Question 1

Consider the following class hierarchy:
Public class Vehicle
{
Private String type;
Public Vehicle(String type)
{
This.type = type;
}
Public String getType()
{
Return type;
}
}
Public class LandVehicle extends Vehicle
{
Public LandVehicle(String type)
{
) . .
}
}
Public class Auto extends LandVehicle
{
Public Auto(String type)
{
) . .
}
}
Which of the following code fragments is NOT valid in Java?

Accepted Answer

In Java, a variable of a subclass type can be assigned an object of the subclass or any of its superclasses. However, a variable of a superclass type cannot be assigned an object of a subclass. This is because the superclass does not have all the methods and properties of the subclass.

Therefore, option D is not valid because a LandVehicle variable cannot be assigned a Vehicle object, which is a superclass of LandVehicle. The other options are valid because they involve creating an object of Auto, which is a subclass of LandVehicle and Vehicle, and assigning it to a variable of the same or a superclass type.

Question 2

Consider the following code snippet:
Public abstract class Employee
{
Public abstract void setSalary();
) . .
}
You wish to create a subclass named Programmer. Which of the following is the correct way to declare this subclass?

Accepted Answer

The correct way to declare a subclass of an abstract class is to use the "extends" keyword and provide an implementation for the abstract method with the correct access modifier.

Question 3

Consider the following code snippet:
Public class Motorcycle extends Vehicle
{
Private String model;
) . .
Public Motorcycle(int numberAxles, String modelName)
{
Model = modelName;
Super(numberAxles);
}
}
What does this code do?

Accepted Answer

The answer of Consider the following code snippet:
Public class Motorcycle...

Question 4

Consider the following code snippet:
Vehicle aVehicle = new Auto();
AVehicle.moveForward(200);
If the Auto class inherits from the Vehicle class, and both classes have an implementation of the moveForward method with the same set of parameters and the same return type, which statement is correct?

Accepted Answer

Since aVehicle is an instance of Auto, calling aVehicle.moveForward(200) will cause the Auto class's implementation of moveForward to be executed due to method overriding.

Question 5

Consider the following code snippet:
Employee anEmployee = new Programmer();
AnEmployee.increaseSalary(2500);
If the Programmer class inherits from the Employee class, and both classes have an implementation of the increaseSalary method with the same set of parameters and the same return type, which statement is correct?

Accepted Answer

Since anEmployee is an object of the Programmer class, when the increaseSalary method is called, the implementation in the Programmer class will be executed. This is because of the concept of polymorphism, where a subclass can override the behavior of a method in the superclass.

Question 6

Consider the following code snippet:
Public class Motorcycle extends Vehicle
{
Private String model;
) . .
Public Motorcycle(int numberAxles, String modelName)
{
Super(numberAxles);
Model = modelName;
}
}
What does this code do?

Accepted Answer

The code snippet is creating a Motorcycle class that extends the Vehicle class. In the constructor of the Motorcycle class, the keyword "super" is used to invoke the constructor of the parent Vehicle class, passing in the parameter "numberAxles". This means that the Motorcycle class inherits the number of axles from the Vehicle class constructor. Additionally, the constructor of the Motorcycle class also initializes its own model variable with the value of the "modelName" parameter.

Question 7

A class that cannot be instantiated is called a/an ____.

Accepted Answer

Abstract classes are defined with the intention of being inherited by other classes. They cannot be instantiated on their own, but rather serve as a base for other classes to build on.

Question 8

Consider the following code snippet:
Public abstract class Machine
{
Public abstract void setRPMs();
) . .
}
You wish to create a subclass named PolisherMachine. Which of the following is the correct way to declare this subclass?

Accepted Answer

Since Machine is an abstract class with an abstract method setRPMs(), the subclass PolisherMachine must implement this method. Therefore, the correct way to declare the subclass is by using the extends keyword to inherit the abstract class and providing an implementation for the setRPMs() method. Therefore, option D is correct. Option A and C also implement the Machine interface but do not inherit from the abstract class, which is not required or recommended in this case. Option B is incorrect since the keyword "void" should be lowercase in the method declaration.

Question 9

Consider the following code snippet:
Public class Motorcycle extends Vehicle
{
) . .
Public Motorcycle(int numberAxles)
{
Super(numberAxles);
}
}
What does this code do?

Accepted Answer

The code defines a Motorcycle class that extends the Vehicle class and has a constructor that takes an integer parameter. This constructor uses the "super" keyword to call the constructor of its superclass, passing it the integer parameter. Therefore, the constructor of the Vehicle class is invoked from within the constructor of the Motorcycle class.

Question 10

A class from which you cannot create objects is called a/an ____.

Accepted Answer

An abstract class cannot be instantiated, which means you cannot create objects from it. It serves as a blueprint/template for its derived classes that it is inherited.

Question 11

If a class has an abstract method, which of the following statements is NOT true?

Accepted Answer

An abstract class cannot be instantiated, so you cannot construct an object directly from it.

Question 12

Consider the following code snippet:
Employee anEmployee = new Programmer();
AnEmployee.increaseSalary(2500);
Assume that the Programmer class inherits from the Employee class, and both classes have an implementation of the increaseSalary method with the same set of parameters and the same return type. Which class's increaseSalary method is to be executed is determined by ____.

Accepted Answer

The method to be executed is determined by the actual object type, not the variable type or the hierarchy of classes. The variable type only determines what methods can be called on that object (based on the methods available in its declared type), but at runtime, the actual object type (in this case, Programmer) determines which implementation of the method will be executed.

Question 13

Consider the following code snippet:
Vehicle aVehicle = new Auto();
AVehicle.moveForward(200);
Assume that the Auto class inherits from the Vehicle class, and both classes have an implementation of the moveForward method with the same set of parameters and the same return type. Which class's moveForward method is to be executed is determined by ____.

Accepted Answer

The method that is executed is determined by the actual object type, not the variable's type or the hierarchy of the classes. Since aVehicle is a reference variable of type Vehicle, but it references an object of type Auto, the moveForward method of the Auto class will be executed. This is known as runtime polymorphism or dynamic method dispatch.

Question 14

Consider the following code snippet:
Public class Motorcycle extends Vehicle
{
) . .
Public Motorcycle(int numberAxles)
{
Super.numberAxles;
}
}
What does this code do?

Accepted Answer

The code defines a Motorcycle class that extends the Vehicle class and has a constructor that takes an integer parameter. This constructor uses the "super" keyword to call the constructor of its superclass, passing it the integer parameter. Therefore, the constructor of the Vehicle class is invoked from within the constructor of the Motorcycle class.

Question 15

Which of the following statements about abstract methods is true?

Accepted Answer

An abstract method is a method that is declared but not implemented in the current class. It has a name, parameters, and a return type specified in its signature, but it does not have any code in the body of the method. The purpose of an abstract method is to provide a common interface for all its concrete subclasses, which must implement the abstract method.

Question 16

Which of the following statements about classes is true?

Accepted Answer

A concrete class can be instantiated, while an abstract class cannot be instantiated. However, it can be subclassed to create a concrete class, which can be instantiated.

Question 17

Consider the following code snippet:
Vehicle aVehicle = new Auto();
AVehicle.moveForward(200);
Assume that the Auto class inherits from the Vehicle class, and both classes have an implementation of the moveForward method with the same set of parameters and the same return type. The process for determining which class's moveForward method to execute is called ____.

Accepted Answer

The process described is called dynamic lookup, where the correct implementation of a method is determined at runtime based on the actual object type rather than the reference type. This is also known as dynamic dispatch or late binding.

Question 18

Which of the following statements about classes is NOT true?

Accepted Answer

You cannot override methods in a class declared with the keyword final, so statement B is not true. The other statements are true: A) You cannot create an object from a class declared with the keyword final, C) You cannot extend a class declared with the keyword final, and D) You cannot create subclasses from a class declared with the keyword final.

Question 19

Consider the following code snippet:
Public class Auto extends Vehicle
{
) . .
Public Auto(int numberAxles)
{
Super(numberAxles);
}
}
What does this code do?

Accepted Answer

The code creates a subclass called Auto that extends the superclass Vehicle. It includes a constructor that takes an integer parameter and calls the constructor of the superclass with that parameter using the keyword "super". This invokes the constructor of the Vehicle class from within the constructor of the Auto class.

Question 20

Consider the following code snippet:
Public class Motorcycle extends Vehicle
{
) . .
Public Motorcycle(int numberAxles)
{
Super(numberAxles); //line #1
}
}
If the line marked "//line #1" was missing, which of these statements would be correct?

Accepted Answer

Without the line "//line #1," the compiler would automatically add a call to the default (no-argument) constructor of the superclass, which in this case is the Vehicle class. Therefore, the Motorcycle class constructor would invoke the constructor of the Vehicle class with no parameters.

Quiz 9: Inheritance and Interfaces

Consider the following code snippet: Public abstract class Employee { Public abstract void setSalary() ; ) . . } You wish to create a subclass named Programmer. Which of the following is the correct way to declare this subclass?

Consider the following code snippet: Public class Motorcycle extends Vehicle { Private String model; ) . . Public Motorcycle(int numberAxles, String modelName) { Model = modelName; Super(numberAxles) ; } } What does this code do?

Consider the following code snippet: Vehicle aVehicle = new Auto() ; AVehicle.moveForward(200) ; If the Auto class inherits from the Vehicle class, and both classes have an implementation of the moveForward method with the same set of parameters and the same return type, which statement is correct?

Consider the following code snippet: Public class Motorcycle extends Vehicle { Private String model; ) . . Public Motorcycle(int numberAxles, String modelName) { Super(numberAxles) ; Model = modelName; } } What does this code do?

A class that cannot be instantiated is called a/an ____.

Consider the following code snippet: Public abstract class Machine { Public abstract void setRPMs() ; ) . . } You wish to create a subclass named PolisherMachine. Which of the following is the correct way to declare this subclass?

Consider the following code snippet: Public class Motorcycle extends Vehicle { ) . . Public Motorcycle(int numberAxles) { Super(numberAxles) ; } } What does this code do?

A class from which you cannot create objects is called a/an ____.

If a class has an abstract method, which of the following statements is NOT true?

Consider the following code snippet: Public class Motorcycle extends Vehicle { ) . . Public Motorcycle(int numberAxles) { Super.numberAxles; } } What does this code do?

Which of the following statements about abstract methods is true?

Which of the following statements about classes is true?

Which of the following statements about classes is NOT true?

Consider the following code snippet: Public class Auto extends Vehicle { ) . . Public Auto(int numberAxles) { Super(numberAxles) ; } } What does this code do?

Consider the following code snippet: Public class Motorcycle extends Vehicle { ) . . Public Motorcycle(int numberAxles) { Super(numberAxles) ; //line #1 } } If the line marked "//line #1" was missing, which of these statements would be correct?