what is UML , what is Unified Modeling Language

What is UML?

Ans:

“The Unified Modeling Language (UML) is a language for specifying, visualizing, constructing, and documenting the artifacts of software systems, as well as for business modeling and other non-software systems”.— OMG UML Specification.
“UML is a graphical notation for modeling various aspects of software systems.” — William H. Mitchell (whm)

Why use UML?

Why use a graphical notation of any sort?
Facilitates construction of models that in turn can be used to:

Reason about system behavior
Present proposed designs to others
Document key elements of design for future understanding

Which graphical notation should be used?
UML has become the de-facto standard for modeling object oriented systems.

UML is extensible and method-independent.
UML is not perfect, but it’s good enough.

What are the different types of UML Diagrams?

Ans:

There are several types of UML diagrams:
Use-case Diagram
Shows actors, use-cases, and the relationships between them.
Class Diagram
Shows relationships between classes and pertinent information about classes themselves.
Object Diagram
Shows a configuration of objects at an instant in time.
Interaction Diagrams
Show an interaction between a group of collaborating objects.
Two types: Collaboration diagram and sequence diagram
Package Diagram
Shows system structure at the library/package level.
State Diagram
Describes behavior of instances of a class in terms of states,stimuli, and transitions.
Activity Diagram
Very similar to a flowchart—shows actions and decision points, but with the ability to accommodate concurrency.
Deployment Diagram
Shows configuration of hardware and software in a distributed system.

Class diagram basics :
Consider a simple class to represent a point on a Cartesian plane:

class Point {
         private double x, y;
         Point(double x, double y) { this.x = x; this.y = y; }
         public double getX( ) { return x; }
         public double getY( ) { return y; }
         public double getDistFromOrigin( ) { ... }
}

class Point {

private double x, y;

Point(double x, double y) { this.x = x; this.y = y; }

public double getX( ) { return x; }

public double getY( ) { return y; }

public double getDistFromOrigin( ) { ... }

}

The corresponding UML class diagram:

Three compartments are shown: class name, attributes, operations.
Member visibility is indicated with + (public) and – (private); these are called visibility adornments.

UML permits the detail of a diagram to vary based on the intended use. The compartments with attributes and/or operations can be omitted.
Here is a less detailed diagram for the Point class:

Here’s a version with only the operations shown:

Here is a class that represents rectangles using a point, a width, and a height:

class Rectangle {
     private double width, height;
     private Point ULCorner;
     public Rectangle(Point ULC, double w, double h) { ... }
     public double getArea() { ... }
     public Point getCenter() { ... }
}

class Rectangle {

private double width, height;

private Point ULCorner;

public Rectangle(Point ULC, double w, double h) { ... }

public double getArea() { ... }

public Point getCenter() { ... }

}

A class diagram showing the relationship between Rectangle and Point:

The link between Rectangle and Point indicates:

There is a field in Rectangle that references a Point.
Point has no knowledge of Rectangle. (We’d expect no references to Rectangle in the source for Point.)
Point’s role with respect to Rectangle is that of “ULCorner”.

Class diagrams—aggregation

When a class may reference several instances of another class, the link between the two classes is shown with a diamond on the end of the aggregate class.
Here is a class diagram showing the relationships between Triangle,Polygon, Rectangle, and Point classes:

It can be seen that:

An instance of Triangle references three instances of Point.
An instance of Polygon references at least one, and a potentially unlimited number of Point instances.
Aside from Triangle, Polygon, and Rectangle knowing of Point, no classes have any knowledge of any other classes.
Note that “3” and “1..*” are multiplicity specifications.

Object diagrams
An object diagram shows a configuration of objects at a point in time.
After execution of this code,

Point corner = new Point(3,4);
Rectangle r = new Rectangle(corner, 5, 10);

the situation can be depicted with this object diagram:

If desired, various elements may be omitted:

Note that an underlined name in the name compartment is the indication that an object is at hand.

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UML -I

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