What are the main types of UML diagrams?

The most common are the class diagram (structure: classes, attributes, methods, relationships), sequence diagram (object interactions over time), use case diagram (functionality from the user’s view), activity diagram (process and workflow flow), state machine diagram (an object’s states and transitions), and component diagram (high-level building blocks and their interfaces).

Can I generate a class diagram from code?

This generator works from a text description, not from uploaded source files. It does not parse your repository. To get a class diagram for existing code, write out the class names, their main attributes and methods, and how they relate (or paste a short summary of them) into the description, and the tool draws the diagram from that.

What is the difference between a class diagram and an ER diagram?

A class diagram describes object-oriented structure: classes with attributes and methods, inheritance, and associations. An ER diagram describes database structure: entities with attributes and keys, and relationships with cardinality, with no methods. Use a class diagram for application code and an ER diagram for database tables.

How do you show composition in a class diagram?

Draw a line between the whole and the part with a filled diamond at the whole end. It means the part cannot exist without the whole and is usually destroyed with it, for example an Order and its OrderItems. Add multiplicity on the part end, such as 1..*, to show how many parts the whole contains.

How do you show visibility in a UML class diagram?

Put a symbol in front of each attribute or method: + for public, - for private, # for protected, and ~ for package visibility. For example, "- email: String" is a private attribute and "+ getTotal(): Decimal" is a public method.

What is a UML class diagram?

A class diagram shows the static structure of a system: its classes, each with a name, attributes (data), and methods (behavior), plus the relationships between them. It is the most widely used UML diagram and is the usual starting point for modeling a data model or object-oriented design.

What are the relationships in a class diagram?

Association is a plain link where one class uses another. Inheritance (a hollow triangle arrow) means one class is a specialized type of another. Aggregation (a hollow diamond) is a whole–part relationship where the part can exist independently. Composition (a filled diamond) is a stronger whole–part relationship where the part cannot exist without the whole.

What is the difference between aggregation and composition?

Both are whole–part relationships. In aggregation the part can exist on its own — a Team has Players, but the players remain if the team disbands (hollow diamond). In composition the part’s lifetime is tied to the whole — a House has Rooms, and the rooms cease to exist if the house does (filled diamond).

Is this UML diagram generator free?

You can try it free: new accounts get a small free allowance of generations (see the pricing page for the current numbers). Free exports carry a watermark; paid plans remove it and add more generations and exports for design docs, slides, and pull requests.

UML Class Diagram Generator and Other UML Diagrams

Q: How do you generate a UML diagram from text?

Describe the diagram you want in plain English — name the UML diagram type (class, sequence, use case, activity, state, or component) and list the key elements and how they relate. The generator draws it with standard UML notation, clear labels, and clean connectors, then lets you export it.

Describe your classes in plain English and get a UML class diagram with attributes, methods, and relationships.

Class, sequence, use case, activity, state & componentFrom a plain-English descriptionStandard UML notation and relationshipsExport for design docs & slides

Hand-drawn example, not generated output. Describe your own system above to generate yours. Example class diagram for an online shop. Association, composition (filled diamond), and inheritance (hollow triangle), as described in the guide below.

UML Diagram Generator

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UML Diagram Examples

Class, sequence, use case, activity, state, and component diagrams

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Class Diagram

Classes with attributes and methods, linked by inheritance, aggregation, and composition.

class-diagramoopinheritance

Use Case Diagram

Actors, use case ovals, and a system boundary — the classic way to capture requirements.

use-caseecommercerequirements

Sequence Diagram

Lifelines and messages over time — how objects collaborate to handle a request.

sequence-diagramapiinteraction

Activity Diagram

Actions, decisions, forks, and swimlanes — a UML flowchart for a process or workflow.

activity-diagramworkflowprocess

State Machine Diagram

States and labeled transitions show how an object changes over its lifecycle.

state-machinelifecycletransitions

Component Diagram

Components and their interfaces — a high-level map of how a system is assembled.

component-diagramarchitecturemicroservices

What is a UML diagram?

UML (Unified Modeling Language) is the standard visual language for modeling software. A UML diagram uses an agreed set of shapes and connectors — boxes for classes, ovals for use cases, lifelines for objects — so that any engineer can read the design the same way. UML lets a team picture a system before they build it: its structure, its behavior, and how its parts interact. This generator turns a plain-English description into a clean, correctly notated UML diagram, so you can capture a design without hand-drawing every box and arrow.

The main UML diagram types

Class diagram: the structure of a system — its classes, their attributes and methods, and the relationships between them. The most-used UML diagram.
Sequence diagram: how objects interact over time, with lifelines and the messages passed between them to complete a scenario.
Use case diagram: the system’s functionality from the user’s point of view — actors, use cases, and a system boundary. Great for requirements.
Activity diagram: the flow of a process or workflow, with actions, decisions, and parallel branches — essentially a UML flowchart.
State machine diagram: the states an object moves through and the events that trigger each transition over its lifecycle.
Component diagram: the high-level building blocks of a system and the interfaces they provide and require — useful for architecture.

How to read a UML class diagram

A class is drawn as a box split into three compartments: the class name on top, its attributes (data) in the middle, and its methods (behavior) at the bottom. A visibility marker precedes each member — plus (+) for public, minus (−) for private, hash (#) for protected. Attributes are usually written name: type, and methods as name(parameters): returnType. Read a class diagram top to bottom for what an object is and does, then follow the connectors to see how it relates to the rest of the system.

Class diagram relationships: association, inheritance, aggregation, composition

Association: a plain line meaning two classes have a structural link (for example, a Customer places Orders). Multiplicities such as 1 or 0..* show how many.
Inheritance (generalization): a hollow triangle arrow pointing to the parent class. A Dog is an Animal, so it inherits the parent’s attributes and methods.
Aggregation: a hollow diamond for a whole–part relationship where the part can exist on its own — a Team has Players, but the players survive if the team disbands.
Composition: a filled diamond for a stronger whole–part relationship where the part cannot exist without the whole — a House has Rooms, and the rooms are gone if the house is.

How to make a UML class diagram

Pick the classes: list the main nouns in your requirements. For an online shop that gives Customer, Order, OrderItem, and Product.
Fill in the three compartments: class name on top, attributes in the middle (for example, Customer: name: String, email: String), methods at the bottom (for example, Order: addItem(product, qty), getTotal(): Decimal). Prefix each member with + for public, - for private, or # for protected.
Connect the classes: a Customer places many Orders (association, multiplicity 1 to 0..*); an Order is made of OrderItems that cannot exist without it (composition, filled diamond); each OrderItem refers to one Product (association); a DigitalProduct and a PhysicalProduct both inherit from Product (inheritance, hollow triangle). A class that only uses another temporarily, such as Order using a PaymentGateway, is a dependency, drawn as a dashed arrow.
Add multiplicity at each end of a line: 1 means exactly one, 0..1 optional, * or 0..* zero or more, 1..* one or more. Then review the diagram for classes that have no relationships and for relationships that are missing.
To generate it here, paste a description such as "class diagram for an online shop: Customer places many Orders; Order is composed of OrderItems; each OrderItem refers to one Product; DigitalProduct and PhysicalProduct inherit from Product". The tool draws the boxes, compartments, and connectors.

Class diagram vs. ER diagram

A class diagram models objects in code: classes with attributes and methods, plus inheritance and other object-oriented relationships. An entity-relationship (ER) diagram models data in a database: entities with attributes and keys, linked by relationships with cardinality, and it has no methods. The two often describe the same domain, such as Customer and Order, from different angles. Use a class diagram when designing application code, and an ER diagram when designing tables.

Generate a UML diagram from a description

Describe what you need in plain English — for example, “a class diagram for a library with Book, Member, and Loan classes, where a Member can have many Loans,” or “a sequence diagram for a user logging in through an API gateway.” Name the diagram type you want (class, sequence, use case, activity, state, or component) and list the key elements and how they relate. The generator draws the diagram with correct UML notation, clear labels, and clean connectors, ready to export and drop into a design doc, slide, or pull request.

Using UML in software design

UML is a thinking and communication tool. Sketch a class diagram to agree on the data model before writing code; use a sequence diagram to nail down an API interaction or trace a bug across services; draw a use case diagram to align with stakeholders on scope; reach for an activity or state diagram when the logic is a process or a lifecycle. A diagram does not have to be exhaustive to be useful — even a rough UML view often surfaces missing relationships and edge cases early, when they are cheapest to fix. Generate one in seconds, then refine the description until the design reads right.

Frequently Asked Questions

Need a different view of the same system? Try the sequence diagram generator for message flows, the state machine diagram generator for object lifecycles, or the ER diagram generator for database design.