Objects: Instances of Classes
Help Questions
AP Computer Science A › Objects: Instances of Classes
If Car is a class, which statement best explains the relationship between the Car class and the concept of inheritance in Java?
The Car class is a subclass of the Object class, inheriting its fundamental methods.
The Car class must be a superclass to at least one other class, such as ElectricCar.
The Car class cannot participate in inheritance unless it is declared as public static.
The Car class inherits its attributes from the objects that are created from it.
Explanation
Unless explicitly stated otherwise, every class in Java automatically extends the Object class. This means the Car class is a subclass in the universal Java class hierarchy and inherits methods like toString() and equals() from its ultimate superclass, Object.
In Java, what is the role of the Object class?
It is the class from which all primitive data types like int and double are derived.
It is a special class that cannot be instantiated and is used only for defining program structure.
It is the direct or indirect superclass of all other classes, providing a set of common behaviors.
It is a utility class that contains static methods for common mathematical and string operations.
Explanation
Every class in Java implicitly or explicitly inherits from the Object class. This means all objects, regardless of their class, are part of a single class hierarchy and inherit a common set of methods, such as toString() and equals().
A class is a formal implementation of attributes and behaviors. How do these concepts relate to an object created from that class?
The class defines the attributes and behaviors, and an object is a specific instance that possesses those characteristics.
The object is a list of behaviors, and the class is a collection of attributes.
The object's behaviors define its attributes, and its attributes define its class.
The class is a single attribute, and the object is a single behavior.
Explanation
A class serves as the blueprint, defining what attributes (e.g., color, size) and behaviors (e.g., move, calculate) an object will have. An object is a concrete realization of that blueprint, with its own specific values for the attributes.
Consider the following Java class and code; after executing it, what is the state of checking's balance?
// Bank account management example
class BankAccount {
private String owner;
private double balance;
public BankAccount(String owner, double startingBalance) {
this.owner = owner;
balance = startingBalance;
}
public void deposit(double amount) {
balance += amount;
}
public boolean withdraw(double amount) {
if (amount <= balance) {
balance -= amount;
return true;
}
return false;
}
public double getBalance() {
return balance;
}
}
class Main {
public static void main(String[] args) {
BankAccount checking = new BankAccount("Ava", 200.0);
BankAccount savings = new BankAccount("Ben", 500.0);
checking.deposit(50.0);
savings.withdraw(100.0);
checking.withdraw(120.0);
// (No printing here)
}
}
```
The balance is $200.0.
The balance is $250.0.
The balance is $130.0.
The balance is $80.0.
Explanation
This question tests AP Computer Science A skills: understanding objects as instances of classes and their interaction through methods. Objects in Java are instances of classes, which define fields and methods; methods allow objects to perform actions or modify state. In this scenario, the checking account starts with $200.0, then deposit(50.0) adds $50 to make it $250, and withdraw(120.0) subtracts $120, resulting in a final balance of $130. Choice A is correct because it reflects the updated balance after all method calls, showing accurate tracking of state changes through multiple operations. Choice C is incorrect because it only accounts for the deposit, ignoring the withdrawal. To help students: Trace through each method call step-by-step, updating the balance after each operation. Emphasize that each object maintains its own state independently. Watch for: students who only track some operations or confuse the states of different objects.
Consider the following Java class and code; after executing it, what is the state of acct2's balance?
class BankAccount {
private String owner;
private double balance;
public BankAccount(String owner, double startingBalance) {
this.owner = owner;
balance = startingBalance;
}
public void deposit(double amount) {
balance += amount;
}
public boolean withdraw(double amount) {
if (amount <= balance) {
balance -= amount;
return true;
}
return false;
}
public double getBalance() {
return balance;
}
}
class Main {
public static void main(String[] args) {
BankAccount acct1 = new BankAccount("Ava", 100.0);
BankAccount acct2 = acct1; // two references to the same object
acct2.deposit(40.0);
acct1.withdraw(10.0);
System.out.println(acct2.getBalance());
}
}
```
40.0
90.0
100.0
130.0
Explanation
This question tests AP Computer Science A skills: understanding objects as instances of classes and their interaction through methods. Objects in Java are instances of classes, which define fields and methods; methods allow objects to perform actions or modify state. In this scenario, acct2 = acct1 creates two references pointing to the same BankAccount object, not two separate objects. When acct2.deposit(40.0) is called, it modifies the shared object to have balance $140.0, and acct1.withdraw(10.0) further modifies it to $130.0. Choice A is correct because both references point to the same object, so all modifications affect the single shared balance. Choice B is incorrect because it assumes acct1 and acct2 are separate objects with independent balances. To help students: Use memory diagrams to visualize reference variables pointing to objects. Emphasize the difference between creating new objects versus creating new references. Watch for: the common misconception that assignment creates a copy of the object rather than copying the reference.
Which statement best describes a class in object-oriented programming?
A named storage location in memory that holds a single primitive value or an object reference.
A sequence of instructions that performs a specific task and can be called from other parts of a program.
A blueprint or template that defines the attributes and behaviors for a type of object.
A specific, concrete entity that exists in memory during program execution.
Explanation
A class serves as a blueprint for creating objects. It defines the common properties (attributes) and actions (behaviors) that all objects of that type will have. An object is a specific entity (A), a method is a sequence of instructions (C), and a variable is a named storage location (D).
In the context of Java, which of the following best defines an object?
A specific instance of a class, having its own state and access to the behaviors defined by its class.
A set of source code files that can be compiled into a runnable program.
A keyword in Java that is used to define the fundamental structure of a data type.
A formal description of a method's name, parameters, and return type.
Explanation
An object is created from a class and represents a tangible instance of that class. Each object has its own state (values for its instance variables) but shares the behaviors (methods) defined in the class. A set of source files is a project (A), the class keyword defines the structure (B), and a method's description is its signature (D).
Consider a class named Robot. Which statement accurately describes the relationship between the Robot class and objects created from it?
Only one Robot object can be created from the Robot class.
The Robot class is an object itself, and no other objects can be created from it.
Multiple Robot objects can be created, and each object is an independent instance with its own state.
All Robot objects created from the class share the same state and attributes.
Explanation
A class is a template from which multiple, distinct objects can be created. Each object is an instance of the class and maintains its own state (the values of its instance variables) independently of other objects.
Based on the Car class definition, if two Car objects, car1 and car2, are created, which statement is true?
The model and year are part of the Car class itself, not the individual objects.
Both car1 and car2 must have the same values for model and year.
car1 will have its own model and year attributes, and car2 will also have its own model and year attributes.
If the model of car1 is changed, the model of car2 will automatically change to the same value.
Explanation
model and year are instance variables. This means that every instance (object) of the Car class gets its own copy of these variables. The state of one object is independent of the state of another object of the same class.
Which of the following code segments correctly declares a variable that is capable of holding a reference to a Student object?
Student newStudent;
Student = newStudent;
Student newStudent = Student();
new Student newStudent;
Explanation
The correct syntax to declare a reference variable is ClassName variableName;. This creates a variable named newStudent of type Student that can hold a reference to a Student object. This statement does not create the object itself.