Objects

Question 1

What does an object represent?

Answer 1

Different individuals in the system, such as, but not exclusively people. Animals, plants and inanimate objects like cars can also be described as objects.

Question 2

What is the relationship between objects called?

Answer 2

A link. In UML, a normal line is used to show that two objects are linked.

Question 3

What are the characteristics of an object?

Answer 3

The characteristics of an object include structural characteristics (attributes) and its behavior (its operations).

Question 4

Can two objects have two different sets of operations?

Answer 4

No, operations are identical for all objects of a class and are therefore usually described exclusively for the class.

Question 5

Can two objects have the same set of attributes?

Answer 5

No, objects have different attributes from one another, but share the same behavior (operations). If two people share the same name and date of birth, they would have to be represented as two objects, such as maxMiller1 and maxMiller2.

Question 6

What does multiplicity indicate?

Answer 6

The multiplicity of an attribute indicates how many values an attribute can contain. For example, how many students (objects) can attend a lecture.

Question 7

What does this multiplicity notation mean? [1..*]

Answer 7

The minimum limit is 1, there is no upper limit

Question 8

What does {ordered} mean in a class context?

Answer 8

A fixed order of attribute values, for example, an address should follow the order: street, street number, zip code, city, country.

Question 9

What does {unique} and {non-unique} mean in a class context?

Answer 9

Specifies whether duplicates of attributes are allowed or not. An address, for example, should only be entered once and is therefore unique.

Question 10

Define how operations are characterized

Answer 10

Operations are characterized by their name, their parameters and the type of their return value.

Question 11

What does a parameter with an inward direction indicate?

Answer 11

If a parameter has the direction in, this indicates that when the operation is used, a value is expected from this parameter.

Question 12

What does a parameter with an outward direction indicate?

Answer 12

The specification of the direction out expresses that after the execution of the operation, the parameter has adopted a new value. If an operation should have multiple return values rather than just one, you can express this using multiple parameters with the direction out.

Question 13

What does a parameter with an inout direction indicate?

Answer 13

This indicates a combined input/output parameter. If no direction is specified, in is the default value.

Question 14

What is used to realize the important concept of information hiding?

Answer 14

Visibilities such as public, private and protected are used to realize information hiding.

Question 15

Why is setting the correct visibility for an attribute important?

Answer 15

Setting the correct visibility ensures that unauthorized access of the attribute is not possible.

Question 16

What does “public visibility” mean? Illustrated via +

Answer 16

Access by objects of any classes is permitted

Question 17

What does “private visibility” mean? Illustrated via -

Answer 17

Access only within the object itself is permitted

Question 18

What does “protected visibility” mean? Illustrated via #

Answer 18

Access by objects of the same class and its subclasses is permitted

Question 19

What does “package visibility” mean? Illustrated via ~

Answer 19

Access by objects whose classes are in the same package is permitted

Question 20

What is a class variable?

Answer 20

As opposed to instance variables, class variables are only created once per class. They can also be referred to as static attributes or class attributes. One example of a class variable is interest on a bank account. This is set once for all the objects in that class. The current balance of the bank account is in this case an instance variable, since it state changes within objects and is different for different objects.

Question 21

What is a class operation?

Answer 21

A class operation is a static operation that can be used even if no instance of the corresponding class has been created. For a person class, getting the amount of objects in the class can be accessed via get.PCounter, which is called directly via the class and not via an instance. This would correspond to a static variable called pCounter or person counter.

Question 22

What is a binary association?

Answer 22

A binary association allows us to associate the instances of two classes with one another. For example, professor and student.

Question 23

What does the following navigability mean?

A <—–> B

Answer 23

A can access the visible attributes and operations of B and vice versa. The two objects can navigate freely between each other.

Question 24

What does the following navigability mean?

A x——> B

Answer 24

This is a non-navigable association. A can access visible attributes and operations of B, but B can not access attributes and operations of A.

Question 25

What does a multiplicity of association specify?

Answer 25

Multiplicities of associations are given as an interval in the form minimum...maximum. They specify the number of objects that may be associated with exactly one object of the opposite side.

Question 26

What is an N-ary association?

Answer 26

If more than two partner objects are involved in a relationship, this is modeled with an n-ary association. (a hollow diamond in the center)

Question 27

What is an association class?

Answer 27

An association class is used when you want to assign attributes or operations to the relationship between one or more classes, rather than to a class itself. The association class is represented by a class and an association that is connected by a dashed line.

Question 28

If a duplicate is required for an association class, at least one association end must be identified as...

Answer 28

{non-unique}. If this property is not specified explicitly, the default value {unique} is assumed.

Question 29

What is an aggregation?

Answer 29

An aggregation is a special form of association that is used to express that instances of one class are parts of an instance of another class. UML differentiates between two types: shared aggregation and composition.

Question 30

What does a shared aggregation express?

Answer 30

A shared aggregation expresses a weak belonging of the parts to a whole, meaning that parts also exist independently of the whole.

Question 31

What does a composition express?

Answer 31

The use of a composition expresses that a specific part can only be contained in at most one composite object at one specific point in time. This results in a maximum multiplicity of 1 at the aggregating end. If the composite object is deleted, its parts are also deleted.

Question 32

When is generalization used?

Answer 32

Generalization is to highlight commonalities between classes, meaning that common characteristics don't have to be defined multiple times. A professor is, for example, an instance of a research associate. An assistant is also a research associate, meaning that the assistant and professor both can be generalized as research associates and then divided into their relevant subclasses, with research associate as a superclass.

Question 33

What is a subclass?

Answer 33

A subclass is an instance of a superclass. The subclass inherits all attributes and operations from the superclass.

Question 34

Can an abstract class be instantiated?

Answer 34

No, an abstract class can not be instantiated, since it is abstract. Only non-abstract subclasses of the class can be instantiated.

Question 35

What does multiple classification mean?

Answer 35

A class that is an instance of multiple classes, which is allowed in UML. The classes don't have to be associated in an inheritance way. A tutor, for example is both a university employee and a student. The tutor inherits from both the employee class and the student class.

Question 36

What does overlapping generalization mean?

Answer 36

In an overlapping generalization set, an object may be an instance of multiple subclasses simultaneously.

Question 37

What does disjoint generalization mean?

Answer 37

An object may be an instance of a maximum of one subclass.

Question 38

What does complete and incomplete generalization mean?

Answer 38

In a complete generalization set, each instance of the superclass must be an instance of at least one of the subclasses. In incomplete generalization sets, this is not necessary. In complete generalizations, every possible type of subclass is defined, in incomplete generalizations, all are not defined.

Question 39

What would a complete generalization of the class "vehicle" contain?

Answer 39

That all subclasses of vehicle are known and depicted in the diagram. If the subclasses are "car" and "motorcycle", no other types of vehicles could be added later.

Question 40

What is an interface?

Answer 40

An interface is a contract that defines a set of operations (behaviors) that a class must implement - without providing the implementation details. A class says "here's how I work", an interface says "here's what I promise you can ask me to do".

Question 41

What is the purpose of an interface?

Answer 41

Interfaces are used to specify common behavior that multiple classes can share, even if otherwise unrelated. They enable polymorphism, code that can work with "anything that implements this interface" rather than with one specific class. A "printable" interface can be used if several, unrelated classes need to be printable, without the classes having to be otherwise related, such as "Document" and "Photo".

Question 42

What are the four primitive data types pre-defined in UML?

Answer 42

Boolean, integer, UnlimitedNatural and String.

Question 43

What is an enumeration?

Answer 43

Enumerations are data types whose values are defined in a list. For example, the enumeration AcademicDegree can list bachelor, master, phd, i.e. all known academic degrees in a system.

Question 44

If you see <> Date in a UML diagram, is that a user-defined data-type or a primitive data type?

Answer 44

Date is a user-defined data type. Primitive data types are denoted with <>.

Question 45

What are the attributes of an enumeration called?

Answer 45

Literals