContents(string fileContents)
{
//writes the fileContents to disk
}
protected string loadFileContents()
{
string fileContents = string.Empty;
//loads the fileContents and returns it as a string
return fileContents;
}
}
class JSONFileContext : FileContext
{
public new void SaveData(object data)
{
//logic to create filecontents from 'data'
base.writeFileContents(string.Empty);
}
public new object LoadData()
{
object data = null;
var fileContents = loadFileContents();
//Logic to turn fileContents into a data object
return data;
}
}
class SQLiteContext : IOContext
{
public object LoadData()
{
object data = null;
//logic to read data into the data object
return data;
}
public void SaveData(object data)
{
//logic to save the data object in the database
}
}
}
Interfaces
An interface is a contract defining how to interact with an object. They are useful to express how your internals intend to interact with an object. Following Dependency Inversion your public API would have all parameters expressed with interfaces. You don't care how it does what you need it to do, just that it does exactly what you need it to do.
Example: You may simply need a Vehicle to transport goods, you don't care about the particular mode of transport.
Inheritance
Inheritance is an extension of a particular implementation. That implementation may or may not satisfy a particular interface. You should expect an ancestor of a particular implementation only when you care about the how.
Example: You may need a Plane implementation of a vehicle for fast transport.
Composition
Composition can be used as an alternative to inheritance. Instead of your class extending a base class, it is created with objects that implement smaller portions of the main class's responsibility. Composition is used in the facade pattern and decorator pattern.
Example: You may create a DuckBoat (DUKW) class that implements LandVehicle and WaterVehicle which both implement Vehicle composed of Truck and Boat implementations.
Answers
Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing any interface then it is inheritance ? We are not using its code.
Interfaces are not inheritance. Implementing an interface expresses that you intend for your class to operate in the way that is defined by the interface. Inheritance is when you have a common ancestor, and you receive the same behavior (inherit) as the ancestor so you do not need to define it.
Q2. If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritance ?
Interfaces do not achieve multiple inheritance. They express that a class may be suitable for multiple roles.
Q3. Anyhow what is the benefit of using Interfaces ? They are not having any code. We need to write code again and again in all classes we implement it.
One of the major benefits of interfaces is to provide separation of concerns:
You can write a class that does something with another class without caring how that class is implemented.
Any future development can be compatible with your implementation without needing to extend a particular base class.
In the spirit of DRY you can write an implementation that satisfies an interface and change it while still respecting the open/closed principal if you leverage composition.
Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing an interface then it is inheritance? We are not using its code.
It is not equal inheritance. It is just similiar. Let me explain:
VolvoV3 extends VolvoV2, and VolvoV2 extends Volvo (Class)
VolvoV3 extends VolvoV2, and VolvoV2 implements Volvo (Interface)
line1: Volvo v = new VolvoV2();
line2: Volvo v = new VolvoV3();
If you see only line1 and line2 you can infer that VolvoV2 and VolvoV3 have the same type. You cannot infer if Volvo a superclass or Volvo is an interface.
Q2. If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritances?
Now using interfaces:
VolvoXC90 implements XCModel and Volvo (Interface)
VolvoXC95 implements XCModel and Volvo (Interface)
line1: Volvo a = new VolvoXC90();
line2: Volvo a = new VolvoXC95();
line3: XCModel a = new VolvoXC95();
If you see only line1 and line2 you can infer that VolvoXC90 and VolvoXC95 have the same type (Volvo). You cannot infer that Volvo is a superclass or Volvo is an interface.
If you see only line2 and line3 you can infer that Volvo95 implements two types, XCModel and Volvo, in Java you know that at least one has to be an interface. If this code was written in C++, for instance, they could be both classes. Therefore, multiple inheritances.
Q3. Anyhow, what is the benefit of using Interfaces? They are not having any code. We need to write code again and again in all classes we implement it.
Imagine a system where you use a VolvoXC90 class in 200 other classes.
VolvoXC90 v = new VolvoXC90();
If you need to evolve your system to launch VolvoXC95 you have to alter 200 other classes.
Now, imagine a system where you use a Volvo interface in 10,000,000 classes.
// Create VolvoXC90 but now we need to create VolvoXC95
Volvo v = new VolvoFactory().newCurrentVolvoModel();
Now, if you need to evolve your system to create VolvoXC95 models you have to alter only one class, the Factory.
It is a common sense question. If your system is composed only of few classes and have few updates use Interfaces everywhere is counterproductive. For big systems, it can save you a lot of pain and avoid risk adopting Interfaces.
I recommend you read more about S.O.L.I.D principles and read the book Effective Java. It has good lessons from experienced software engineers.
Interfaces are made so that a class will implement the functionality within the interface and behave in accordance with that interface.
I'm developping a server application in Java. The server need two type of server classes. These classes have some methods in common, the code in these methods is exactly the same. So i create an abstract super-class containing all the shared code, and both classes are inheriting it. But, there is some part of the code that need to be precised by subclasses. I mean that the superclass "rely" on subclasses methods.
Here is a purified example of what i mean:
public abstract class AbstractServer
{
public void loadConfig(String configPath)
{
//Load the configuration file.
//This code is exactly the same for subclasses.
}
public void startRMI(int port)
{
//Create an empty RMI registry.
//This part also need to be identical.
//Here' where the superclass "rely" on subclasses.
fillRegistry(); //Call the method overwritten by subclasses.
}
/**
Bind remote objects in the RMI registry
*/
protected abstract void fillRegistry(); //This method will be overriten by subclasses.
}
I feel that it's really bad to make it like that, but i can't find another cleaner way to do it.
So, what i want is some advice on how i could make it better.
Thanks, and sorry for my bad english.
Your approach is just fine. Stick with it buddy.
I feel your 'philosophical need' to understand it. Base class 'relying' on the subclass is fine as long as the base class is abstract. It knows that some things have to be registered at this point, but it doesn't have the faintest clue about what exactly to be registered. So the high-level process is encoded in the base class with 'holes' that can be plugged in by the derived class. The high level process and the position of the 'hole' itself is valuable and this justifies the implementation of the base class. The derived classes just follow the fundamental OO principle of 'coding by difference' and plugs the 'holes'.
Looks about right to me after your edits (assuming that you left out the Exception throwing part for readability) :)
All three methods would need to raise exceptions in a real world case.
Super class is inherited by sub-class. You can write methods in super class which you want to make common and leave it untouched. For the other part of code which you want it to be overwritten by sub classes define other set of methods in super class. write methods in sub-classes also. when u call method from sub-class u can put to call super-class method's
in short u have to write methods in sub class to over write the methods of superclass.
I would also make sure that your superclass is actually abstract. In this snippet it isn't. Overall though, looks decent.
Also consider declaring any instance variables in your superclass that classes that extend it will need as well.
First, there is nothing wrong with requiring subclasses' implementation in abstract (base) classes. It's just something that should not get abused, IMO. However, if I had to avoid it, I would make the ServerClass not abstract at all, and define every method of it. Instead, I would create RegistryFactory classes and pass them to the ServerClass :
class ServerClass {
public void startRMI(int port, RegistryFactory rf) {
// ...
rf.fillRegistry(this);
}
}
interface RegistryFactory {
/**
* Implement this method
*/
public void fillRegistry(ServerClass server);
}
public class RMIRegistryFactory implements RegistryFactory {
public void fillRegistry(ServerClass server) { /* ... */ }
}
Or something like that.
Your approach is fine, but it needs a simple improvement to make it perfect - make the startRMI() method final:
public final void startRMI(int port) {
fillRegistry();
}
This way you will prevent that someone overrides it (maybe because of not knowing that everything in startRMI() should be reused and that only fillRegistry() has to be customized).
Your solution generally matches the template method design pattern:
The template method is a method in a superclass, usually an abstract
superclass, and defines the skeleton of an operation in terms of a
number of high-level steps. These steps are themselves implemented by
additional helper methods in the same class as the template method.
The helper methods may be either abstract methods, for which case
subclasses are required to provide concrete implementations, or hook
methods, which have empty bodies in the superclass. Subclasses can
(but are not required to) customize the operation by overriding the
hook methods. The intent of the template method is to define the
overall structure of the operation, while allowing subclasses to
refine, or redefine, certain steps. (Wikipedia)
Given the above, the method startRMI() is a template method which defines the skeleton of an operation by using a number of high-level steps (in your case it's only one step but this doesn't make a difference). The method fillRegistry() in your example is a high-level step - it's defined as an abstract method in the superclass and has a concrete implementation in the superclasses.
On the other side, if you would override the method startRMI() in a subclass, this would not be OK anymore. That's why you should make it final to avoid confusion - this way someone who creates a subclass will know that he must implement fillRegistry() (since it's abstract) but should not change the implementation of startRMI (since it's final).
Since this is a commonly used design pattern, I wouldn't worry at all if this solution is OK, a lot of people are doing it like that and everyone who knows design patterns will recognize it, I think it feels very natural even for developers who don't know the design pattern.