AP Computer Science Unit 9 Test Review Practice Questions

Last Updated on August 9, 2024

AP Computer Science Unit 9 Test Review Practice Questions. AP Computer Science Principles (CSP) Unit 9: Inheritance Review Practice Test 2024: College Board’s Advanced Placement® Program (AP®) Computer Science Principles (CSP) Unit 9.

In Unit 9, “Inheritance,” you focus on extending existing classes and creating hierarchies that allow you to reuse code efficiently. This unit covers the principles of inheritance, method overriding, polymorphism, and how to work with class hierarchies. Here’s an overview of the key topics and sample multiple-choice questions (MCQs).

AP Computer Science Unit 9 Test Review Practice Questions

1

AP Computer Science Unit 9 Test Review Practice Questions

AP Computer Science Unit 9
Test Review Practice Questions
On The Exam: 5%–7.5% of exam score

1 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Of the methods shown, how many different nonconstructor methods can be invoked by a SavingsAccount object?

(A) 1

(B) 2

(C) 3

(D) 4

(E) 5

2 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Which of the following correctly implements the default constructor of the SavingsAccount class?

I interestRate = 0; super();

II super(); interestRate = 0;

III super();

(A) II only

(B) I and II only

(C) II and III only

(D) III only

(E) I, II, and III

super(); calls the default constructor of BankAccount, which initializes balance.

interestRate = 0; initializes the interestRate variable.

The default constructor should initialize interestRate after calling super(); or handle it directly if super() is called.

3 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Which is a correct implementation of the constructor with parameters in the SavingsAccount class?

(A) balance = acctBalance; interestRate = rate;

(B) getBalance() = acctBalance; interestRate = rate;

(C) super(); interestRate = rate;

(D) super(acctBalance); interestRate = rate;

(E) super(acctBalance, rate);

The correct implementation for the constructor with parameters in SavingsAccount should call the BankAccount constructor with super(acctBalance) to initialize balance, and then set interestRate

4 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Which is a correct implementation of the CheckingAccount constructor?

I super(acctBalance);

II super(); deposit(acctBalance);

III deposit(acctBalance);

(A) II only

(B) II only

(C) III only

(D) II and III only

(E) I, II, and III

5 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Which is correct implementation code for the withdraw method in the CheckingAccount class?

(A) super.withdraw(amount); if (balance < MIN_BALANCE) super.withdraw(FEE);

(B) withdraw(amount); if (balance < MIN_BALANCE) withdraw(FEE);

(C) super.withdraw(amount); if (getBalance() < MIN_BALANCE) super.withdraw(FEE);

(D) withdraw(amount); if (getBalance() < MIN_BALANCE) withdraw(FEE);

(E) balance -= amount; if (balance < MIN_BALANCE) balance -= FEE;

6 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Redefining the withdraw method in the CheckingAccount class is an example of:

(A) method overloading.

(B) method overriding.

(C) downcasting.

(D) dynamic binding (late binding).

(E) static binding (early binding).

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

7 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Use the following for Questions 7–9.

A program to test the BankAccount, SavingsAccount, and CheckingAccount classes has these declarations:

BankAccount b = new BankAccount(1400);
BankAccount s = new SavingsAccount(1000, 0.04);
BankAccount c = new CheckingAccount(500);

Which method call will cause an error?

(A) b.deposit(200);

(B) s.withdraw(500);

(C) c.withdraw(500);

(D) s.deposit(10000);

(E) s.addInterest();

8 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Use the following for Questions 7–9.

A program to test the BankAccount, SavingsAccount, and CheckingAccount classes has these declarations:

BankAccount b = new BankAccount(1400);
BankAccount s = new SavingsAccount(1000, 0.04);
BankAccount c = new CheckingAccount(500);

In order to test polymorphism, which method must be used in the program?

(A) Either a SavingsAccount constructor or a CheckingAccount constructor

(B) addInterest

(C) deposit

(D) withdraw

(E) getBalance

Explanation:

To test polymorphism, a method defined in the superclass and overridden in the subclass should be used. withdraw is overridden in CheckingAccount.

9 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Use the following for Questions 7–9.

A program to test the BankAccount, SavingsAccount, and CheckingAccount classes has these declarations:

BankAccount b = new BankAccount(1400);
BankAccount s = new SavingsAccount(1000, 0.04);
BankAccount c = new CheckingAccount(500);

Which of the following will not cause a ClassCastException to be thrown?

(A) ((SavingsAccount) b).addInterest();

(B) ((CheckingAccount) b).withdraw(200);

(C) ((CheckingAccount) c).deposit(800);

(D) ((CheckingAccount) s).withdraw(150);

(E) ((SavingsAccount) c).addInterest();

Explanation:

Casting c (which is a CheckingAccount) to CheckingAccount is valid. Method deposit is inherited from BankAccount and not specific to CheckingAccount.

10 / 20

Questions 1–10 refer to the BankAccount, SavingsAccount, and CheckingAccount classes defined below:

public class BankAccount
{

private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double acctBalance)
{ balance = acctBalance; }
public void deposit(double amount)
{ balance += amount; }
public void withdraw(double amount)
{ balance -= amount; }
public double getBalance()
{ return balance; }

}

public class SavingsAccount extends BankAccount
{

private double interestRate;
public SavingsAccount()
{ /* implementation not shown */ }
public SavingsAccount(double acctBalance, double rate)
{ /* implementation not shown */ }
public void addInterest() //Add interest to balance
{ /* implementation not shown */ }

}

public class CheckingAccount extends BankAccount
{

private static final double FEE = 2.0;
private static final double MIN_BALANCE = 50.0;
public CheckingAccount(double acctBalance)
{ /* implementation not shown */ }
/** FEE of $2 deducted if withdrawal leaves balance less
* than MIN_BALANCE. Allows for negative balance. */
public void withdraw(double amount)
{ /* implementation not shown */ }

}

Use the following for Questions 7–9.

A program to test the BankAccount, SavingsAccount, and CheckingAccount classes has these declarations:

BankAccount b = new BankAccount(1400);
BankAccount s = new SavingsAccount(1000, 0.04);
BankAccount c = new CheckingAccount(500);

A new method is added to the BankAccount class.

/** Transfer amount from this BankAccount to another BankAccount.
* Precondition: balance > amount
* @param another a different BankAccount object
* @param amount the amount to be transferred
*/

public void transfer(BankAccount another, double amount)
{
withdraw(amount);
another.deposit(amount);
}

A program has these declarations:

BankAccount b = new BankAccount(650);
SavingsAccount timsSavings = new SavingsAccount(1500, 0.03);
CheckingAccount daynasChecking = new CheckingAccount(2000);

Which of the following will transfer money from one account to another without error?

I b.transfer(timsSavings, 50);

II timsSavings.transfer(daynasChecking, 30);

III daynasChecking.transfer(b, 55);

(A) I only

(B) II only

(C) III only

(D) I, II, and III

(E) None

Explanation:

All these transfers involve valid BankAccount instances and will work because the transfer method accepts any BankAccount as the target.

11 / 20

Consider these class declarations:

public class Person
{
...
}
public class Teacher extends Person
{
...
}

Which is a true statement?

I Teacher inherits the constructors of Person.

II Teacher can add new methods and private instance variables.

III Teacher can override existing private methods of Person.

(A) I only

(B) II only

(C) III only

(D) I and II only

(E) II and III only

Explanation:

I: Constructors are not inherited. Teacher must explicitly define its own constructors.

II: Teacher can add new methods and private instance variables.

III: Private methods in Person are not accessible in Teacher, so they cannot be overridden.

12 / 20

Which statement about abstract classes and interfaces is false?

(A) An interface cannot implement any non-default instance methods, whereas an abstract class can.

(B) A class can implement many interfaces but can have only one superclass.

(C) An unlimited number of unrelated classes can implement the same interface.

(D) It is not possible to construct either an abstract class object or an interface object.

(E) All of the methods in both an abstract class and an interface are public.

Explanation:

E: Methods in an abstract class can have any access modifier (public, protected, or private), whereas methods in an interface are implicitly public. Not all methods in an abstract class are required to be public.

13 / 20

Consider the following hierarchy of classes:

A program is written to print data about various birds:

public class BirdStuff
{

public static void printName(Bird b)
{ /* implementation not shown */ }
public static void printBirdCall(Parrot p)
{ /* implementation not shown */ }
//several more Bird methods
public static void main(String[] args)
{

Bird bird1 = new Bird();
Bird bird2 = new Parrot();
Parrot parrot1 = new Parrot();
Parrot parrot2 = new Parakeet();
/* more code */

}

}

Assuming that none of the given classes is abstract and all have default constructors, which of the following segments of /* more code */ will not cause an error?

(A) printName(parrot2); printBirdCall((Parrot) bird2);

(B) printName((Parrot) bird1); printBirdCall(bird2);

(C) printName(bird2); printBirdCall(bird2);

(D) printName((Parakeet) parrot1); printBirdCall(parrot2);

(E) printName((Owl) parrot2); printBirdCall((Parakeet) parrot2);

explanation:

A: printName(parrot2) is valid because parrot2 is a Parakeet, which is a Parrot and hence a Bird. Casting bird2 to Parrot is also valid because bird2 is actually a Parrot.

B: Casting bird1 to Parrot will cause an error because bird1 is a Bird and not a Parrot.

C: printBirdCall(bird2) will cause an error because bird2 is of type Bird, not Parrot.

D: Casting parrot1 to Parakeet will cause an error because parrot1 is a Parrot, not a Parakeet.

E: Casting parrot2 to Owl will cause an error because parrot2 is a Parakeet, not an Owl.

14 / 20

Refer to the classes below for Questions 14 and 15.

public class ClassA
{
//default constructor not shown ...
public void method1()
{ /* implementation of method1 */ }
}
public class ClassB extends ClassA
{
//default constructor not shown ...
public void method1()
{ /* different implementation from method1 in ClassA*/ }
public void method2()
{ /* implementation of method2 */ }
}

The method1 method in ClassB is an example of:

(A) method overloading.

(B) method overriding.

(C) polymorphism.

(D) information hiding.

(E) procedural abstraction.

15 / 20

Refer to the classes below for Questions 14 and 15.

public class ClassA
{
//default constructor not shown ...
public void method1()
{ /* implementation of method1 */ }
}
public class ClassB extends ClassA
{
//default constructor not shown ...
public void method1()
{ /* different implementation from method1 in ClassA*/ }
public void method2()
{ /* implementation of method2 */ }
}

Consider the following declarations in a client class:

ClassA ob1 = new ClassA();
ClassA ob2 = new ClassB();

Which of the following method calls will cause an error?

I ob1.method2();

II ob2.method2();

III ((ClassB) ob1).method2();

(A) I only

(B) II only

(C) III only

(D) I and III only

(E) I, II, and III

16 / 20

Use the declarations below for Questions 16–18.

public abstract class Solid
{

private String name;
//constructor
public Solid(String solidName)
{ name = solidName; }
public String getName()
{ return name; }
public abstract double volume();

}
public class Sphere extends Solid
{

private double radius;
//constructor
public Sphere(String sphereName, double sphereRadius)
{
super(sphereName);
radius = sphereRadius;
}
public double volume()
{ return (4.0/3.0) * Math.PI * radius * radius * radius; }

}
public class RectangularPrism extends Solid
{

private double length;
private double width;
private double height;
//constructor
public RectangularPrism(String prismName, double l, double w, double h)

{
super(prismName);
length = l;
width = w;
height = h;
}

public double volume()
{ return length * width * height; }

}

A program that tests these classes has the following declarations and assignments:

Solid s1, s2, s3, s4;
s1 = new Solid("blob");
s2 = new Sphere("sphere", 3.8);
s3 = new RectangularPrism("box", 2, 4, 6.5);
s4 = null;

How many of the above lines of code are incorrect?

(A) 0

(B) 1

(C) 2

(D) 3

(E) 4

17 / 20

Use the declarations below for Questions 16–18.

public abstract class Solid
{

private String name;
//constructor
public Solid(String solidName)
{ name = solidName; }
public String getName()
{ return name; }
public abstract double volume();

}
public class Sphere extends Solid
{

private double radius;
//constructor
public Sphere(String sphereName, double sphereRadius)
{
super(sphereName);
radius = sphereRadius;
}
public double volume()
{ return (4.0/3.0) * Math.PI * radius * radius * radius; }

}
public class RectangularPrism extends Solid
{

private double length;
private double width;
private double height;
//constructor
public RectangularPrism(String prismName, double l, double w, double h)

{
super(prismName);
length = l;
width = w;
height = h;
}

public double volume()
{ return length * width * height; }

}

Which is false?

(A) If a program has several objects declared as type Solid, the decision about which volume method to call will be resolved at run time.

(B) If the Solid class were modified to provide a default implementation for the volume method, it would no longer need to be an abstract class.

(C) If the Sphere and RectangularPrism classes failed to provide an implementation for the volume method, they would need to be declared as abstract classes.

(D) The fact that there is no reasonable default implementation for the volume method in the Solid class suggests that it should be an abstract method.

(E) Since Solid is abstract and its subclasses are nonabstract, polymorphism no longer applies when these classes are used in a program.

Explanation:

E: Polymorphism applies even when the superclass is abstract. The subclasses can still be used polymorphically.

18 / 20

Use the declarations below for Questions 16–18.

public abstract class Solid
{

private String name;
//constructor
public Solid(String solidName)
{ name = solidName; }
public String getName()
{ return name; }
public abstract double volume();

}
public class Sphere extends Solid
{

private double radius;
//constructor
public Sphere(String sphereName, double sphereRadius)
{
super(sphereName);
radius = sphereRadius;
}
public double volume()
{ return (4.0/3.0) * Math.PI * radius * radius * radius; }

}
public class RectangularPrism extends Solid
{

private double length;
private double width;
private double height;
//constructor
public RectangularPrism(String prismName, double l, double w, double h)

{
super(prismName);
length = l;
width = w;
height = h;
}

public double volume()
{ return length * width * height; }

}

Here is a program that prints the volume of a solid:

public class SolidMain
{
/** Output volume of Solid s. */
public static void printVolume(Solid s)
{
System.out.println("Volume = " + s.volume() + " cubic units");
}
public static void main(String[] args)
{
Solid sol;
Solid sph = new Sphere("sphere", 4);
Solid rec = new RectangularPrism("box", 3, 6, 9);
int flipCoin = (int) (Math.random() * 2); //0 or 1
if (flipCoin == 0)
sol = sph;
else
sol = rec;
printVolume(sol);
}
}

Which is a true statement about this program?

(A) It will output the volume of the sphere or box, as intended.

(B) It will output the volume of the default Solid s, which is neither a sphere nor a box.

(C) A ClassCastException will be thrown.

(D) A compile-time error will occur because there is no implementation code for volume in the Solid class.

(E) A run-time error will occur because of parameter type mismatch in the method call printVolume(sol).

Explanation:

The program correctly instantiates Sphere and RectangularPrism objects and assigns them to Solid references. The printVolume method will call the appropriate volume method based on the actual object type (either Sphere or RectangularPrism).

19 / 20

Consider the Computable interface below for performing simple calculator operations:

public interface Computable
{

/** Return this Object + y. */
Object add(Object y);
/** Return this Object - y. */
Object subtract(Object y);
/** Return this Object * y. */
Object multiply(Object y);

}

Which of the following is the least suitable class for implementing Computable?

(A) LargeInteger //integers with 100 digits or more

(B) Fraction //implemented with numerator and denominator of type int

(C) IrrationalNumber //nonrepeating, nonterminating decimal

(D) Length //implemented with different units, such as inches, centimeters, etc.

(E) BankAccount //implemented with balance

Explanation:

E: BankAccount is less suitable for implementing Computable because operations like addition, subtraction, and multiplication of bank account balances are not straightforward and might involve considerations beyond simple arithmetic, such as interest rates or transaction fees.

20 / 20

Refer to the Player interface shown below for Questions 20

public interface Player
{

/** Return an integer that represents a move in a game. */
int getMove();
/** Display the status of the game for this Player after
* implementing the next move. */
void updateDisplay();

}

HumanPlayer is a class that implements the Player interface. Another class, SmartPlayer, is a subclass of HumanPlayer. Which statement is false?

(A) SmartPlayer automatically implements the Player interface.

(B) HumanPlayer must contain implementations of both the updateDisplay and getMove methods, or be declared as abstract.

(C) It is not possible to declare a reference of type Player.

(D) The SmartPlayer class can override the methods updateDisplay and getMove of the HumanPlayer class.

(E) A method in a client program can have Player as a parameter type.

Explanation:

C: It is indeed possible to declare a reference of type Player. For example: Player p = new HumanPlayer();.

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Topics Overview

Using Common Attributes and Behaviors

Superclasses: General classes that provide common attributes and behaviors.
Subclasses: Specialized classes that inherit from superclasses and extend or modify their functionality.

Defining and Overriding Methods

Method Overriding: Redefining a method in a subclass that was already defined in its superclass.
Method Signature: The method name, return type, and parameters.

Creating References Using Inheritance Hierarchies

Superclass References: Variables that can reference objects of a subclass type.
Subclass Objects: Instances of subclasses that can be referenced using superclass variables.

Associating Subclass Objects with Superclasses to Create Polymorphism

Polymorphism: The ability for objects of different subclasses to be treated as objects of their superclass.
Dynamic Binding: The process by which the Java runtime determines the method to be invoked.