UML Class Diagrams Overview

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Questions and Answers

What is the consequence of choosing aggregation when a client gets a reference to the position or velocity of a projectile?

The client can change these quantities without asking the projectile.

Explain the concept of Composition in object-oriented programming.

An object of type X that is composed of an object of type Y means X has-a Y object and owns the Y object, having exclusive access to it.

What does it mean when it's said that the X object has exclusive access to its Y object in Composition?

It means that the X object will generally not share references to its Y object with clients.

Why is a deep copy required in Composition when a constructor has a Y parameter?

A deep copy is required to ensure that the X object has exclusive access to its Y object. Signup and view all the answers

What must a default constructor do in Composition?

The default constructor must create a suitable Y object. Signup and view all the answers

In Composition, what should a constructor do when it has a Y parameter?

The constructor must first deep copy and then validate the Y object. Signup and view all the answers

What is a UML class diagram used for?

A UML class diagram is used for visualizing the design of a software system. Signup and view all the answers

What does an open arrow at the end of a dashed line in a UML class diagram represent?

A dependency relationship. Signup and view all the answers

How is aggregation different from composition in terms of ownership?

Composition implies ownership, while aggregation does not imply ownership. Signup and view all the answers

Give an example of aggregation with a Person class.

In a Person class, having a name and a date of birth as attributes. Signup and view all the answers

What relationship does association model in UML class diagrams?

The has-a relationship. Signup and view all the answers

Explain the 'lollipop' notation used in UML class diagrams.

It indicates that a class implements an interface. Signup and view all the answers

What does a solid line connecting two classes in a UML class diagram represent?

An association relationship. Signup and view all the answers

What type of relationship is dependency in UML class diagrams?

Dependency is the weakest relationship between classes. Signup and view all the answers

How is aggregation different from association?

Aggregation and composition are stronger forms of association, with ownership implications. Signup and view all the answers

What happens in aggregation when a client modifies an attribute of the shared object?

The modification is reflected in all instances sharing the object. Signup and view all the answers

What is a privacy leak in a class?

A privacy leak occurs when a class exposes a reference to a non-public field that is not a primitive or immutable. Signup and view all the answers

How can a privacy leak affect the composition of an object?

A privacy leak can break composition as the object no longer owns its attribute. Signup and view all the answers

What are the consequences of a privacy leak in a CreditCard class?

A client could set the expiry date of the CreditCard to before the issue date. Signup and view all the answers

Why is it important to prevent privacy leaks in a Period class?

To ensure that the end of the period cannot be set before the start of the period. Signup and view all the answers

What are the key points in the recipe for immutability in Java?

Do not provide methods that alter the object's state, make fields final and private, prevent inheritance, and prevent clients from obtaining a reference to mutable fields. Signup and view all the answers

Why should a class prevent clients from obtaining a reference to any mutable fields?

To maintain the immutability of the class and prevent unexpected changes to its state. Signup and view all the answers

How does using composition in a Period class help prevent privacy leaks?

By creating new Date objects in accessors and mutators instead of returning references to the original Date objects. Signup and view all the answers

What is the difference between aggregation and composition when dealing with arrays?

Aggregation with arrays of reference types can lead to complexity, while composition involves better ownership and control over array elements. Signup and view all the answers

How can a privacy leak affect the object's class invariants?

A privacy leak can make it impossible to guarantee class invariants. Signup and view all the answers

What is the risk of not preventing privacy leaks in a Java class?

The object's state can become inconsistent and other objects may control its data. Signup and view all the answers

Study Notes

Aggregation and Composition

Aggregation: Clients get a reference to the position or velocity of a projectile. The client can modify the projectile's state directly, leading to unexpected behavior.
Composition: A strong "has-a" relationship. An X object owns a Y object. The Y object exists only within the scope of the X object.
Exclusive Access: An X object has exclusive access to its Y object, meaning no other object can directly access or modify the Y object.
Deep Copy: A constructor that accepts a Y parameter needs a deep copy to prevent shared mutable state between objects.
Default Constructor: In Composition, a default constructor must initialize the Y object with a properly constructed instance.
Constructor with Y Parameter: A constructor with a Y parameter should receive a Y object and copy it to avoid shared state.
UML Class Diagram: A diagram that visualizes the relationships between classes in a program.
Open Arrow: Represents a dependency relationship. The class with the arrow depends on the target class.
Ownership: Composition represents strong ownership, while aggregation represents a weaker form of association.
Aggregation Example:
- A Person class can aggregate a list of addresses. Even if the Person object is deleted, addresses objects continue to exist independently.
Association: Represents a relationship between two classes. This relationship can be either a "has-a" or "uses-a" relationship.
Lollipop Notation: Represents a reference from one class to another. The lollipop is connected to the class that holds the reference.
Solid Line Connection: Represents a "has-a" relationship between two classes.
Dependency Relationship: One class needs the services of another class to function correctly.
Aggregation vs. Association: Association is a looser relationship than aggregation.
Aggregation Modification: If a client modifies an attribute of the shared object in aggregation, the change is reflected in all objects sharing that object.
Privacy Leak: Occurs when a class exposes a reference to its mutable fields.
Privacy Leak Impact: Privacy leaks can compromise the object's invariants and create unpredictable behavior.
CreditCard Privacy Leak: If a CreditCard class leaks its mutable fields, hackers could access and modify sensitive information like card details.
Period Privacy Leak: Preventing privacy leaks in a Period class ensures its internal state remains consistent and that incorrect dates are not generated.
Immutability Recipe:
- Make all fields private.
- Avoid using mutable data structures.
- Use defensive copying when necessary.
Mutable Fields: Clients should not be allowed to obtain references to mutable fields to avoid potential modification.
Composition and Privacy Leaks: Using composition in a Period class helps prevent privacy leaks by encapsulating the internal state and providing controlled access.
Aggregation vs. Composition with Arrays: Both composition and aggregation can be used with arrays, but composition provides stronger ownership and control over the array data.
Privacy Leaks and Invariants: Privacy leaks can violate the object's class invariants, leading to unexpected behavior and potential errors.
Risk of Privacy Leaks: Not preventing privacy leaks in a Java class can compromise the class's integrity, introduce vulnerabilities, and lead to unpredictable behavior.

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Description

Learn about Unified Modelling Language (UML) class diagrams, which are used in software engineering to visualize the design of a software system. This quiz covers the structure of a class including its name, fields, and methods.