UML Class Diagram Relationships

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💨

Dependency

Temporary "uses-a" — the weakest relationship

🔍 Relationship Spectrum — Weakest to Strongest

WEAK

STRONG

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Dependency

Temporary "uses-a" — the weakest relationship

Dependency is the weakest UML relationship. It means one class temporarily uses another — typically as a method parameter, local variable, or return type. The dependent class does NOT store a reference as a field; the usage is confined to a single method scope. A change in the depended-upon class may affect the dependent class, because it relies on its interface. In UML, dependency is drawn as a dashed line with an open arrowhead pointing from the dependent class to the class it depends on.

Key Ideas

Weakest relationship — temporary usage, not permanent storage
UML notation: dashed line with open arrowhead (- - - ->)
Always directed: Class A depends on Class B (A - - -> B)
Caused by method parameters, local variables, or return types
No field reference — the object is used and released within method scope
A change in Class B may break Class A (A depends on B's interface)

Strengths

✦Keeps coupling minimal — classes are loosely connected
✦Easy to refactor since the relationship is not structural
✦Makes the "uses" intent explicit in the diagram
✦Good for utility and helper class relationships

Weaknesses

△Often overlooked in diagrams because it is so lightweight
△Students frequently confuse dependency with association
△Too many dependencies on one class signals a design smell
△Hard to track in large codebases without tool support

Used in:A Driver class that takes a Car parameter in its drive() method has a dependency on Car. The Driver does not own a Car — it borrows one temporarily. In a ride-sharing app, the DriverService depends on PaymentGateway only during processPayment().

Code Snippets

Quiz

Dependency Quiz

QUIZ 1/6

class Printer {
    void print(Document doc) {
        System.out.println(doc.getText());
    }
}

Which of the following Java code patterns represents a UML dependency relationship?

Diagrams

All Six UML Relationships at a Glance

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Association vs Aggregation vs Composition

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House Multiplicity Example

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Practice

Classify each property into the correct UML relationship type:

EXERCISE 1/5

Dashed line with open arrowhead

Temporary usage in method scope

Solid line with unfilled triangle

Filled diamond on the whole side

Parts have independent lifetimes

Dashed line with unfilled triangle

Hollow diamond on the whole side

"implements" keyword in Java

Dependency

Generalization

Realization

Aggregation

Composition

UML Relationship Myths

EXERCISE 2/5

Dependency stores a reference as a field in the dependent class.

A class can implement multiple interfaces (multiple realization) in Java.

In aggregation, destroying the whole also destroys the parts.

The diamond (hollow or filled) always points toward the "whole" class.

Generalization and realization use the same arrowhead (open triangle) — the line style differs.

Association multiplicity "0..*" and "*" mean different things.

Complete the Java Relationship Code

EXERCISE 3/5

class Printer {
    // Dependency: temporary usage
    void print( doc) {
        System.out.println(doc.getText());
    }
}

class Dog  Animal {
    // Generalization: inheritance
}

class PayPalProcessor  PaymentService {
    // Realization: interface implementation
}

class House {
    // Composition: owns rooms exclusively
    private  List<Room> rooms;
}