Inheritance

Question 1

What is inheritance?

Answer 1

Inheritance is a concept in object-oriented programming where a class can inherit properties and behaviors from another class.

Question 2

What is an is-a relationship?

Answer 2

An is-a relationship is a type of relationship in which a subclass is a more specific version of its superclass.

Question 3

Give an example of an is-a relationship.

Answer 3

A mammal is an animal. This is an example of an is-a relationship because a mammal is a more specific version of the general class type animal.

Question 4

What is a superclass?

Answer 4

A superclass is a general class type that has properties and behaviors that can be inherited by its subclasses.

Question 5

What is a subclass?

Answer 5

A subclass is a more specific version of a superclass that inherits properties and behaviors from its superclass.

Question 6

Why do we use superclasses and subclasses in object-oriented programming?

Answer 6

We use superclasses and subclasses to factor out common behaviors and attributes into a superclass, which can be inherited by its subclasses. This reduces code redundancy and makes it easier to maintain and update the code.

Question 7

What does the extends keyword do?

Answer 7

The extends keyword is used in object-oriented programming to create inheritance relationships between classes. It indicates that a subclass is inheriting properties and behaviors from a superclass.

Question 8

What does the code “public class DVD extends Playable” mean?

Answer 8

This code is creating a class called DVD that is inheriting properties and behaviors from the Playable class. The DVD class is a subclass of the Playable class.

Question 9

What does the code “public class CD extends Playable” mean?

Answer 9

This code is creating a class called CD that is inheriting properties and behaviors from the Playable class. The CD class is a subclass of the Playable class.

Question 10

What are the benefits of inheritance?

Answer 10

The benefits of inheritance in object-oriented programming include:
All data and methods are located in one location, making the code easier to manage.
Maintenance is easier because changes made to the superclass are automatically inherited by its subclasses.
Avoidance of code redundancy and reimplementation of features.
Data and operations are available to all subclasses, making it easier to reuse code.
Exceptions in Java form an inheritance hierarchy, making it easier to handle errors in code.

Question 11

Why does inheritance make maintenance easier?

Answer 11

Inheritance makes maintenance easier because changes made to the superclass are automatically inherited by its subclasses. This means that instead of making changes to each subclass individually, you can make changes to the superclass, and those changes will propagate to all of its subclasses.

Question 12

How does inheritance avoid reimplementation of features?

Answer 12

Inheritance avoids reimplementation of features by allowing subclasses to inherit properties and behaviors from their superclass. This means that if a superclass already has a certain feature implemented, its subclasses do not need to reimplement that feature.

Question 13

How does the inheritance hierarchy of exceptions work in Java?

Answer 13

In Java, exceptions form an inheritance hierarchy, which means that each exception class is a subclass of the Throwable class. This allows for more specific handling of exceptions, with subclasses inheriting properties and behaviors from their parent exception classes.

Question 14

What is a superclass?

Answer 14

A superclass is a class that provides methods and attributes that are common to all of its subclasses.

Question 15

What is the role of a superclass in object-oriented programming?

Answer 15

The role of a superclass in object-oriented programming is to provide common behaviors and attributes that can be inherited by its subclasses. This reduces code redundancy and makes it easier to maintain and update the code.

Question 16

Can references store instances of subclasses?

Answer 16

Yes, references can store instances of subclasses. This is because a subclass is a type of its superclass, and can therefore be treated as an instance of that superclass.

Question 17

Can methods in the superclass implement default behaviors unless overridden by the subclass?

Answer 17

Yes, methods in the superclass can implement default behaviors unless overridden by the subclass. This means that subclasses can inherit the default behavior of a superclass method, or they can choose to override that behavior with their own implementation.

Question 18

What is a subclass?

Answer 18

A subclass is a class that inherits the methods and attributes of its superclass, and can also add its own additional methods and attributes.

Question 19

How does a subclass inherit methods and attributes from its superclass?

Answer 19

A subclass inherits methods and attributes from its superclass by extending the superclass. This means that the subclass has access to all of the methods and attributes of the superclass, and can use them as if they were its own.

Question 20

Can a subclass override methods in its superclass?

Answer 20

Yes, a subclass can override methods in its superclass. This is done by giving the method in the subclass the exact same declaration as the method in the superclass. This allows the subclass to provide its own implementation of the method, rather than using the implementation from the superclass.

Question 21

Can you call public (and protected) methods of a superclass from a subclass?

Answer 21

Yes, you can call public (and protected) methods of a superclass from a subclass. This is because subclasses have access to all of the public and protected methods and attributes of their superclass.

Question 22

What is the “protected” keyword in Java?

Answer 22

In Java, the “protected” keyword is an access modifier that allows a subclass to access a member (variable or method) of its superclass. Protected members can be accessed by any subclass that is on the extends path in the class hierarchy.

Question 23

How does “protected” differ from “private”?

Answer 23

The “private” keyword in Java limits access to a member to only the class in which it is defined, while the “protected” keyword allows any subclass to access the member as long as it is on the extends path in the class hierarchy.

Question 24

What is “super” in Java?

Answer 24

In Java, “super” is a keyword that refers to the superclass of a class. It can be used to call methods or access variables in the superclass from a subclass.

Question 25

How is “super” used to call a method in the superclass?

Answer 25

To call a method in the superclass from a subclass, you can use the “super” keyword followed by the name of the method you want to call. For example, “super.myMethod()” will run “myMethod” in the superclass.

Question 26

How is “super” used to call a constructor in the superclass?

Answer 26

To call a constructor in the superclass from a subclass, you can use the “super” keyword followed by the appropriate arguments for the constructor you want to call. For example, “super(…)” can be used to call the constructor of the superclass with the specified parameters. This is useful for reusing initialization code in the superclass when constructing a subclass.

Question 27

What is the problem that can occur when both a superclass and its subclass have a method with the exact same name, return type, and parameters?

Answer 27

The problem that can occur is a name conflict, where the compiler may not know which method to use in a given context.

Question 28

Why would a developer want to define a method with the same name, return type, and parameters in both a superclass and its subclass?

Answer 28

A developer might want to do this to provide default behavior for all objects of the superclass while also providing more specific behavior for objects of the subclass.

Question 29

What is the benefit of defining default behavior for objects of the superclass?

Answer 29

The benefit is that it reduces code duplication and makes the code more maintainable.

Question 30

What is the benefit of providing more specific behavior for objects of the subclass?

Answer 30

The benefit is that it allows for greater customization and flexibility in how objects of the subclass behave compared to objects of the superclass.

Question 31

When the “play()” method is called on an object that is an instance of a superclass with a matching method in a subclass, which method gets called?

Answer 31

The method that gets called depends on the actual object that the variable is referring to at runtime. If the object is an instance of the subclass, then the subclass method will be called. Otherwise, the superclass method will be called.

Question 32

What is method overriding?

Answer 32

Method overriding is when a subclass provides a specific implementation for a method that is already defined in its superclass.

Question 33

Why is method overriding useful?

Answer 33

Method overriding is useful because it allows for greater customization and flexibility in how objects of the subclass behave compared to objects of the superclass.

Question 34

When a method is overridden in a subclass, which version of the method gets called when the method is called on an object of that subclass?

Answer 34

When a method is overridden in a subclass, the version of the method in the subclass gets called when the method is called on an object of that subclass, independent of the type of the reference, because that is the “real” type of the object.

Question 35

What is reference casting?

Answer 35

Reference casting is the process of transforming a reference of one data type into another data type.

Question 36

Can you cast any object into any type of reference?

Answer 36

No, you can only cast an object into its own type or any of its superclasses. For example, if you have an object of type DVD, you can cast it into the DVD class, the Playable class, the Media class, or the Product class, but not into the Book class or any other unrelated types.

Question 37

What happens if you try to cast an object into an incompatible type?

Answer 37

If you try to cast an object into an incompatible type, a ClassCastException is thrown at runtime.

Question 38

How can reference casting be used in practice?

Answer 38

Reference casting can be used to access methods and attributes that are specific to a subclass, but not defined in the superclass. For example, if you have a reference of type Playable, you can cast it into a DVD reference to access the “getDirector()” method, which is not defined in the Playable class.

Question 39

What is Object in Java?

Answer 39

Object is a built-in class in Java that is the root of all class hierarchies. It is a reference type that can refer to any object in Java.

Question 40

What is the purpose of Object in Java?

Answer 40

The purpose of Object in Java is to provide a common interface for all objects in Java, regardless of their specific types. Because every class in Java is a subclass of Object, Object provides a universal set of methods and attributes that can be used on any object.

Question 41

What are some common methods and attributes of Object?

Answer 41

Some common methods and attributes of Object include toString(), hashCode(), equals(), getClass(), and wait().

Question 42

How is Object used in practice?

Answer 42

Object is used extensively in Java programming to write generic code that can work on any type of object. For example, you can create a method that takes an Object parameter and performs some action on it, without knowing the exact type of the object. This allows you to write more flexible and reusable code that can handle a wide range of objects.

Question 43

What is method overloading?

Answer 43

Method overloading is a feature in Java that allows us to define multiple methods with the same name but different parameters.

Question 44

What are the benefits of method overloading?

Answer 44

Method overloading can make code more flexible and easier to use, since it allows us to define methods that can accept different types of inputs without needing to come up with different method names for each case. This can make code more readable and easier to maintain.

Question 45

How does method overloading work?

Answer 45

When we call a method that has been overloaded, Java looks at the number and types of the arguments that are passed in and matches them to the closest matching method signature. If there is no exact match, Java tries to find the closest possible match by converting the arguments to match the method signature as closely as possible.

Question 46

What are some things to keep in mind when overloading methods?

Answer 46

When overloading methods, it’s important to make sure that the method signatures are distinct enough that Java can tell them apart. This usually means having different parameter types or numbers of parameters. It’s also important to make sure that the method behavior is consistent across all overloaded versions of the method, so that users can expect the same results regardless of which version they use.

Question 47

What is constructor overloading?

Answer 47

Constructor overloading is when we define multiple constructors for a class, each with different parameters.

Question 48

Why is constructor overloading useful?

Answer 48

Constructor overloading allows us to create objects of the same class with different initial values or configurations, depending on the parameters passed to the constructor.

Question 49

How is constructor overloading similar to using the “super” keyword?

Answer 49

Just like using “super”, constructor overloading allows us to reuse code from a superclass or from other constructors in the same class.

Question 50

How do you distinguish between different overloaded constructors?

Answer 50

The different constructors must have different parameter lists. This can be achieved by changing the number or types of parameters.

Question 51

Can constructors have different return types?

Answer 51

No, constructors do not have a return type, not even void.

Question 52

What is an abstract class?

Answer 52

An abstract class is a class that cannot be instantiated and is used to provide a common interface for its subclasses. It can contain both abstract and non-abstract methods.

Question 53

Why would you use an abstract class?

Answer 53

You would use an abstract class when you want to define a set of methods that subclasses must implement but don’t want to provide an implementation in the abstract class itself.

Question 54

What is an abstract method?

Answer 54

An abstract method is a method that is declared but not implemented in an abstract class. It has no body and ends with a semicolon instead of curly braces. Subclasses of the abstract class must provide an implementation of the abstract method.

Question 55

Can an abstract class be instantiated?

Answer 55

No, an abstract class cannot be instantiated because it is not complete. It must be extended by a subclass that provides an implementation for all its abstract methods.

Question 56

What is the difference between an abstract class and an interface?

Answer 56

An abstract class can have both abstract and non-abstract methods, while an interface can only have abstract methods. Also, a class can only extend one abstract class, but can implement multiple interfaces. Finally, an abstract class can have instance variables, while an interface cannot.

Question 57

What is an interface in Java?

Answer 57

An interface in Java is a way to enforce certain operations that a class must implement. It contains only abstract methods and does not have any data.

Question 58

How is an interface different from an abstract class?

Answer 58

An abstract class can contain both abstract and concrete methods, whereas an interface contains only abstract methods. An abstract class can have data members, but an interface cannot have any data members.

Question 59

How do you define an interface in Java?

Answer 59

To define an interface in Java, you use the interface keyword, followed by the name of the interface, and then a block of abstract method declarations.

Question 60

How do you implement an interface in Java?

Answer 60

To implement an interface in Java, you use the implements keyword, followed by the name of the interface, when you define a class. The class must then provide an implementation for all of the methods declared in the interface.

Question 61

Can a Java class implement multiple interfaces?

Answer 61

Yes, a Java class can implement multiple interfaces, but it can only inherit from one superclass.

Question 62

What is the purpose of an interface in Java?

Answer 62

The purpose of an interface in Java is to provide a way to enforce certain behavior without specifying the implementation. By defining an interface, you can create a contract that any class implementing that interface must follow. This can make your code more flexible and easier to maintain.

Question 63

Can an interface be instantiated?

Answer 63

No, an interface cannot be instantiated.

Question 64

Can an interface act as a reference type?

Answer 64

Yes, an interface can act as a reference type.

Question 65

Can an object be assigned to an interface variable?

Answer 65

Yes, an object can be assigned to an interface variable as long as the object implements the interface.

Question 66

What happens if an object is assigned to an interface variable that does not implement the interface?

Answer 66

If an object is assigned to an interface variable that does not implement the interface, a compile-time error will occur.

Question 67

Can an interface variable call methods that are defined in the interface?

Answer 67

Yes, an interface variable can call methods that are defined in the interface.

Question 67

Can an interface variable call methods that are defined in the interface?

Answer 67

Yes, an interface variable can call methods that are defined in the interface.

Question 68

Can an interface variable call methods that are implemented differently by different objects that implement the interface?

Answer 68

Yes, an interface variable can call methods that are implemented differently by different objects that implement the interface, as long as the methods are defined in the interface.

Question 69

Can an interface variable be used to access fields of an object that implements the interface?

Answer 69

No, an interface variable cannot be used to access fields of an object that implements the interface, since interfaces do not define fields.