For a UML Class Diagram, if a class A has a composition relationship with another class B, does it work like inheritance? I know that the class A cannot exist without class b but does class A inherit the attributes and methods from class B?
Composition over inheritance
In OO design, a common advice is to prefer composition over inheritance. This might mislead to think that there is a relation between these two different concepts:
Inheritance is about a relation between classes.
Composition is a about relations between objects of classes.
Even more misleading: the "composition" used in the general OO literature corresponds in general to a simple UML association and is not to be understood as restrictive as the UML "composition".
Inheritance is about generalisation and specialisation
If class D inherits from B, it means that D specialises a more general concept B:
It has all the properties and operations of B (without need to repeat them in the diagram),
It can have additional properties and operations that are not relevant for B, and
It can perform some operations of B differently.
Any object of class D is also an object of class B for its full lifetime. This is why we speak of a "Is a" relationship.
Naive example (just for the illustration):
Composition is about association of objects of very different classes
If class B "composes" (has a composition association with) C, it means that the B object can be associated with different C objects over its lifetime. Anf that objects of class C :
always belong to a B object that owns it. This is why we sometimes speak of a "has a" relationship;
will not survive its owning B object .
Caution: UML composition is about an exclusive ownership. In the OO literature, the term "composition" often refers to weaker associations without any ownership or even exclusivity.
Naive example (just for the illustration):
Inheritance and composition are two different concepts. If they were the same there would not be the need to have different notation.
Compostion is about lifetime of objects. Composed elements will be destroyed the moment the owner is destroyed. But each instance can (and likely will) be something completely different than the composing object. The well known example is a car: When you shredder a car its composed wheels will be shreddered as well. Only if you detach them beforehand they can live on.
Inheritance means that instances will have operations and attributes on their own defined in the parent class. This works in each single instance. There's no good real worl example for inheritance (genes work different and inherited assets also don't help here).
if a class A has a composition relationship with another class B
Lets suppose you mean A <*>--- B or A <*>---> B rather than the reverse (B <*>--- A or B <*>---> A), because this is not very clear in your question.
class A cannot exist without class b
It is not about classes but about instances of these classes, and B can have instances independently of that composition, so the limited life is not for all instances of B.
does it work like inheritance? does class A inherit the attributes and methods from class B ?
No.
A inherits B means A is a B, this is absolutely not the case having A <*>--- B or A <*>---> B or the reverse.
Then independently of the visibility of the operations you cannot apply the operations of A to the instances of B nor the reverse, except of course if you also have an inheritance more that the composition.
About the attributes of B depending on the way the composition is implemented it is possible they are part of the corresponding instance of A, this is the case for instance in C++ not using a pointer for the composition. But anyway that does not means these attributes of B are attributes of A, to access them it is needed to first access of the instance of B though the compositions, and that also supposes an instance of A has the right to directly access these attributes of B, so they are typically public.
Note also you can have A<*>--->B<*>--->A etc and fortunately that does implies A inherits B and B inherits A which is impossible
Related
The above statement is taken from www.uml-diagrams.org section Assocation ends from the rather beginning.
Is there a real-world example for this?
What exactly means owned by association itself?
What is association class or how is it implement in code?
Here's my attempt. I also was unsure about the nature of an association in the past and until not so long ago where I sort of grasped it. For the full pain you have to go through pp. 197 of UML 2.5 (11.5 Associations)
What is an association?
When you have a number of classes (we may call them A, B, etc. here for brevity. They are designed to be somewhat self-consistent. So you have properties and operations which work with the latter. If our A is some mathematical class you may have a well working black box. Fine.
Now, when any class is in need of another (e.g. the B class needs objects of type A to have that math toolbox handy) you create some reference. In a programming language that will be some variable holding a reference to an instance of the needed class. That's the moment where you have an association.
In UML you can denote such an association as a property typed with the other class. Or (!) you draw a line for an association between both class boxes and place the role name towards the referenced class (telling the name of the variable the programmer shall use). This being quite simple got me some headaches since I saw notations with both property and role notation. I have not fosteres the UML spec as tho what this would mean, but I established a rule I was also teaching that this is not allowed. Tools like EA do not support roles in a correct way which forces you to manually maintain your diagrams. That is, if you have an association role from B to A and the diagram does not contain A then you should show the role as property (see my note about dot notation below). But once you place A on that diagram you should see the assocation line with the role name, but not the property (pretty confusing I admit).
Note: Dot notation
Since UML 2.5 (or was it 2.1?; but see p. 200 of UML 2.5) the OMG guys introduced the so called dot notation. That is, you have a little dot at the association line end along where the role name is. This dot indicates that the role name is actually the property of the referencing class. Pretty much what I was talking about above. And what I thought was the intention. So, I have no idea what a not-owned property would be in any case. May be a good question to be asked.
Associaton class
Assume you have class C which reflects some kind of relation between A and B (and eventually D, E, etc.). Doing that makes it an association class. It's doing that by exactly the same technique as above, just for multiple classes. (If a class has multiple relations to other classes it's not necessarily an association class, but only if it's main purpose is to set these into some relation. YMMV)
So here you have the case where an association owns the role since you have a real object. The association explained above exists only as a reference in the programming world, not a complex storage structure like an association class.
First of all, you should understand that, as opposed to the explanation of #qwerty_so, in data/information modeling, relations between two objects (called "links" in UML) are a first-class citizen. Logically/conceptually, they exist next to objects. In terms of OO implementation, they may exist either within one or both of the two participating objects in the form of object references, or outside of the two participating objects in the form of another object representing the "link".
Likewise, in data/information modeling, an Assocation, which classifies "links", is a first-class citizen that logically/conceptually exists next to the involved Classes. A binary Assocation is implementated either within one or both of the two (Java/C#/JS/etc.) classes involved in the form of a reference property (or possibly two mutually inverse reference properties), or outside of the two classes in the form of another class representing the Assocation.
In the case where a binary association has one ownership dot at one of its ends, it is unidirectional and implemented as a reference property in the corresponding class, while in the case where it has ownership dots at both of its ends, it is bidirectional and implemented as two mutually inverse reference properties, one in each of the involved classes.
What exactly means owned by association itself?
An Assocation End of a binary Assocation can be owned either by the Class on the opposite side or by the Assocation itself. In the former (more common) case, the Assocation End corresponds to a reference property of the (Java/C#/JS/etc.) class on the opposite side. In the former (less common) case, the Assocation End corresponds to a reference property of the (Java/C#/JS/etc.) class that represents the Assocation.
In principle, there could be an OOP language that supports Assocations as first-class citizens, having a keyword "assocation", in addition to "class".
What is association class or how is it implement in code?
The need for an Association Class arises when the "links" of an Association have to be characterized with the help of attributes, or when they have to be processed with the help of special operations. In such a case, an Association Class allows defining these attributes or operations. An Association Class is implemented in the form of a (Java/C#/JS/etc.) class that has corresponding reference properties for each of the Association's ends.
I'm trying to understand how inheritance with association work between the superclass and its subclasses. Here are the classes with their relationships:
my question here if I created an object from class C, will it inherit also from class A as below? and why?
No. It won't inherit anything. B inherits from A so C will see a B with operations/attributes inherited from A.
If I (class C) use a cleaning device B that inherited from Sponge (A) I will see/use a soft cleaning device. If B inherited from steel it would be steel wool. If inherting from sand it would be sand paper. All cleaning devices with different usability depending from where they inherit. But I wouldn't change in any way. Only the way I would "clean" things. (I know, a horrible example)
N.B. As #www.admiraalit.nl commented your object diagram does not match the class diagram.
which one is not an object association a) simple association
b) inheritance
c) Aggregation
d) Association
First you need to know what an association is.
And association in O.O.P defines a relationship between classes of objects that allows one object instance to cause another to perform an action on its behalf (Wikipedia ,2016)
Read more about it here: https://en.wikipedia.org/wiki/Association_(object-oriented_programming)
This means that you can call objects from one class into another and allow it to "mutate" perform some logic/action on behalf of another object.
I think the answer to your question is c) Aggregation. This is because aggregation in used in Unified Modelling Language (UML). This is when one object is Strongly Composed of another. This means it is made-up of many objects. In this, the object isn't taking permission to call that object, it is composed of them which means that it can call them directly.
a) and d) contain the word "Association" already so that shouldn't be the answer.
b) Inheritance is when you extend a parent/super class into a subclass. This is when the superclass contains objects and methods which can be called by the subclass. The difference with the subclass is that it can have it's own unique parameters for the objects (which the superclass doesn't have).
I am reviewing my knowledge in object-oriented programming. Under the relationship between classes topic, I have encountered some relationships which are a bit ambiguous to me.
I know dependency "uses-a" and inheritance "is-a" but I'm a bit unfamiliar with Aggregation, Composition, Association and Direct Association; also, which of them is "has-a" relationship. Some use Aggregation interchangeably with Association.
What is Direct Association? Also, what is Composition? In UML diagrams, the arrows that represents them are different. I would be really thankful if you could clear these things out for me.
Please note that there are different interpretations of the "association" definitions. My views below are heavily based on what you would read in Oracle Certification books and study guides.
Temporary association
A usage inside a method, its signature or as a return value. It's not really a reference to a specific object.
Example: I park my Car in a Garage.
Composition association
A so-called "STRONG relationship": The instantiation of the linked object is often hard
coded inside the constructor of the object. It cannot be set from
outside the object. (Composition cannot be a many-to-many
relationship.)
Example: A House is composed of Stones.
Direct association
This is a "WEAK relationships". The objects can live independent and there are usually setters or other ways to inject the dependent objects.
Example: A Car can have Passengers.
Aggregation association
Very similar to a Direct association. It's also a "WEAK relationship" with independent objects. However here the associated objects are a crucial part of the containing object.
Example: A Car should have Tires.
Note: Both Direct associations and Aggregation associations are often generalized as "Associations". The difference is rather subtle.
The whole point of OOP is that your code replicates real world objects, making your code readable and maintainable.
1. Association
Association is: Class A uses Class B.
Example:
Employee uses Bus/train Services for transportation.
Computer uses keyboard as input device
And in In UML diagram Association is denoted by a normal arrow head.
2. Aggregation
Class A contains Class B, or Class A has an instance of Class B.
An aggregation is used when life of object is independent of container object. But still container object owns the aggregated object.
So if we delete class A that doesn't mean that class B will also be deleted. E.g. none, or many, teachers can belong to one or many departments.
The relationship between Teachers and Departments is aggregation.
3. Composition
Class A owns Class B.
E.g. Body consists of Arm, Head, Legs. BankAccount consists of Balance and TransactionHistory.
So if class A gets deleted then also class B will get deleted.
Direct association has nothing in common with the other three. It does not belong to UML at all, it is the IBM requirements modelling term.
As for others,
Association A->B is a child of Dependency. Association means, that A (or its instance) has some easy way to get to instance of B. For example, a.x.y.b. Or by function, or by some local variable. Or by a direct reference or pointer, or something else (there are many languages in the world). As you see, there is no strict border between dependency and association.
One of attributes of Association is Aggregation, it can have values: None, shared (often incorrectly called aggregation), and composition.
If A (or instance) has some (or one) instances of B so, that destroying of association means the destroying of B instances, it is the composition.
If you or a tool author had decided, that some has-a relationship, that is weaker that composition, needs to be specially shown, you can use shared aggregation. Usually it is some collections of references to B in A.
There are some more interesting attributes of associations. Look here if you are interested.
An association between object types classifies relationships between objects of those types. For instance, the association Person-isEmployedBy-Enterprise may classify the relationships PeterMiller-isEmployedBy-IBM, SusanSmith-isEmployedBy-IBM and SarahAnderson-isEmployedBy-Google between the objects PeterMiller, SusanSmith and SarahAnderson of type Person as well as Google and IBM of type Enterprise. In other words, associations are relationship types with two or more object types participating in them. An association between two object types is called binary. While binary associations are more common, we may also have to deal with n-ary associations, where n is a natural number greater than 2. For instance, Person-isTreatedIn-Hospital-for-Disease is a 3-ary ("ternary") association between the object types Person, Hospital and Disease.
I guess that with "direct association" you mean a directional (or directed) association, which is an association (with a domain class and a range class) that represents a reference property in its domain class. Such a directional association has an "ownership dot" at its target end.
Please see this book chapter for more about associations.
And see my answer to this SO question for an explanation of aggregations and compositions.
I'm familiar with the practical aspects of OOP. Now I'm writing a technical document and I'm questioning my use of terms.
Take this class hierarchy. Each class has zero, one, or more (multiple inheritance) classes it directly derives from. For B, this is A. I'd say that A is B's base class or superclass (interchangeably).
Conversely, a class can have any number of classes directly deriving from it. I'd say that B and C are A's derived classes or subclasses, again interchangeably.
Yet there must also be a term to refer to all classes a class directly or indirectly inherits from. How would you call {A, B} from D's perspective?
And I'm missing another term for all classes directly or indirectly derived from a class. How would you call {B, C, D, E} from A's perspective?
Good questions. In this case, A is the root class, as all other classes inherit from it. All objects descend from A, or are either direct or indirect subclasses of A. The opposite is also true. B is a direct superclass of D and A is an indirect superclass. I suppose you could use the word ancestor as the opposite of descend/descendant, but I haven't seen this before.