UML class diagrams

Summary

In this tutorial, Bear introduces UML class diagrams, starting with basic characteristics like class representation and attributes. He uses a zoo example to explain concepts like inheritance, where subclasses like tortoise inherit properties from a superclass 'animal'. Bear also covers relationships, including association, aggregation, and composition, and discusses visibility with examples like private and public. He illustrates these with a zoo scenario and a real-world online shopping cart system, explaining multiplicity and class behaviors. The tutorial aims to simplify UML diagrams for better understanding.

Takeaways

• 📚 UML class diagrams are used to represent the structure of a system, focusing on the relationships between classes.
• 🐘 Classes in UML diagrams are depicted with a rectangle divided into three sections: class name, attributes, and methods.
• 🔑 Attributes are key data points that describe instances of a class and are placed in the middle section of the class diagram.
• 🐾 Methods, also known as operations or functions, represent the behaviors of a class and are placed in the bottom section.
• 🔍 Visibility indicators (+, -, #, ~) determine the accessibility of attributes and methods within the class structure.
• 🌐 Inheritance is a relationship where a subclass inherits attributes and methods from a superclass, represented by an open arrow.
• 🔗 Association is a basic relationship between two classes, depicted by a simple line, indicating a connection without dependency.
• 🤝 Aggregation is a type of association where a whole is made up of parts, but the parts can exist independently of the whole, noted with an open diamond.
• 🏠 Composition is a strong form of aggregation where the parts cannot exist without the whole, indicated by a closed diamond.
• 🔢 Multiplicity defines the number of instances one class can relate to another, using notations like '1', '0..*', or specific ranges.

Q & A

What are the basic characteristics of UML class diagrams?

The basic characteristics of UML class diagrams include classes, which are represented by a rectangle with three sections: the top section for the class name, the middle for attributes, and the bottom for methods. Each class can have visibility indicators for its attributes and methods, which can be private (-), public (+), protected (#), or package (~).

How do you represent an instance of a class in UML?

An instance of a class in UML is represented by creating an object with the class name followed by the specific values for its attributes.

What is the purpose of attributes in a class?

Attributes in a class are significant pieces of data that describe each instance of the class. They are also known as fields, variables, or properties.

What is the difference between a class and an instance in UML?

A class in UML is a blueprint for a set of objects, defining their properties and behaviors. An instance is a specific occurrence of a class, with actual values for its attributes.

How are methods represented in a UML class diagram?

Methods in a UML class diagram are represented in the bottom section of the class rectangle, and they are described by their name, followed by parentheses which may include parameters and their data types.

What does visibility mean in the context of UML class diagrams?

Visibility in UML class diagrams refers to the accessibility of attributes and methods. It determines whether they can be accessed from outside the class or only within the class and its subclasses.

What is inheritance and how is it represented in UML?

Inheritance is a relationship between two classes where one class (subclass) inherits attributes and methods from another class (superclass). It is represented in UML by an open arrow from the subclass to the superclass.

What is the difference between an abstract class and a concrete class in UML?

An abstract class in UML is a class that cannot be instantiated and is used as a base for other classes. It is often used to define common attributes and methods. A concrete class is a class that can be instantiated and does not inherit from an abstract class.

What are the different types of relationships that can exist between classes in UML?

The different types of relationships in UML include inheritance, association, aggregation, and composition. Each relationship has its own notation and represents different types of connections between classes.

How is multiplicity used in UML class diagrams?

Multiplicity in UML class diagrams is used to define the number of instances one class can have with another. It is represented by numbers or symbols near the association lines to indicate the range of possible connections.

What is the difference between aggregation and composition in UML?

Aggregation in UML is a relationship where a part can exist outside the whole, while composition is a relationship where a part cannot exist without the whole. Aggregation is represented with an open diamond, and composition with a closed diamond.

Can you provide an example of how to use a diagramming tool like Lucidchart for UML class diagrams?

Using a diagramming tool like Lucidchart for UML class diagrams involves signing up for an account, selecting the UML class diagram template, and then adding classes, attributes, methods, and relationships according to the UML syntax.

Outlines

00:00

📚 Introduction to UML Class Diagrams

Bear introduces the topic of UML class diagrams, starting with basic characteristics such as classes, attributes, and methods. He uses a zoo system as an example to explain these concepts. Classes represent things in the system, attributes describe instances of a class with data like name, id, and age, and methods define behaviors. Bear also discusses visibility, explaining public, private, protected, and package-private access levels for attributes and methods. The paragraph concludes with a brief mention of using a diagramming tool like Lucidchart for creating UML diagrams.

05:02

🔗 Exploring Relationships in UML

Bear delves into the relationships between classes in UML, using the zoo example to illustrate inheritance, where subclasses like tortoise, otter, and slow loris inherit attributes from a superclass 'animal'. He explains the concept of abstraction, where the 'animal' class is not instantiated but serves to simplify the model. Bear then moves on to association, aggregation, and composition, describing how they represent different types of connections between classes. Multiplicity is introduced as a way to define numerical constraints on relationships, with examples ranging from one-to-one to zero-to-many. The paragraph ends with a real-world example of an online shopping cart system, highlighting how these concepts apply in practice.

10:04

🛒 Real-World Application of UML Class Diagrams

In the final paragraph, Bear applies the concepts of UML class diagrams to a real-world scenario: an online shopping cart system. He describes the 'user' class with its attributes and methods, and how subclasses like 'customer' and 'administrator' inherit from 'user' while having their unique attributes and methods. Bear explains composition relationships, where parts like 'shopping cart', 'shipping info', and 'order details' cannot exist without the 'customer'. He also revisits multiplicity, illustrating how a customer can have multiple orders, but an order can only belong to one customer. The tutorial concludes with an invitation to learn more about diagramming on Lucidchart's training platform.

Keywords

💡UML Class Diagrams

UML (Unified Modeling Language) Class Diagrams are visual representations used to model and design the structure of a system by showing the system's classes, their attributes, operations, and the relationships between them. In the video, UML class diagrams are the central theme, with the instructor explaining how to create and interpret them using examples like a zoo system to illustrate the concepts.

💡Class

A class in UML represents a blueprint for creating objects, defining the properties (attributes) and behaviors (methods) that the objects of the class will have. The video uses the example of an 'Animal' class to explain how classes are depicted in diagrams and how they can represent real-world entities within a system.

💡Attributes

Attributes in a class diagram are the properties or characteristics of the class that describe the state of an object. They are often depicted in the middle section of a class box. The script gives examples such as 'name', 'id', and 'age' for an 'Animal' class, showing how attributes help identify specific instances of a class.

💡Methods

Methods, also known as operations or functions, are the behaviors or actions that instances of a class can perform. They are depicted in the bottom section of a class box. The video script uses 'setName' as an example of a method that could be used to change an animal's name, illustrating how methods represent the dynamic aspects of a class.

💡Visibility

Visibility in UML class diagrams refers to the accessibility of an attribute or method, determining whether it can be accessed from outside the class. The video explains different visibility levels such as private (-), public (+), protected (#), and package (~), using these to control the encapsulation of the class's data and behaviors.

💡Inheritance

Inheritance is a relationship between two classes where one class (subclass) inherits attributes and methods from another class (superclass). The video uses the zoo example to demonstrate how 'Tortoise', 'Otter', and 'Slow Loris' can be subclasses of an 'Animal' superclass, inheriting common attributes without duplication.

💡Abstract Class

An abstract class in UML is a class that is not intended to be instantiated on its own but serves as a base for other classes. The 'Animal' class in the video is described as abstract because it represents a general concept from which specific animal types are derived, but it is not used to create objects directly.

💡Association

Association in UML class diagrams represents a connection between two classes that work together but do not have a strong ownership relationship. The script uses the relationship between 'Otter' and 'Sea Urchin' to illustrate a simple association where the otter eats sea urchins, but there's no mandatory dependency.

💡Aggregation

Aggregation is a type of association that indicates a whole-part relationship where the part can exist independently of the whole. In the video, 'Creep' (a group of tortoises) is shown as an aggregate of 'Tortoise', meaning tortoises can be part of a creep or exist on their own.

💡Composition

Composition is a strong form of association where the part cannot exist without the whole. The video uses the example of 'Visitor Center', 'Lobby', and 'Bathroom' to explain composition, where if the visitor center is destroyed, the lobby and bathroom are also considered destroyed, as they are integral parts of the whole.

💡Multiplicity

Multiplicity in UML class diagrams specifies the number of objects involved in a relationship. The video provides examples such as a 'Visitor Center' having exactly one 'Lobby' and a variable number of 'Bathrooms', illustrating how multiplicity constraints are used to define the quantity of objects in relationships.

Highlights

Introduction to UML class diagrams and their basic characteristics.

Use of a zoo system as an example to illustrate class diagrams.

Explanation of classes and how to represent them in UML.

Importance of attributes in identifying instances of a class.

Description of visibility and its impact on attribute and method accessibility.

Different visibility types: private, public, protected, and package.

Introduction to methods and their role in defining class behavior.

Formatting conventions for attributes and methods in UML.

The concept of inheritance and its application in class diagrams.

Use of the animal class as a superclass in the zoo example.

Explanation of abstract classes and their representation in UML.

Introduction to association relationships between classes.

Aggregation and composition relationships in UML with examples.

Concept of multiplicity and its role in defining class relationships.

Real-world example of a UML class diagram for an online shopping cart.

Explanation of customer and administrator classes in the shopping cart example.

Illustration of composition relationships in the shopping cart system.

Discussion on multiplicity in the context of customer orders.

Conclusion of the tutorial and invitation to learn more at training.lucid.co.

Transcripts

Hi, my name is Bear, and today I'll be teaching you about UML class diagrams.

We'll start with some of the basic characteristics.

Then we'll talk about relationships

and finally we'll finish up by going through a few examples together.

All right, let's talk about some of the basic characteristics of class diagrams.

To help explain these characteristics, I'm going to make up an example

to help us illustrate.

So let's say we're building a system for a zoo.

And keep in mind, these examples I'm going to use probably wouldn't

ever find their way into an actual program, but humor me

as I use these concepts to make it easier to understand.

So in our zoo, we want to describe the different things that are in the system.

You represent these things through classes,

and a class is depicted with this shape here.

So what's in a zoo?

Well, a ton of animals.

So we can create a class for our animals.

To do that, you just write the name of the class in this top section.

If our class is animal, an instance of that class would be a specific animal.

So the question is, how would you go about

identifying each instance of that class?

You do that through attributes.

An attribute is a significant piece of data containing values

that describe each instance of that class.

They're also known as fields, variables or properties,

and they go in the middle section here.

So for our animal class, we could create attributes like name, id, and age.

That way we could identify a specific instance of the animal class like Ruth

id number 304, age 114.

These need to be formatted a certain way, though.

You'll start with visibility, which we'll talk about later.

The name of the attribute, beginning with a lowercase letter.

Then you follow it with a colon and the data type for the name.

We'd want to return a string

and we can format the other attributes the same way, except

that we'd want to return an integer, since these are numbers.

Now that we've got some attributes

for our animal class, we get to the bottom section here.

This is where you put methods which are also known as operations or functions.

Methods allow you to specify any behavioral features of a class.

So we might ask ourselves what are some different behaviors of this animal class?

Maybe we'd want to be able

to change our animals names, like Ruth should actually be called Rita.

So let's create a function called setName.

We can also create a method for eating since all of our animals eat.

Methods also need to be formatted a certain way.

You start with visibility, which we'll talk about next.

Then the method beginning with a lowercase letter.

Next, you put parentheses to signify the function you're going to program later.

You can also add variables and the data type in here,

but in most cases it's not really necessary.

We'll add visibility in parentheses to the eat method as well.

Now let's talk about visibility.

The visibility of an attribute or method sets the accessibility

for that attribute or method.

So right now we have a minus sign for all of these, which indicates

that each of these attributes and methods are private.

They can't be accessed by any other class or subclass.

The exact opposite is the plus sign, which means an attribute

or method is public and can be accessed by any other class.

Another visibility type is indicated by the hash,

which means an attribute or method is protected.

These can only be accessed by the same class or its subclasses.

And finally there's the tilde or the squiggly, as I like to call it.

This sets the visibility to package or default, which means it can be used

by any other class as long it’s in the same package.

But that one is rarely ever used.

In most cases, your attributes are going to be private

or protected, and methods are often public.

Let's quickly review these basics with another example.

Let's make a class for employee.

We could give an employee attributes like name,

employee ID,

phone number

and department.

We'll want all of these attributes to be private and we could create a simple

method like updating a phone number which we'll go ahead and set to public.

So you might have noticed that I'm using

a diagraming application to create these UML class diagrams.

The same principles apply if you're using pen and paper.

But a diagraming software makes it much easier.

The diagraming application that I'm using today is Lucidchart.

You can sign up for a free account with your email address

and then you'll be able to follow along as we make these class diagrams together.

So the next thing we'll need to cover

are the different relationships that exist between classes.

The first type of relationship that we'll describe is inheritance.

And I'm going to keep going with the zoo example

because it makes it easy to understand the logic of these relationships.

We'll get to a more

technical real world example just a little later.

Okay.

So inheritance, let's say, in our zoo, the only animals we have

are tortoises, otters and the lesser known but nonetheless amazing slow loris.

In our system, we want to distinguish each of them as their own class.

So we'll make three classes for a tortoise, otter, and slow loris.

Now, instead of duplicating attributes for name, id and age,

we can make these classes into subclasses of the animal class

by drawing open arrows like this.

This is an inheritance relationship.

We're saying that these subclasses inherit all the attributes

and methods of the superclass.

You could also use the terms child and parent class.

So our order class is going to inherit the attributes of name, age and ID.

And then we could also add an attribute specific to the otter like whisker length.

One of the advantages of inheritance is that if we wanted to change or add

an attribute for all animals, we wouldn't have to go in and make

that change to tortoise and then otter and then slow loris.

We just make the change to the animal class

and it applies across all of its subclasses.

in this scenario, we also have what's called abstraction.

Animal is an abstract class because in our system,

any time we want to instantiate one of our classes, it's

going to be a tortoise, otter, or slow loris.

We wouldn't instantiate the animal class itself.

The animal class is just a way to simplify things and keep the code dry

so you don't repeat yourself.

So to show that this is an abstract class,

we'll put the name in italics.

You could put the class name inside these things as well, but I prefer italics.

Okay.

Another type of relationship is association.

So if we had a class for sea urchin, we could draw an association

which is just depicted by a simple line between otter and sea urchin.

And we could say otter eats sea urchin.

There's no dependency between them.

It's just a basic association relationship, and it's pretty simple.

The next type of relationship is aggregation.

It's a special type of association that specifies a whole and its parts.

So to continue with our zoo example,

again, this is just to help explain the logic.

Let's create a new class for a group of tortoises.

A group of tortoises is called a creep.

So here's our creep class and it's got a relationship with tortoise.

Any of our zoo's tortoises could be part of a creep, but they don't have to be.

A tortoise could leave the group at any point and still exist on its own.

That type of relationship where a part can exist outside

the whole is aggregation, and we note it with an open diamond.

There's also a relationship where the part can't exist outside the whole.

That's called composition.

To illustrate this, I'm going to create a few new classes.

Let's just say we have several different visitor centers in our zoo,

and each of those visitor centers has a lobby and a bathroom.

Now, if one of our visitor centers was torn down, the lobby

and the bathroom of that visitor center would also be destroyed.

Those rooms couldn't exist apart from the visitor center that is contained in.

That’s composition.

When a child object wouldn't be able to exist without its parent object.

We note a composition relationship with a closed diamond.

Another important

concept when we talk about relationships in UML is multiplicity.

Multiplicity allows you to set numerical constraints on your relationships.

For example, let's say we want to specify

that our visitor centers are going to have just one lobby.

We simply write the number one here, meaning

there can be one and only one lobby per visitor center.

But for bathrooms, maybe we want to make it so that there's

at least one bathroom per visitor center.

But leave the option to have as many as you'd like.

We'd use this notation to denote one or many bathrooms.

Other types of multiplicity are 0 to 1, which is an optional relationship.

N representing the specific amount, which in our example was one,

but it could be any other number depending on your use case.

Zero to many.

One to many.

Or a specific range.

Hopefully our zoo examples have helped explain those concepts.

But I want to show you what a real world example would look like.

This is a UML class diagram for an online shopping cart.

And if you want to look at the diagram with me, just click the link.

In the top right corner,

you can

see that this system has several classes and relationships.

So let's walk through a couple of them.

We'll start with the user class.

It's got attributes for user ID, password, login status and register date.

You've got the different return types on the right and on the left.

The visibility which is set to private.

You can see how the values returned by those attributes

would specifically describe an instance of the user class.

Down below we have a public method of verify

log in returning a boolean and this makes sense.

Right? Methods are behaviors of a class.

So if you were to log into your user account,

there's a function in place that verifies your login credentials.

Let's move on to the customer class.

This arrow tells us that customer is a child of user,

so customer inherits all the attributes and methods of the user class.

And same thing for the administrator class.

Both of these inherit from user, but also have their own

specific attributes and methods

like administrator can update the catalog, but the customer can't.

Stemming from customer, there are also several lines with the closed end diamond.

So if you recall, these are composition relationships

which mean that the parts cannot exist without the whole.

in the instance of the customer class, if that customer’s account was destroyed,

his shopping cart would be destroyed as well and all of his orders would be lost.

They can't exist outside of the customer.

The same applies for the shipping info and order details.

If there's no order, there's not going to be any order details or shipping info.

The last thing we'll look at in this example is multiplicity.

You can see that a customer can have zero or many orders.

Makes sense, right?

You could create a customer account for an online store but never buy anything.

Or you could be a frequent customer and place several different orders.

And then on the flip side, an order can belong to only one customer.

It'd be pretty confusing if a specific order with a unique order ID

was duplicated across several different customers.

And here you can see a 1 to 1 relationship.

Each order has one and only one order details

and order details belongs to one and only one order.

Okay, that wraps up our tutorial for UML class diagrams.

If you're interested in learning more about diagraming, processes, systems

and organizations, visit training.lucid.co I hope to see you there.