A model is a simplification of reality. We build model so that we can better understand the system we are developing and it often expose opportunity for simplification and reuse. Model help us to communicate the desired structure and behavior of the system. Model give us a  template that guide us in constructing a system.

UML

The 'UML' is a modeling language. A modeling language is a language whose vocabulary and rule focus on the conceptual and physical  representation of a system. It helps in communicating the conceptual model of a program to others.
                               Second , it is difficult to understand the class hierarchy by staring  at the code. UML helps in such case.
                               Third , if the developer who cut the code  never  wrote down the model that are in his or her mind, that information is  lost forever or at best , only partially retractable from the implementation once that developer moved on .
                           UML address the third issue also. Each symbol in the UML notation is a well defined semantic . In this manner one developer can write to another developer or even another tool can interpret the model unambiguously.
                            UML address the documentation of a system's architecture and  all of its detail. The UML also provides a language for expressing requirements and tests. UML provides a language for modeling the activities of project planning and release management.
 
                Building blocks of UML
                            the  things ,relations and diagrams are the three building block of the UML.
 
                                     

Things are the abstraction that are first class citizen in a model ; relationship ties  these things together ; diagram  group interesting collection of things.

The things (building block -one)
 
                       the  things are basically of  four types

1. Structural things :--
                              are the nouns of the UML model , representing
elements that are either conceptual or physical

There are 7 kinds of structural things.

1) Class :- is a description of a set of objects that share the attributes and semantics. A class implements one or more interfaces .
               Graphically class is denoted as a rectangle in following manner .
 
                                            

2) Interface :- An interface is a collection of operation that specify a service of a class or component. It defines a set of operation specifications ( that is , there signature ) [ thus the externally visible behavior of that element] but never a set of operation implementation.
                  Graphically it is denoted by a circle ; together with its name.

3) Collaboration :- defines an interaction and is a society of roles and other elements that work together to provide some co-operative behavior that's bigger than the sum of all the elements .
                     Graphically - ellipse with dashed lines usually  including only its name.

[    an example
 import java.awt.Graphics;

class HelloWord extend java.applet.applet{

public void paint (Graphics g) {

g.drawString("HelloWord",10,10);

}

}

[ modeling after coding is called reverse engineering ]

                 Here import java,awt.Graphics directly available to the code that  follows ( java.awt prefix specifies the  Java package in which the class Graphics lives )
                              The second line of code  class HelloWord extend java.applet.applet  introduces a new class named applet which lives in the  package java.applet ]

Here HelloWord collaborates directly with only two classes namely Applet and Graphics .
 

4) Use case :- is a description of set of sequence of actions that a system performs that yield an  observable result of value to a particular actor .
                        Graphically  an ellipse with solid lines , usually including only its name.
                         Use case describes what a system does but does not specify how it does all.
                    An actor represents a coherent set of roles that user of use cases play when interacting  with these use case, communicate with one another , each one possibly sending and  receiving messages.
 
                                              
 

5)Active class :- is just like a class except its objects represents elements whose behavior is concurrent with other elements.
                  Graphically:- similar to the class but with heavy line.
(An active class is an class- whose objects own one or more processes or threads and therefore   can initiate control activity )

6) Component:- is a physical and replaceable part of a system that conforms to and provides the  realization of a set of interfaces.
               A component typically represents the physical packaging of otherwise logical elements , such as classes , interfaces , and collaborations.
                     Graphically :- a component is rendered as a rectangle with tubes usually including only its name . It is usually have higher level of abstraction than   classes.
 
                                                
 

PHYSICAL VIEW OF APPLET :-
 
                                                

7) NODE :- is a physical element that exist at runtime and represents a computational resources , generally having at least some memory  and often , processing capabilities set of components  may reside on node or migrate from node to node
                    Graphically :- a node is rendered as a cube , usually including name.

2) Behavioral Things :- are dynamic parts of UML models. These are the verbs of a model , representing behavior over time and space

              There are two primary kinds of behavioral things :-

1) Interaction :- is a behavior that comprise a set of message exchanged among a set of objects within a particular context to accomplish a  specific purpose.
            Graphically a message is rendered as a directed line , almost
always including the name of its operation.

2) State machine :- is a behavior that specifies the sequence
of states an object or an interaction  goes through during its lifetime in response to events together with its response to those events.
                    A state machine involves a number of other elements ,
including states , transition ( the flow from state to state ), events (things  that trigger a transition and activity ( the response to a transition)
                             Graphically , a state is rendered as a rounded
rectangle usually including its name and its sub states.
 
                                                   

 
3) Grouping Things :- are the organizational parts of UML model . These  are the boxes into which a model can be decomposed.
           A package is a general purpose mechanism for organizing elements into groups structural things behavioral things and even other  grouping things may be placed in a package.
            Graphically a package is renders as a tabbed folder usually including only its name and sometimes its contents .

4 Annotational things :- are the explanatory parts of UML models . These are the comments you may apply to describe , illuminate and  remark about any element in a model .

Graphically a note is rendered as a rectangle with Dog-eared corner ; together with a textual or graphical comment.
 
 
 

                             BUILDING BLOCK-2

There are four kinds of relation ships in the UML.

1) Dependency :- is a semantic relationship between two things in which a change to one thing ( the independent things)
                   Graphically it is shown by dashed possibly directed toward,
independent thing.

2) Generalization :- is specialization generalization relationship in  which objects of the specialized elements ( the child ) are substitutable for  objects of the generalization element( the parent)  ( Here the child has all the structure and behavior of a parent  and may has some addition properties )
                                      Graphically a generalization on relationship is rendered as a  solid line with a hollow arrowhead pointing to the parent.
                                           
 
3) Realization :- is a semantic relationship between classifiers , wherein one classifier specifies guarantees to carryout . Realization relationship is somewhat a cross between dependency  and generalization and its notation is a combination of the notation for dependency and generalization .
                    Graphically a realization is rendered as a dashed line with a large open arrowhead pointing to the classifier that specifies the contact.
exp.- In Java program . by using a keyword interface , you can  fully abstract a class interface from its implementation . Interface are  syntactically similar to classes, but they lack instance variable ( i.e. there instance can't be created ) and their  methods are declared .without any body(i.e. they are abstract    method - they always overridden access

interface name {

return type ( int/ void ...) method name

( parameter list );

}

The interface gets realized only when it is implemented
by some class . A class may implement a number of interfaces and an interface
can be  implemented by a number of classes .

4)Association :- An association is a structural relationship that specifies that object of one thing are connected to object of another . Given  an association connection two classes ., you can navigate from an object of  one class to an object of another class and vice versa.
                                          Graphically an association is rendered as a solid line  connecting the same or different classes . Use association when you want to  show structural relationship .
 
                             Beyond this basic form ,there are four adornments that apply

to association .

NAME - To avoid ambiguity , you use the name to describe the  nature of relationship.
You can give a direction to the name by providing a  direction triangle that points the direction you intend to read the name.

ROLE:-When a class participates in association it has a specific role that it plays in that relation ; a role is just the face the  class at the near end of the association presents to the class at the other end  of association.

MULTIPLICITY :- An association represents structural relationship
When you state ( number like shown in figure ) multiplicity at one end of the association , you are specifying that for each  object of that class at the opposite end , there must be that many objects at  the near end.

                                 Here for each company their must be one or more  person.

other multiplicity notation

0....1 zero or one

(1) exactly one

0....* zero or many

0..1,3..4,6..* ant # other than 2 & 5

AGGREGATION :- When one class represents a larger things ( the whole ) which consists of smaller things ( the parts ) .This kind of relationship is called aggregation is really just
a special kind of association and is specified by adorning a plain  association with an open diamond at the whole end , as shown in  figure .

                              BUILDING BLOCKS -3

DIAGRAM

1) class diagram :- shown a set of classes , interface and
collaboration and their relationship .

2) object diagram :- shows a set of object and their  relationship

3) use case diagram :- a set of use cases and actors.

              Interaction diagram :- shows an interaction , consisting of a  set of objects & their relationship , including the message that may be  dispatched among them.

4) A sequence diagram :- is an interaction diagram that
  emphasizes time-ordering

5) Collaboration diagram :- is an interaction diagram that  emphasizes the structural organization of the objects that send and  receive message.

6) A state chart diagram :- shows a state machine , consisting of  states, transition events and activities .

7) Activity diagram :- is an special kind of a state chart  diagram that shows the flow from activity to activity within system.

8) Component diagram :- shows the organization and dependencies

among a set of components.

9) Deployment diagram :- shows the configuration of run-time  processing nodes and the components that live on them .

                                   
 
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