Question 1

Consider the following code snippet: public abstract class Employee
{
Public abstract void setSalary();
) . .
}
You wish to create a concrete 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 2

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

Accepted Answer

If a class has an abstract method, it is not mandatory to implement it in the abstract class. However, since the class is not fully implemented, you cannot create an instance of it.

Question 3

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

Accepted Answer

An abstract class cannot be instantiated directly. It is meant to be subclassed, and its abstract methods must be implemented in the derived classes.

Question 4

Consider the Counter class below. public class Counter
{
Public int count = 0;
Public int getCount()
{
Return count;
}
Public void increment()
{
Count++;
}
}
Using the class above and the variables declared below, what is the value of num1.equals(num2)?
Counter num1 = new Counter();
Counter num2 = new Counter();

Accepted Answer

In Java, the default implementation of equals() method in Object class compares memory addresses. Since num1 and num2 are two different instances, num1.equals(num2) will return false unless the equals method is overridden in the Counter class.

Question 5

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 actual object type determines which method is executed. In this case, since `aVehicle` is an instance of `Auto`, the `moveForward` method of the `Auto` class will be executed.

Question 6

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 actual object type determines which method is executed. Since anEmployee is an instance of Programmer, the increaseSalary method of the Programmer class will be executed.

Question 7

Consider the classes shown below: public class Parent
{
Public int getValue()
{
Return 24;
}
Public void display()
{
System.out.print(getValue() + " ");
}
}
Public class Child extends Parent
{
Public int getValue()
{
Return -7;
}
}
Using the classes above, what is the output of the following lines of code?
Parent kid = new Child();
Parent adult = new Parent();
Kid)display();
Adult.display();

Accepted Answer

The method `display()` in `Parent` calls `getValue()`. For `kid`, which is a `Child` object, the overridden `getValue()` in `Child` is called, returning -7. For `adult`, the `Parent`'s `getValue()` is called, returning 24.

Question 8

Suppose the abstract class Message is defined below public abstract class Message {
Private String value;
Public Message(String initial)
{
Value = initial;
}
Public String getMessage()
{
Return value;
}
Public abstract String translate();
}
A concrete subclass of Message, FrenchMessage, is defined. Which methods must FrenchMessage define?

Accepted Answer

The answer of Suppose the abstract class Message is defined...

Question 9

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

Dynamic lookup refers to the process of determining which method implementation to execute at runtime, based on the actual object's class, not the reference type.

Question 10

Which of the following statements about classes is true?

Accepted Answer

The keyword final in a class declaration indicates that the class cannot be subclassed, but objects can still be created from it. This means statement A is true, but statements B, C, and D are false.

Question 11

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

Accepted Answer

An abstract class is a class that cannot be instantiated on its own and is meant to be subclassed.

Question 12

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

The increaseSalary method of the Programmer class will be executed because of method overriding in object-oriented programming. The actual method that gets called is determined by the object's runtime type, which is Programmer in this case.

Question 13

Consider the classes shown below: public class Parent
{
Public int getValue()
{
Return 24;
}
Public void display()
{
System.out.print(getValue() + " ");
}
}
Public class Child extends Parent
{
Public int getValue()
{
Return -7;
}
}
Using the classes above, what is the output of the following lines of code?
Child kid = new Child();
Parent adult = new Parent();
Kid)display();
Adult.display();

Accepted Answer

The `display` method in `Parent` calls `getValue()`. For `kid.display()`, `getValue()` from `Child` is called, returning -7. For `adult.display()`, `getValue()` from `Parent` is called, returning 24.

Question 14

Consider the Counter class below. public class Counter
{
Public int count = 0;
Public int getCount()
{
Return count;
}
Public void increment()
{
Count++;
}
}
Using the class above and the variables declared below, what is the value of num1.equals(num2)?
Counter num1 = new Counter();
Counter num2 = num1;

Accepted Answer

The variable num2 is assigned the reference of num1, so both num1 and num2 point to the same object. Therefore, num1.equals(num2) will return true because they reference the same object.

Question 15

Which of the following statements about classes is true?

Accepted Answer

You can create an object from a concrete class that has a defined implementation, but not from an abstract class that only declares methods without implementation. However, you can create an object reference whose type is an abstract class and assign it to an object of a concrete subclass that extends the abstract class.

Question 16

Consider the Counter class below. public class Counter
{
Public int count = 0;
Public int getCount()
{
Return count;
}
Public void increment()
{
Count++;
}
}
Using the class above and the variable declared below, what is the value of num.toString()?
Counter num = new Counter();

Accepted Answer

The default implementation of toString() in Java returns a string consisting of the class name, the "@" sign, and the unsigned hexadecimal representation of the hash code of the object. Since toString() is not overridden in the Counter class, this default behavior applies.

Question 17

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

The moveForward method of the Auto class will be executed because the object is instantiated as an Auto, and method overriding allows the subclass method to be called.

Question 18

Consider the following code snippet: public abstract class Machine
{
Public abstract void setRPMs();
) . .
}
You wish to create a concrete subclass named PolisherMachine. 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 the subclass must provide an implementation for the abstract method with the correct signature, which is "public void setRPMs()".

Question 19

Which of the following statements about abstract methods is true?

Accepted Answer

An abstract method has a name, parameters, and a return type, but no code in the body of the method. This allows the method to be defined in a subclass with different behavior, while still maintaining the same method signature.

Question 20

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

The code fragment D is not valid because a Vehicle object cannot be directly assigned to a LandVehicle reference, as Vehicle is a superclass of LandVehicle.

Quiz 9: Inheritance

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

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

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

Which of the following statements about classes is true?

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

Which of the following statements about classes is true?

Consider the Counter class below. public class Counter { Public int count = 0; Public int getCount() { Return count; } Public void increment() { Count++; } } Using the class above and the variable declared below, what is the value of num.toString() ? Counter num = new Counter() ;

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

Which of the following statements about abstract methods is true?