UML: Dependency, Association, Aggregation, Composition, Generalization, Realization

UML describes various ways objects and classes can relate, including Dependency, Association, Aggregation, Composition, Generalization, and Realization.

Dependency

Dependency means element A changes affect element B, but not vice‑versa; B depends on A. It is shown with a dashed arrow pointing to the depended‑on element.

Characteristics: one‑way, often appears as local variables, method parameters, or static method calls in Java/C++.

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

Association

Association is a structural, usually bidirectional, relationship where one class knows another class's attributes and methods. In code it is implemented via member variables.

public class Disciple {
    // ...
}

public class TangSeng {
    protected: list<Disciple> tdlist;
}

Aggregation

Aggregation is a special form of association that represents a whole‑part relationship (has‑a). The part can exist independently of the whole.

public class Engine {
    // ...
}
public class Tire {
    // ...
}
public class Car {
    protected: Engine engine;
    protected: Tire tyre[4];
}

Composition

Composition is a stronger form of aggregation where the part’s lifecycle is bound to the whole; when the whole is destroyed, its parts are destroyed as well.

class Limb {
    // ...
}
class Person {
    protected: Limb limb[4];
}

Generalization

Generalization models inheritance (is‑a) relationships between a more general element and a more specific one. In Java it is expressed with the extends keyword.

Realization

Realization models the contract between an interface (the definition) and a class that implements it. In Java it is expressed with the implements keyword.

Differences Between Relationships

Aggregation vs. Composition : Both are whole‑part relationships, but in aggregation the part can exist independently of the whole, while in composition the part’s lifecycle is tied to the whole.

Association vs. Aggregation : Association is a generic structural link; aggregation adds the whole‑part semantic.

Association vs. Dependency : Association represents a stronger, often long‑term structural link, whereas dependency is a weak, temporary usage relationship.

Generalization vs. Realization : Generalization models inheritance between classes (or interfaces), while realization models a class fulfilling an interface contract.

Overall, the strength of these relationships can be ordered as: Composition > Aggregation > Association > Dependency.