Chapter 12 - Further abstraction techniques

Question 1

We may create these within abstract classes or interfaces.

in cases where it may be impossible to define the implementation of a method because it is solely or heavily dependent upon its subclasses.

Answer 1

where would we
Use abstract methods

Question 2

This is a further abstraction technique in Java that often contains only abstract method definitions and no implementations.

Answer 2

What is an
interface?

Question 3

describe
Re-factoring

Answer 3

refers to improving the structure of the code while keeping the functionality the same

Question 4

What are 3 advantages of using the @Override annotation when overriding a method of a superclass or implementing an interface in Java?

Answer 4

advantages include:
1. It states our intention that we are indeed meant to override a superclass method or implement an abstract method from an interface.
2. If we create an overridden method with an incorrect signature that does not match that of a method in the superclass or interface, the Java compiler will tell us that “method does not override or implement a method from a supertype,” allowing us to catch our error at compile time.
3. If a method is overridden by many subclasses, and we decide to change its header in the base superclass or interface, locating the subclasses that have overridden this method is easy, as the compiler will flag the places where the header does not match.

Question 5

this type of class is appropriate if we have a collection of classes that are closely related.

For example:
1. BirthdayCard
2. ChristmasCard
3. GreetingCard

could extend the abstract class Card.

Answer 5

In general when would an abstract class be appropriate?

Question 6

advantages include:
1. It states our intention that we are indeed meant to override a superclass method or implement an abstract method from an interface.
2. If we create an overridden method with an incorrect signature that does not match that of a method in the superclass or interface, the Java compiler will tell us that “method does not override or implement a method from a supertype,” allowing us to catch our error at compile time.
3. If a method is overridden by many subclasses, and we decide to change its header in the base superclass or interface, locating the subclasses that have overridden this method is easy, as the compiler will flag the places where the header does not match.

Answer 6

What are 3 advantages of using the @Override annotation when overriding a method of a superclass or implementing an interface in Java?

Question 7

What is a question to ask when deciding whether to use an interface or an abstract class?

Answer 7

“Does this type need concrete elements such as instance fields, constructors, and method bodies?”

If yes, then an abstract class would be appropriate because interfaces cannot have instance fields or constructors.

Question 8

public int compareTo(Pet otherPet)
{
    return (int) (weightInGrams() - otherPet.weightInGrams());
}

Answer 8

Can you provide an example implementation of the Comparable interface method compareTo()

Question 9

What is inherited when we extend
abstract classes?

Answer 9

When we extend these types of classes, we inherit the type and a partial implementation.

Question 10

The compareTo() method is used to compare two objects and determine their order.

It returns an int value that indicates whether one object comes before or after or is at the same level as another object.

Answer 10

How is the compareTo() method of the Comparable interface used in Java?

Question 11

Three benefits of Inheritance and Interfaces

Answer 11

Inheritance:
1. less code duplication - due to the inheritance of fields and methods
2. polymorphic variables and method calls - where the dynamic type of a variable at runtime is what determines the behavior

Although interfaces do not help gain the first benefit, they can be used to gain the benefit of polymorphic variables by using them as a variable type (static type).

Question 12

How is the compareTo() method of the Comparable interface used in Java?

Answer 12

The compareTo() method is used to compare two objects and determine their order.

It returns an int value that indicates whether one object comes before or after or is at the same level as another object.

Question 13

In general when would an abstract class be appropriate?

Answer 13

this type of class is appropriate if we have a collection of classes that are closely related.

For example:
1. BirthdayCard
2. ChristmasCard
3. GreetingCard

could extend the abstract class Card.

Question 14

To declare this, it is prefixed with the keyword “abstract” and has no body. Instead, it is terminated with a semicolon.

For example:
abstract public void methodName(params);

Answer 14

write a
declaration of an abstract method

Question 15

Some languages allow for multiple inheritance, while others do not. Java allows for a limited use of multiple inheritance through interfaces.

Answer 15

Does java allow for multiple inheritance?

Question 16

What are the possible int values returned by the compareTo() method in Java?

Answer 16

This method of Comparable interface can return one of three int values:
1. a negative value if this object comes before the second
2. a positive value if the second object comes before this object
3. and zero if both objects belong in the same position.

Question 17

this can be accomplished with:
interfaceName.super.methodName(…)

Answer 17

how do we
call a default method of an interface

Question 18

This is a class that typically has no implementation but states what any implementations of the specification must do.

In Java, an interface does just this by defining abstract methods.

Answer 18

What is a specification in Java?

Question 19

Can you provide an example implementation of the Comparable interface method compareTo()

Answer 19

public int compareTo(Pet otherPet)
{
    return (int) (weightInGrams() - otherPet.weightInGrams());
}

Question 20

No, these do not have instance fields

Answer 20

can
interfaces
contain instance fields

Question 21

What is the purpose of abstract classes with abstract methods?

Answer 21

They force subclasses to implement the abstract methods, ensuring consistency across different subclass implementations.

For an abstract subclass to become a concrete class, it must implement all inherited abstract methods.

Question 22

in this scenario it may be wiser to opt for an interface as Subtypes are not then bound to a single supertype as is the case with abstract classes.

This leads to less coupling and greater code flexibility and room for extensibility.

Answer 22

Why might it be wiser to opt for an interface instead of an abstract class if instance fields or constructors are not required?

note that either could be used

Question 23

1. To implement multiple inheritance
2. To use the interface as a supertype and the highest abstraction with no implementation - We want to treat objects that are instances of a number of different classes in a uniform way
3. To implement a specification - We want to allow each of those concrete classes to decide how they should implement the specification. We only define an action that concrete classes should have not the actual behaviour

Answer 23

What are three motivations for creating an interface?

Question 24

What is inherited when we extend
concrete classes

Answer 24

When we extend this type of class, we inherit all implementation of the class as well as its type.

Question 25

to accomplish this: `Import java.util.Collections`

Answer 25

what is the statement to **import the Collections class**

Question 26

describe an **abstract method**

Answer 26

These are methods that have a header but no body. That is, they have an interface but no implementation.

Question 27

This class in Java is a utility class that provides many methods for manipulating the contents of collections, such as lists and sets.

Answer 27

What is the Collections class in Java?

Question 28

What are the **general benefits of inheritance?**

Answer 28

The general benefits of this are: 1. **code inheritance** -where we may inherit common code such as fields and methods 2. **subtyping** - where we may inherit the type of the superclass which allows for polymorphism and specialization of a type.

Question 29

describe the solution where we have two classes with identical methods except the data they use comes from thier class constant

Answer 29

the solution here is: 1. Keep the class constant in the subclass 2. Redefine the method in the superclass 3. Use an abstract getter method in the superclass and a concrete getter method in the subclass to access the subclass class constant.

Question 30

can **interfaces** contain constructors

Answer 30

no, these do not contain constructors

Question 31

a method with either of these keywords within an interface must have a body with an implementation

Answer 31

within an interface methods with the static or default kewyord must have what

Question 32

The Comparable interface in Java can be implemented by other classes using the syntax `public class MyClass implements Comparable` The type given in the Comparable interface will determine the type of object being compared.

Answer 32

How is the Comparable interface implemented in Java?

Question 33

When we extend this type of class, we inherit all implementation of the class as well as its type.

Answer 33

What is inherited when we extend **concrete classes**

Question 34

Does java allow for multiple inheritance?

Answer 34

Some languages allow for multiple inheritance, while others do not. Java allows for a limited use of multiple inheritance through interfaces.

Question 35

How does an interface differ from an abstract class?

Answer 35

While both are abstractions, an interface has its own set of rules and a class can implement multiple interfaces, whereas it can only extend one abstract class.

Question 36

What is the **Comparable interface** in Java?

Answer 36

This is used when we have objects that can be sorted in some way and has one method called compareTo() that returns an int value indicating whether one object belongs before or after or at the same level as another object.

Question 37

This method of Comparable interface can return one of three int values: 1. a negative value if this object comes before the second 2. a positive value if the second object comes before this object 3. and zero if both objects belong in the same position.

Answer 37

What are the possible int values returned by the compareTo() method in Java?

Question 38

refers to how easy the code is to keep code functional and adapt to changes.

Answer 38

describe **maintainability**

Question 39

How is the Comparable interface implemented in Java?

Answer 39

The Comparable interface in Java can be implemented by other classes using the syntax `public class MyClass implements Comparable` The type given in the Comparable interface will determine the type of object being compared.

Question 40

This is when a class inherits directly from two or more superclasses.

Answer 40

What is **multiple inheritance?**

Question 41

Can a class without abstract methods be declared as abstract?

Answer 41

Yes, a class without abstract methods may be declared as abstract. This allows us to prevent the instantiation of the superclass, only allowing its subclasses to be instantiated. For example, we would not have an instance of a vehicle, but rather we would only want to instantiate subclasses of vehicle.

Question 42

within an interface methods with the static or default kewyord must have what

Answer 42

a method with either of these keywords within an interface must have a body with an implementation

Question 43

What are **default methods** in Java interfaces?

Answer 43

these were added in Java 8 to allow interfaces to have implementation for methods. When a method is defined using this keyword in an interface, it will have an implementation and that implementation will be inherited by all classes that implement or inherit from the interface. It's important to note that the functionality of this method type is rather limited, as the interface has no constructor or instance fields and so there is no state that can be examined or manipulated by the default method. In general, interfaces will be limited to using only abstract methods, but these can be useful in certain cases.

Question 44

this is appropriate if we have a collection of classes from different hierarchies that are not closely related but share some common characteristic. For example, we might have an interface Sendable that can be implemented by cards, letters, parcels, emails, furniture, and food boxes. They can all be sent, but how they are sent depends solely on the concrete implementation.

Answer 44

in general, when would an interface be appropriate?

Question 45

When we implement these, we usually only inherit the type and no implementations

Answer 45

What is inherited when we implement **interfaces**

Question 46

Why might it be wiser to opt for an interface instead of an abstract class if instance fields or constructors are not required? note that either could be used

Answer 46

in this scenario it may be wiser to opt for an interface as Subtypes are not then bound to a single supertype as is the case with abstract classes. This leads to less coupling and greater code flexibility and room for extensibility.

Question 47

in general, when would an interface be appropriate?

Answer 47

this is appropriate if we have a collection of classes from different hierarchies that are not closely related but share some common characteristic. For example, we might have an interface Sendable that can be implemented by cards, letters, parcels, emails, furniture, and food boxes. They can all be sent, but how they are sent depends solely on the concrete implementation.

Question 48

describe **extensibility**

Answer 48

refers to how easy it is to add new features to the code

Question 49

What happens if a subclass of an abstract class does not provide concrete implementations?

Answer 49

If a subclass of an abstract class does not provide concrete implementations, then it itself will become an abstract class (an abstract subclass).

Question 50

When might we define an interface instead of using an abstract class?

Answer 50

When there is a common characteristic between classes but implementing through inheritance such as an abstract class makes no sense since the classes may only be related through this single characteristic and nothing else. For example: The class is weighable (the concrete classes are weighable) The class is drawable (the concrete classes are drawable) The class is an actor (the concrete classes are actors)

Question 51

where would we **Use abstract methods**

Answer 51

We may create these within abstract classes or interfaces. in cases where it may be impossible to define the implementation of a method because it is solely or heavily dependent upon its subclasses.

Question 52

What happens when **a class is defined as abstract?**

Answer 52

No instances of that class can be created, and calling the new keyword on this class will result in a compile-time error.

Question 53

describe an **astract class**

Answer 53

These are classes that are not intended to use for creating instances of the class. Their purpose is to serve as a superclass for subclasses.

Question 54

This is used when we have objects that can be sorted in some way and has one method called compareTo() that returns an int value indicating whether one object belongs before or after or at the same level as another object.

Answer 54

What is the **Comparable interface** in Java?

Question 55

What is inherited when we implement **interfaces**

Answer 55

When we implement these, we usually only inherit the type and no implementations

Question 56

"Does this type need concrete elements such as instance fields, constructors, and method bodies?" If yes, then an abstract class would be appropriate because interfaces cannot have instance fields or constructors.

Answer 56

What is a question to ask when deciding whether to use an interface or an abstract class?

Question 57

what is **Computational modelling**

Answer 57

This is the act of modelling or simulating complex or real world systems within a simulation

Question 58

all methods within this: 1. abstract 2. concrete 3. static will have public visibility, therefore the public keyword may be omitted

Answer 58

what visibility do all methods have within an **interface**

Question 59

How is the sort() method used in the Collections class?

Answer 59

To use the sort() method in the Collections class, you simply call the method on the collection you want to sort, like this: `Collections.sort(aCollection);` Note: The sort() method in the Collections class can be used to sort any collection that implements the Comparable interface.

Question 60

Can concrete classes have abstract methods?

Answer 60

No, allowing an abstract method within a concrete class would mean that we would not be able to create an instance of that class since it lacks an implementation of the method.

Question 61

describe **maintainability**

Answer 61

refers to how easy the code is to keep code functional and adapt to changes.

Question 62

What happens when **a subclass does not implement all inherited abstract methods?**

Answer 62

The subclass itself becomes an abstract class (abstract subclass), and we will not be able to create an instance of that class since it does not have an implementation for inherited abstract methods.

Question 63

can **interfaces** contain instance fields

Answer 63

No, these do not have instance fields

Question 64

What is an **interface?**

Answer 64

This is a further abstraction technique in Java that often contains only abstract method definitions and no implementations.

Question 65

refers to how easy it is to add new features to the code

Answer 65

describe **extensibility**

Question 66

What is **multiple inheritance?**

Answer 66

This is when a class inherits directly from two or more superclasses.

Question 67

within an interface does the abstract keyword have to be given with the abstract method

Answer 67

no, within these Abstract methods do not have to contain the keyword abstract

Question 68

What is a specification in Java?

Answer 68

This is a class that typically has no implementation but states what any implementations of the specification must do. In Java, an interface does just this by defining abstract methods.

Question 69

What is the Collections class in Java?

Answer 69

This class in Java is a utility class that provides many methods for manipulating the contents of collections, such as lists and sets.

Question 70

These are classes that are not intended to use for creating instances of the class. Their purpose is to serve as a superclass for subclasses.

Answer 70

describe an **astract class**

Question 71

Why can't fields in a superclass be overridden?

Answer 71

Fields in a superclass cannot be overridden because they are not methods. As a result, we cannot set or modify the subclass constant in the superclass.

Question 72

Inheritance: 1. **less code duplication** - due to the inheritance of fields and methods 2. **polymorphic variables and method calls** - where the dynamic type of a variable at runtime is what determines the behavior Although interfaces do not help gain the first benefit, they can be used to gain the benefit of polymorphic variables by using them as a variable type (static type).

Answer 72

Three benefits of Inheritance and Interfaces

Question 73

What are three motivations for creating an interface?

Answer 73

1. **To implement multiple inheritance** 2. **To use the interface as a supertype and the highest abstraction with no implementation** - We want to treat objects that are instances of a number of different classes in a uniform way 3. **To implement a specification** - We want to allow each of those concrete classes to decide how they should implement the specification. We only define an action that concrete classes should have not the actual behaviour

Question 74

in this case the class must override the method, as it must be clear which method should be called the implemnting class can: 1.Override the default method and then call the preferred implementation 2.Override the default method and create its own implementation

Answer 74

what action must be taken if a class inherits from two different interfaces a default method with a matching signature

Question 75

In an inheritance hierarchy, a class inherits from all the classes of its superclass hierarchy, while in multiple inheritance, a class inherits directly from multiple superclasses.

Answer 75

How does multiple inheritance differ from inheritance hierarchy?

Question 76

The general benefits of this are: 1. **code inheritance** -where we may inherit common code such as fields and methods 2. **subtyping** - where we may inherit the type of the superclass which allows for polymorphism and specialization of a type.

Answer 76

What are the **general benefits of inheritance?**

Question 77

describe a **concrete class**

Answer 77

These are classes that must have implementations of all their methods, whereas an abstract class may omit the implementation.

Question 78

what action must be taken if a class inherits from two different interfaces a default method with a matching signature

Answer 78

in this case the class must override the method, as it must be clear which method should be called the implemnting class can: 1.Override the default method and then call the preferred implementation 2.Override the default method and create its own implementation

Question 79

[Chapter 12 glossary quiz](https://learn2.open.ac.uk/mod/quiz/view.php?id=2014943)

Answer 79

ignore

Question 80

To use the sort() method in the Collections class, you simply call the method on the collection you want to sort, like this: `Collections.sort(aCollection);` Note: The sort() method in the Collections class can be used to sort any collection that implements the Comparable interface.

Answer 80

How is the sort() method used in the Collections class?

Question 81

When there is a common characteristic between classes but implementing through inheritance such as an abstract class makes no sense since the classes may only be related through this single characteristic and nothing else. For example: The class is weighable (the concrete classes are weighable) The class is drawable (the concrete classes are drawable) The class is an actor (the concrete classes are actors)

Answer 81

When might we define an interface instead of using an abstract class?

Question 82

These are methods that have a header but no body. That is, they have an interface but no implementation.

Answer 82

describe an **abstract method**

Question 83

What are the differences between **an interface, an abstract class, and a concrete class in Java?**

Answer 83

1. An interface would most likely only contain definitions of methods and contains no implementation for its subclasses. 2. An abstract class would contain partial implementation for its subclasses, such as fields and methods that can be inherited. Inherited abstract methods might also be implemented here. 3. A concrete class will be a full implementation and will have implementations for any abstract methods that have been inherited.

Question 84

no, these do not contain constructors

Answer 84

can **interfaces** contain constructors

Question 85

This is the act of modelling or simulating complex or real world systems within a simulation

Answer 85

what is **Computational modelling**

Question 86

what visibility do all methods have within an **interface**

Answer 86

all methods within this: 1. abstract 2. concrete 3. static will have public visibility, therefore the public keyword may be omitted

Question 87

These are classes that must have implementations of all their methods, whereas an abstract class may omit the implementation.

Answer 87

describe a **concrete class**

Question 88

no, within these Abstract methods do not have to contain the keyword abstract

Answer 88

within an interface does the abstract keyword have to be given with the abstract method

Question 89

what is the statement to **import the Collections class**

Answer 89

to accomplish this: `Import java.util.Collections`

Question 90

refers to improving the structure of the code while keeping the functionality the same

Answer 90

describe **Re-factoring**

Question 91

which type of field is allowed within an interface

Answer 91

this may contain fields that are: **class constants with public visibility** therefore the `public static final` keywords are assumed automatically and may be omitted

Question 92

[Chapter 12 quiz](https://learn2.open.ac.uk/mod/quiz/view.php?id=2014945)

Answer 92

ignore

Question 93

these were added in Java 8 to allow interfaces to have implementation for methods. When a method is defined using this keyword in an interface, it will have an implementation and that implementation will be inherited by all classes that implement or inherit from the interface. It's important to note that the functionality of this method type is rather limited, as the interface has no constructor or instance fields and so there is no state that can be examined or manipulated by the default method. In general, interfaces will be limited to using only abstract methods, but these can be useful in certain cases.

Answer 93

What are **default methods** in Java interfaces?

Question 94

this may contain fields that are: **class constants with public visibility** therefore the `public static final` keywords are assumed automatically and may be omitted

Answer 94

which type of field is allowed within an interface

Question 95

write a **declaration of an abstract method**

Answer 95

To declare this, it is prefixed with the keyword "abstract" and has no body. Instead, it is terminated with a semicolon. For example: `abstract public void methodName(params);`

Question 96

When we extend these types of classes, we inherit the type and a partial implementation.

Answer 96

What is inherited when we extend **abstract classes?**

Question 97

How does multiple inheritance differ from inheritance hierarchy?

Answer 97

In an inheritance hierarchy, a class inherits from all the classes of its superclass hierarchy, while in multiple inheritance, a class inherits directly from multiple superclasses.

Question 98

How do you declare and implement an interface in Java?

Answer 98

To declare an interface, you use the header ``` public interface InterfaceName { } ``` To implement an interface, you use the "implements" clause in the class header, such as ``` public class Fox extends Animal implements Drawable { } ```

Question 99

how do we **call a default method of an interface**

Answer 99

this can be accomplished with: `interfaceName.super.methodName(…)`