UML Relationship Types: Dependency, Association, Aggregation, Composition, Generalization, and Realization

UML defines several ways to describe relationships between objects and classes, including Dependency, Association, Aggregation, Composition, Generalization, and Realization.

Dependency is a one‑way relationship where a change in element A may affect element B; it is shown with a dashed arrow. Types of dependency such as usage, call, parameter, send, instantiate, trace, refine, derive, access, import, friend, and bind are described with UML stereotypes.

Method‑parameter example of a dependency:

public class Person {
    void buy(Car car) {
        // ...
    }
}

Association represents a structural link between classes, typically implemented via member variables; it can be uni‑ or bi‑directional and is shown with a solid line.

Association example in Java/C++:

public class 徒弟 {
    // ...
}

public class 唐僧 {
    protected: list<徒弟> tdlist;
}

Aggregation is a special form of association indicating a whole‑part relationship, depicted with a hollow diamond on the whole side.

Aggregation example:

public class 引擎 {
    // ...
}
public class 轮胎 {
    // ...
}
public class 汽车 {
    protected: 引擎 engine;
    protected: 轮胎 tyre[4];
}

Composition is a stronger whole‑part relationship where the part’s lifecycle is bound to the whole, shown with a filled diamond.

Composition example:

class 肢 {
    // ...
}
class 人 {
    protected: 肢 limb[4];
}

Generalization models inheritance (a “is‑a” relationship) and is drawn with a solid line and a hollow triangle arrow pointing to the general element.

Realization models the implementation of an interface, using a dashed line with a hollow triangle arrow.

These UML relationships help designers capture semantic connections between software elements, while the code snippets illustrate how they manifest in typical Java or C++ implementations.