UML class diagrams

Summary

In this video tutorial, Bear introduces us to the basics of UML class diagrams, focusing on their structure and relationships. The session is broken down into understanding the attributes and methods of classes, the visibility and accessibility of these elements, and relationships like inheritance, associations, aggregations, and compositions in class diagrams. Using a fun zoo example, Bear helps to illustrate these concepts, making it more relatable and easier to grasp. The tutorial concludes with a real-world example of a UML class diagram for an online shopping cart, providing practical insights into how these diagrams function.

Highlights

• Bear introduces UML class diagrams and breaks down their components in a fun and engaging way 🍿.
• Understanding the importance of attributes and methods in defining class characteristics 🎯.
• Learning about class relationships such as inheritance, association, and composition using zoo animals 🦓.
• Detailed walkthrough of class element visibility settings and their implications 👁️.
• Real-world application in an online shopping cart demonstrates the practicality of UML diagrams 🛍️.

Key Takeaways

• UML class diagrams are essential for visually representing the classes and relationships in a system 📊.
• Attributes (fields/variables) and methods (operations/functions) define class characteristics and behaviors 🖋️.
• Relationships in class diagrams include inheritance, association, aggregation, and composition 🔗.
• Visibility controls access to class elements and can be private, public, protected, or package-level 🔐.
• The zoo and online shopping cart examples help illustrate UML concepts in a fun and relatable way 🦁🛒.

Overview

Bear kicks off the tutorial by setting the stage for understanding UML class diagrams, a crucial tool for developers. Using an imaginative zoo system, he explains how classes represent different entities within a system, like animals at a zoo, and how each class can have specific attributes and methods to depict characteristics and behaviors.

As the lesson progresses, Bear introduces relationships between classes, such as inheritance, which allows subclasses like tortoises, otters, and slow lorises to inherit properties from a parent animal class. With charming examples, he covers more complex relationships including association, aggregation, and composition, making the concepts clear and engaging.

The video wraps up with a real-world application: an online shopping cart class diagram. Here, Bear illustrates how UML diagrams are used to manage complex systems, showcasing relationships between user classes and elements like shopping carts and orders. This demonstration highlights the utility of visualizing systems through UML class diagrams, encouraging viewers to adopt this method in their projects.

Chapters

• 00:00 - 00:30: Introduction to UML Class Diagrams In the introduction to UML Class Diagrams, Bear introduces the fundamental concepts and structure of UML class diagrams. The chapter outlines the basic characteristics of class diagrams, which include their components and the way they illustrate systems. Bear uses the example of a zoo system to help explain these characteristics clearly. The session aims to familiarize learners with class diagrams by discussing the fundamental characteristics, exploring the relationships within diagrams, and providing practical examples to reinforce learning. The examples used, while instructional, may not necessarily reflect actual systems, but serve to clarify the discussed concepts.
• 00:30 - 05:00: Basic Characteristics of Class Diagrams This chapter introduces the basic characteristics of class diagrams by using a zoo as an example. It highlights the concept of classes and their representation in diagrams. Specifically, it describes how to create a class for animals by writing the class name in a designated section of the diagram.
• 05:00 - 07:00: Visibility in UML The chapter discusses the concept of 'Visibility' in UML, using 'animal' as the class example. It explains that instances of classes, like a specific animal in this case, can be identified through 'attributes'. Attributes, also known as fields, variables, or properties, contain values that describe each instance of a class and are placed in the middle section of the UML representation.
• 07:00 - 10:00: Example: Employee Class Diagram The chapter on Employee Class Diagram discusses the creation of attributes for an animal class, using examples like name, id, and age to identify instances such as Ruth with id number 304 and age 114. It emphasizes the importance of formatting these attributes correctly, starting with visibility, the attribute name in lowercase, followed by a colon and the data type—such as returning a string for the name. The chapter promises further discussion on visibility, and notes that similar formatting applies to other attributes.
• 10:00 - 18:00: Relationships in UML The chapter explores the concept of relationships in UML (Unified Modeling Language), focusing on the structure of an animal class. It explains how to define attributes, such as names, and methods, also known as operations or functions, which describe behavioral features. An example given is the ability to change an animal's name, such as changing 'Ruth' to 'Rita'.
• 18:00 - 20:30: Real-World Example: Online Shopping Cart The chapter discusses creating functions and methods with a focus on an example related to online shopping carts. A function called 'setName' is introduced along with the concept of creating methods, such as 'eat', which may be applicable for programming virtual animals or similar objects. The narration explains the syntax and structure necessary for defining methods in programming, including the importance of formatting, starting with visibility followed by the method name, parentheses, and optional variables and data types. It is highlighted that, although you can specify variables and their types within the parentheses in method definitions, it is often unnecessary. Emphasis is placed on adding visibility to methods, specifically in the context of an 'eat' method.
• 20:30 - 21:30: Conclusion and Further Learning The chapter discusses the concept of visibility in object-oriented programming, focusing on accessibility modifiers for attributes and methods. It explains that a minus sign indicates a private attribute or method, restricting access to the defining class or subclass. Conversely, a plus sign denotes public attributes or methods, allowing access from any other class.

UML class diagrams Transcription

• 00:00 - 00:30 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
• 00:30 - 01:00 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.
• 01:00 - 01:30 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.
• 01:30 - 02:00 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
• 02:00 - 02:30 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.
• 02:30 - 03:00 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.
• 03:00 - 03:30 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.
• 03:30 - 04:00 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.
• 04:00 - 04:30 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
• 04:30 - 05:00 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.
• 05:00 - 05:30 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.
• 05:30 - 06:00 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.
• 06:00 - 06:30 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
• 06:30 - 07:00 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
• 07:00 - 07:30 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.
• 07:30 - 08:00 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.
• 08:00 - 08:30 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.
• 08:30 - 09:00 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.
• 09:00 - 09:30 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.
• 09:30 - 10:00 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.
• 10:00 - 10:30 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.
• 10:30 - 11:00 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.
• 11:00 - 11:30 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.
• 11:30 - 12:00 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.
• 12:00 - 12:30 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.