UML - Class Diagram

At the high level, there are two things we can see in the UML class diagram-

• Class block - The Box Shape
• The connecting line among the classes

Based on the line type, we can figure how the entities are connected. Let’s go through different type line types for the relation type.

Connector for different type relationship

1. Unfilled arrowhead with a solid line for Inheritance

The direction of the arrow is from the child class to the parent class. The child class is just a specific type of parent class and can be replaced with one another.

2. Unfilled arrowhead with a broken line for Interface-Implementation

This arrow is used to show interface implementation. The direction is from the implementation class to the interface.

3. Filled arrowhead with a solid line for Strong association

class Dependent {
    Provider provider;

    Dependent(Provider provider) {
        this.provider = provider;
    }
}

class Provider { }

In the code snippet, the Dependent class is storing a reference of the Provider class. So the Dependent and the Provider class have a strong association.

The direction of the arrow is from the dependent to the dependee.

Note: Sometimes, you will also see association without any arrow.

4. Filled arrowhead with a broken line for Dependency

class Dependent {
    public void someMethod(Provider provider) {}
}

class Provider { }

A dependency is much weaker than an association. Similar to the association, the direction of the arrow is from the dependent to the dependee.

In the example code above, the provider class is passed via a method. However, there are other ways the Dependent class could depend on the Provider class.

• When Dependent class raises or catches an exception of type Provider
• Whenever the Provider class is modified, the Dependent class needs to be modified.
• The dependent class calls a static method from the Provider class.

Note: Sometimes, you will also see association without any arrow.

5. Specific type of association connector

5.1. Solid line with a filled diamond for Composition

The filled diamond shape is placed with the dependent entity.

Composition means The Dependent class owns the Provider class. Therefore, the Provider class cannot exist or has no meaning without the Dependent class.

For example, the relationship between a building and rooms is a type of Composition.

5.2. Solid line with an unfilled diamond for Aggregation

The unfilled diamond shape is placed with the dependent entity.

Aggregation means the Dependent class uses the Provider class. The Provider class exists independently (conceptually) from the Dependent class.

For example, The relationship between a Company and People is a type of Aggregation.