Chapter 10 - Improving structure with inheritance

Question 1

What is the
principle of inheritance
in Java?

Answer 1

this in Java is an abstraction technique that allows us to categorize things based on shared common elements and unique elements.

Question 2

from the following classes create an example
Inheritance hierarchy

Classes: animal, mammal, dog, cat, poodle, dalmation

Answer 2

example:
animal is the superclass, mammal is a subclass, and dog and cat are subclasses of mammal, and poodle and dalmatian are subclasses of dog.

Question 3

describe the 6 steps that take place when an object is initialised

Answer 3

1.Sufficient memory for the new object is allocated.
2.The object’s fields are given default ‘zero’ values: Reference type fields are assignednull, numerical type primitive fields are assigned0, floating point primitives are assigned0.0andbooleanfields are assignedfalse. Achartype field is also assigned a value of0that is equivalent to the null Unicode character.
3.The constructor is called. This might involve also calling another constructor in the same class as the first line of code in the constructor, usingthis().
4.A parent class constructor is called (except for classObject), either explicitly usingsuper()or implicitly.
5.Any initialisation of fields that takes place outside of the constructor (e.g. static constant initialisation) is performed.
6.The rest of the constructor code is executed.

Question 4

What is a
subtype
in Java?

Answer 4

In Java, this is a type that has a supertype. This may be substituted in places where its supertype has been declared or is expected

Question 5

this in Java is a variable that can reference different object types at runtime, including its declared type and any subtypes of the declared type.

Answer 5

What is a
polymorphic variable
in Java?

Question 6

How is
code duplication
a waste of time?

Answer 6

this is a waste of time because creating duplicate code for a new feature that has common code to an existing feature is inefficient.

Question 7

With inheritance in Java, what are subtypes?

Answer 7

these in Java are the subclasses of a superclass.

Question 8

What is a
class diagram
used for and what information will it illustrate?

Answer 8

this diagram is used to illustrate the structure of a system by showing the classes in the system

information included would be:
1. Class name
2. Class fields
3. Class methods

Question 9

this would be a wise decision if we have two constructors that have identical code.

in this case we can have one constructor that sets the duplicate code while the other can set its unique values and call the other constructor for any duplicate code

Answer 9

when would it be wise to
reuse constructor code

Question 10

What is
code duplication

Answer 10

this is where similar data or operations are stored in multiple parts of the source code.

Question 11

this is a constructor placed within the bytecode by java if the class is inheriting and the programmer has not explicitly called its superclass constructor

note:
the call is a zero argument call to the superclass constructor i.e. super()

Answer 11

describe the
The default constructor

Question 12

give 3 notes about calling
this() within a constructor

Answer 12

notes include:
1. must be the first line of code in a constructor
2. It will find and call a constructor with the matching signature in the same class
3. may not be used in the same constructor calling super()

Question 13

these include:
1. Avoids code duplication - identical source code is avoided
2. Code reuse - existing code can be reused. E.g. if a class already exists that contains similar functionality we may be able to create a subclass of this instead of rewriting the code again
3. Easier maintenance - maintenance is easier changes can be made in one place that may be shared by many classes
4. Extendibility - adding new funcionality can be much easier when inheritance has been implemented. E.g. we could add a new feature as a subclass of an already existing and similar superclass

Answer 13

what are 4 advantages that come from using
inheritance

Question 14

describe the
this() keyword

Answer 14

when this is used within a constructor it will search for a constructor in the same class with a matching signature

Question 15

this in Java is a structure where many classes are linked through an inheritance relationship, and subclasses are themselves superclasses for other subclasses.

Answer 15

What is an
inheritance hierarchy
in Java?

Question 16

What is a
**ClassCastException **
in Java?

Answer 16

this in Java occurs when the java runtime system checks whether or not a variable really holds an object of a certain type, and it turns out that the variable actually holds a different subtype other than the one specified in the cast operator.

This can occur if the type specified in the cast operator is incorrect or if the variable actually holds a different subtype.

Question 17

describe the
general rule for variables VS the correct rule

Answer 17

1. The general rule for variables in Java is that their type must match the type of the object they are initialized with.
2. The correct rule for variables in java is that a variable declared with a supertype may be initialised with any subtype of the declared supertype variable

Question 18

What does the
‘extends’ keyword
in Java do?

Answer 18

this keyword in Java is used to specify that a class is inheriting from a superclass.

Question 19

3 methods include:
1. toString() - which returns a string representation of the object
2. equals() - which compares the object to another object for equality
3. hashCode() - which returns a hash code value for the object.

Answer 19

name 3 methods provided by the Object class in Java?

Question 20

Can Java automatically call a superclass constructor for us if we don’t use the ‘super’ keyword?

Answer 20

Yes, Java will automatically call a superclass constructor for us if we don’t use the ‘super’ keyword.

However, this will result in a compile-time error if there is no zero-argument superclass constructor.

Question 21

What is
inheritance
in Java?

Answer 21

this is a feature in Java that allows us to define a class that extends another class, known as the superclass.

Question 22

1. The general rule for variables in Java is that their type must match the type of the object they are initialized with.
2. The correct rule for variables in java is that a variable declared with a supertype may be initialised with any subtype of the declared supertype variable

Answer 22

describe the
general rule for variables VS the correct rule

Question 23

When might casting be necessary in Java?

Answer 23

this is sometimes necessary when the compiler experiences what is known as “type loss”

Question 24

What is a
superclass
in Java?

Answer 24

this is a class that is extended by another class and can contain common code that is inherited by its subclasses.

Question 25

How is
**code duplication **
inflexible?

Answer 25

this is inflexible because changes must be made to every piece of duplicate code if a change is made.

Question 26

In Java, this refers to the ability to pass a subtype object where a supertype is expected, provided that the subtype given is a subtype of the declared supertype.

Answer 26

What does the concept of
**substitution **
refer to in Java?

Question 27

this is where similar data or operations are stored in multiple parts of the source code.

Answer 27

What is
code duplication

Question 28

What is the general rule of
substitution
in Java?

Answer 28

The general rule of this in Java is that you cannot assign a supertype to a subtype.

Question 29

It may be necessary to perform this if a new subclass is added but it differs from its sibling subclasses in that it does not require or should not have some of the fields that it will inherit from its superclass.

Answer 29

When might it be necessary to
**refactor an inheritance hierarchy **
in Java?

Question 30

Can public methods of a superclass be accessed by a subclass in Java?

Answer 30

Yes, any public methods of a superclass can be accessed from a subclass using the simple name, as if it were in the subclass itself.

We don’t need to use dot notation and specify the fully qualified name.

Question 31

The disadvantages of this include:

1. inflexibility
2. error-proneness
3. waste of time

Answer 31

What are the disadvantages of
code duplication?

Question 32

these in Java are the subclasses of a superclass.

Answer 32

With inheritance in Java, what are subtypes?

Question 33

notes include:
1. must be the first line of code in a constructor
2. It will find and call a constructor with the matching signature in the same class
3. may not be used in the same constructor calling super()

Answer 33

give 3 notes about calling
this() within a constructor

Question 34

In Java, when can substitution be used for formal and actual parameters?

Answer 34

In Java, substitution can be used for:

1. formal parameter - can be declared with a supertype
2. actual parameter - can accept the supertype as an argument or any subtype of the expected supertype.

Question 35

this is a class that is extended by another class and can contain common code that is inherited by its subclasses.

Answer 35

What is a
superclass
in Java?

Question 36

What are the members of a class in Java?

Answer 36

Members of a class in Java include fields and methods.

Question 37

What is a
subclass
in Java?

Answer 37

this is a class that extends another class (its superclass) and inherits all of its fields and methods.

Question 38

this is a class that extends another class (its superclass) and inherits all of its fields and methods.

Answer 38

What is a
subclass
in Java?

Question 39

describe the
The default constructor

Answer 39

this is a constructor placed within the bytecode by java if the class is inheriting and the programmer has not explicitly called its superclass constructor

note:
the call is a zero argument call to the superclass constructor i.e. super()

Question 40

What is an
inheritance hierarchy
in Java?

Answer 40

this in Java is a structure where many classes are linked through an inheritance relationship, and subclasses are themselves superclasses for other subclasses.

Question 41

this is a feature in Java that allows us to define a class that extends another class, known as the superclass.

Answer 41

What is
inheritance
in Java?

Question 42

How is
code duplication
error-prone?

Answer 42

this is error-prone because someone else making changes later may not realize that there is duplicate code that also must carry the same changes.

Question 43

during initialisation how do we pass values from a subclass to its superclass in Java?

Answer 43

To achieve this we use the keyword ‘super’ to call the superclass constructor.

Question 44

reasons include:
1. Redundant code in the class
2. There is a risk that in the future another developer may try and process the fields which could result in bad behavior
3. It is untidy and leaves ambiguity in the class

Answer 44

What are the reasons that make it bad practice to let the fields be inherited and not used in a class?

Question 45

this should be used as the first statement within a Java subclass constructor.

Answer 45

In what order should the
**super keyword **
be used in a Java subclass constructor?

Question 46

The general rule of this in Java is that you cannot assign a supertype to a subtype.

Answer 46

What is the general rule of
substitution
in Java?

Question 47

What does the
super
keyword do in Java?

Answer 47

In Java, this keyword is used to call a superclass constructor from a subclass constructor, to properly initialize the superclass.

Question 48

In Java, this is a type which has subtypes.

The subtypes may be substituted in place of the supertype where a supertype has been declared or is expected.

Answer 48

What is a
supertype
in Java?

Question 49

this in Java is the conversion of one type to another type using the cast operator.

Answer 49

What is
**casting **
in Java?

Question 50

using the following code create a scenario where the compiler would experience type loss

Vehicle v;
Car c = new Car();

Answer 50

example:

Vehicle v;
Car c = new Car();
v = c;  // correct
c = v;  // compile error

In the last line we get an error even though we know that v holds an object of the same type as the c variable so this should technically work

Question 51

How is the
**extends keyword **
used in Java?

Answer 51

This keyword is used in the header of a subclass to specify that it is extending a superclass

example:
“public class MessagePost extends Post”.

Question 52

this in Java is an abstraction technique that allows us to categorize things based on shared common elements and unique elements.

Answer 52

What is the
principle of inheritance
in Java?

Question 53

describe
**type loss **

Answer 53

this is a phenomenon experienced by the java compiler

it occurs when a legal assignment has been made but the compiler does not accept it because it sees the declared variable type being incompatible with the type of variable we are attempting to initialise it with.

even though the assignment could be legal

Question 54

To achieve this we use the keyword ‘super’ to call the superclass constructor.

Answer 54

during initialisation how do we pass values from a subclass to its superclass in Java?

Question 55

What are the reasons that make it bad practice to let the fields be inherited and not used in a class?

Answer 55

reasons include:
1. Redundant code in the class
2. There is a risk that in the future another developer may try and process the fields which could result in bad behavior
3. It is untidy and leaves ambiguity in the class

Question 56

the java compiler experiences this for two reasons:

1. java looks at and compiles each line in isolation
2. objects are not created untill runtime

Vehicle v;
Car c = new Car();
v = c; // correct
c = v; // compile error

from the code above the compiler sees that we are trying to initialise a subtype variable with a supertype variable. THIS IS AN ILLEGAL ASSIGNMENT
as far as the compiler is concerned

Answer 56

explain why the java compiler experiences type loss

Question 57

What is a
polymorphic variable
in Java?

Answer 57

this in Java is a variable that can reference different object types at runtime, including its declared type and any subtypes of the declared type.

Question 58

name 3 methods provided by the Object class in Java?

Answer 58

3 methods include:
1. toString() - which returns a string representation of the object
2. equals() - which compares the object to another object for equality
3. hashCode() - which returns a hash code value for the object.

Question 59

The benefit of using these in Java is that it allows for flexibility and code reusability, as variables can be declared with a supertype and then be initialized with that supertype or any of its subtypes.

Answer 59

What is the
benefit of using polymorphic variables
in Java?

Question 60

this is inflexible because changes must be made to every piece of duplicate code if a change is made.

Answer 60

How is
**code duplication **
inflexible?

Question 61

What are the disadvantages of
code duplication?

Answer 61

The disadvantages of this include:

1. inflexibility
2. error-proneness
3. waste of time

Question 62

In what order should the
**super keyword **
be used in a Java subclass constructor?

Answer 62

this should be used as the first statement within a Java subclass constructor.

Question 63

example:

Vehicle v;
Car c = new Car();
v = c;  // correct
c = v;  // compile error

In the last line we get an error even though we know that v holds an object of the same type as the c variable so this should technically work

Answer 63

using the following code create a scenario where the compiler would experience type loss

Vehicle v;
Car c = new Car();

Question 64

this is a waste of time because creating duplicate code for a new feature that has common code to an existing feature is inefficient.

Answer 64

How is
code duplication
a waste of time?

Question 65

What is
UML

Answer 65

this stands for Unified Modeling Language, and it is a visual modeling language used for software engineering.

Question 66

these can’t be called by name in Java because their main purpose is to initialize objects and not to be used like methods.

Answer 66

Why can’t
constructors
be called by name in Java?

Question 67

this stands for Unified Modeling Language, and it is a visual modeling language used for software engineering.

Answer 67

What is
UML

Question 68

What is the
Object class
in Java?

Answer 68

this is a special class in the Java standard library that serves as the superclass of all classes in Java, either directly or indirectly.

it is also the only class in a java program that does not itself have superclass

Question 69

In Java, this is a type that has a supertype. This may be substituted in places where its supertype has been declared or is expected

Answer 69

What is a
subtype
in Java?

Question 70

In Java, this keyword is used to call a superclass constructor from a subclass constructor, to properly initialize the superclass.

Answer 70

What does the
super
keyword do in Java?

Question 71

this is error-prone because someone else making changes later may not realize that there is duplicate code that also must carry the same changes.

Answer 71

How is
code duplication
error-prone?

Question 72

The two purposes include:
1. to allow the declaration of polymorphic variables of type Object, which can accept any object type (not often used but in special cases can be beneficial)
2. to provide common methods that will be available to every created object, such as toString, equals, and hashCode.

Answer 72

What are the two purposes of
having a common superclass of all classes
in Java?

Question 73

1.Sufficient memory for the new object is allocated.
2.The object’s fields are given default ‘zero’ values: Reference type fields are assignednull, numerical type primitive fields are assigned0, floating point primitives are assigned0.0andbooleanfields are assignedfalse. Achartype field is also assigned a value of0that is equivalent to the null Unicode character.
3.The constructor is called. This might involve also calling another constructor in the same class as the first line of code in the constructor, usingthis().
4.A parent class constructor is called (except for classObject), either explicitly usingsuper()or implicitly.
5.Any initialisation of fields that takes place outside of the constructor (e.g. static constant initialisation) is performed.
6.The rest of the constructor code is executed.

Answer 73

describe the 6 steps that take place when an object is initialised

Question 74

when this is used within a constructor it will search for a constructor in the same class with a matching signature

Answer 74

describe the
this() keyword

Question 75

this keyword in Java is used to specify that a class is inheriting from a superclass.

Answer 75

What does the
‘extends’ keyword
in Java do?

Question 76

Chapter 10 glossary quiz

Answer 76

ignore

Question 77

Do access modifiers of superclass members apply to subclasses in Java?

Answer 77

Yes, access modifiers of superclass members still apply to subclasses in Java. For example, private fields of a superclass cannot be accessed by a subclass.

Question 78

When might it be necessary to
**refactor an inheritance hierarchy **
in Java?

Answer 78

It may be necessary to perform this if a new subclass is added but it differs from its sibling subclasses in that it does not require or should not have some of the fields that it will inherit from its superclass.

Question 79

What is the
benefit of using inheritance in Java?

Answer 79

using this allows us to create a class that holds common code (the superclass) and then create subclasses that inherit all of its fields and methods while also implementing their own specialized fields and methods.

by doing this we can avoid the downfalls of code duplication

Question 80

What is
**casting **
in Java?

Answer 80

this in Java is the conversion of one type to another type using the cast operator.

Question 81

What is the
benefit of using polymorphic variables
in Java?

Answer 81

The benefit of using these in Java is that it allows for flexibility and code reusability, as variables can be declared with a supertype and then be initialized with that supertype or any of its subtypes.

Question 82

Can you provide an example of how polymorphic variables can be used in Java?

Answer 82

An example of how polymorphic variables can be used in Java is when two objects are given a superclass, and a loop can be used to loop over the superclass type.

By using polymorphic variables, both subtypes can be held as variables in the loop if the supertype is declared. This allows for more efficient and reusable code.

Question 83

give 2 reasons as to why it is good style to explicitly call the superclass constructor in a Java subclass?

Answer 83

It is good style to explicitly call the superclass constructor in a Java subclass because:

1. it makes it clear that there is always a call to the superclass
2. it ensures that the superclass is properly initialized.

Question 84

this is a phenomenon experienced by the java compiler

it occurs when a legal assignment has been made but the compiler does not accept it because it sees the declared variable type being incompatible with the type of variable we are attempting to initialise it with.

even though the assignment could be legal

Answer 84

describe
**type loss **

Question 85

Are constructors considered as members of a class in Java?

Answer 85

No, constructors are not considered as members of a class in Java.

Question 86

this in Java occurs when the java runtime system checks whether or not a variable really holds an object of a certain type, and it turns out that the variable actually holds a different subtype other than the one specified in the cast operator.

This can occur if the type specified in the cast operator is incorrect or if the variable actually holds a different subtype.

Answer 86

What is a
**ClassCastException **
in Java?

Question 87

Do private fields and methods get inherited by subclasses in Java?

Answer 87

No, private fields and methods are not inherited by subclasses in Java and hence will not be members of any subclasses.

Question 88

Why can’t
constructors
be called by name in Java?

Answer 88

these can’t be called by name in Java because their main purpose is to initialize objects and not to be used like methods.

Question 89

How can a subclass access private fields of its superclass in Java?

Answer 89

A subclass can’t directly access private fields of its superclass in Java.

If access is required, the superclass must create public methods to provide access to these fields.

Question 90

What is the advantage of separating the responsibilities of constructors from methods in Java?

Answer 90

Separating the responsibilities of constructors from methods in Java allows for predictable initialization of objects.

Question 91

In Java, substitution can be used for:

1. formal parameter - can be declared with a supertype
2. actual parameter - can accept the supertype as an argument or any subtype of the expected supertype.

Answer 91

In Java, when can substitution be used for formal and actual parameters?

Question 92

using this allows us to create a class that holds common code (the superclass) and then create subclasses that inherit all of its fields and methods while also implementing their own specialized fields and methods.

by doing this we can avoid the downfalls of code duplication

Answer 92

What is the
benefit of using inheritance in Java?

Question 93

this is a special class in the Java standard library that serves as the superclass of all classes in Java, either directly or indirectly.

it is also the only class in a java program that does not itself have superclass

Answer 93

What is the
Object class
in Java?

Question 94

this is achieved by making a call from one constructor to another constructor using the this() keyword

Answer 94

how do we actually
reuse constructor code

Question 95

What is a
supertype
in Java?

Answer 95

In Java, this is a type which has subtypes.

The subtypes may be substituted in place of the supertype where a supertype has been declared or is expected.

Question 96

explain why the java compiler experiences type loss

Answer 96

the java compiler experiences this for two reasons:

1. java looks at and compiles each line in isolation
2. objects are not created untill runtime

Vehicle v;
Car c = new Car();
v = c; // correct
c = v; // compile error

from the code above the compiler sees that we are trying to initialise a subtype variable with a supertype variable. THIS IS AN ILLEGAL ASSIGNMENT
as far as the compiler is concerned

Question 97

What does the concept of
**substitution **
refer to in Java?

Answer 97

In Java, this refers to the ability to pass a subtype object where a supertype is expected, provided that the subtype given is a subtype of the declared supertype.

Question 98

when would it be wise to
reuse constructor code

Answer 98

this would be a wise decision if we have two constructors that have identical code.

in this case we can have one constructor that sets the duplicate code while the other can set its unique values and call the other constructor for any duplicate code

Question 99

example:
animal is the superclass, mammal is a subclass, and dog and cat are subclasses of mammal, and poodle and dalmatian are subclasses of dog.

Answer 99

from the following classes create an example
Inheritance hierarchy

Classes: animal, mammal, dog, cat, poodle, dalmation

Question 100

This keyword is used in the header of a subclass to specify that it is extending a superclass

example:
“public class MessagePost extends Post”.

Answer 100

How is the
**extends keyword **
used in Java?

Question 101

What are the two purposes of
having a common superclass of all classes
in Java?

Answer 101

The two purposes include:
1. to allow the declaration of polymorphic variables of type Object, which can accept any object type (not often used but in special cases can be beneficial)
2. to provide common methods that will be available to every created object, such as toString, equals, and hashCode.

Question 102

what are 4 advantages that come from using
inheritance

Answer 102

these include:
1. Avoids code duplication - identical source code is avoided
2. Code reuse - existing code can be reused. E.g. if a class already exists that contains similar functionality we may be able to create a subclass of this instead of rewriting the code again
3. Easier maintenance - maintenance is easier changes can be made in one place that may be shared by many classes
4. Extendibility - adding new funcionality can be much easier when inheritance has been implemented. E.g. we could add a new feature as a subclass of an already existing and similar superclass

Question 103

how do we actually
reuse constructor code

Answer 103

this is achieved by making a call from one constructor to another constructor using the this() keyword

Question 104

this is sometimes necessary when the compiler experiences what is known as “type loss”

Answer 104

When might casting be necessary in Java?

Question 105

this diagram is used to illustrate the structure of a system by showing the classes in the system

information included would be:
1. Class name
2. Class fields
3. Class methods

Answer 105

What is a
class diagram
used for and what information will it illustrate?