Chapter 6 Practice Questions

Question 1

What is an object in Java?
a. A data type used for calculations.
b. A blueprint for creating classes.
c. An entity in memory that can store data and perform operations.
d. A way to encapsulate primitive data types.

Answer 1

c
Explanation: Objects are instances of classes that can hold data (fields) and perform actions (methods).

Question 2

Which of the following is true about a class?
a. It is an instance of an object.
b. It defines fields and methods for objects.
c. It cannot be used to create multiple objects.
d. It is the same as a method.

Answer 2

b
Explanation: A class is a blueprint that defines what fields and methods objects of that class will have.

Question 3

What is an instance of a class?
a. A method that belongs to the class.
b. A copy of the class file.
c. A specific object created from a class.
d. A field declared inside a method.

Answer 3

c
Explanation: An instance is a concrete object created from the template provided by a class.

Question 4

TRUE OR FALSE: You can create only one object from a class.

Answer 4

False
Explanation: A program can create as many objects from a class as needed, each being a separate instance.

Question 5

Which of the following is an example of an object you’ve already used in Java?
a. Rectangle
b. int
c. Scanner
d. File

Answer 5

c
Explanation: Scanner is an object provided by the Java API for reading input.

Question 6

What is the purpose of access specifiers in Java?
a. To create methods in a class.
b. To define the visibility of class members.
c. To initialize objects.
d. To restrict object creation.

Answer 6

b
Explanation: Access specifiers control how fields and methods in a class can be accessed by other classes.

Question 7

What will the following code do?

java
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public class Example {
private int value;
public void setValue(int v) {
value = v;
}
public int getValue() {
return value;
}
}
a. Cause an error because value is private.
b. Successfully encapsulate the value field.
c. Fail because the methods are not private.
d. Compile but will not run.

Answer 7

b
Explanation: The code properly uses encapsulation by providing set and get methods for accessing the private value field.

Question 8

Which of these access specifiers restricts access to within the same class?
a. public
b. private
c. protected
d. default

Answer 8

b
Explanation: The private specifier limits access to the field or method within the same class only.

Question 9

TRUE OR FALSE: A private field can be accessed directly by code in another class.

Answer 9

Answer: False
Explanation: Private fields are accessible only within the class they are declared in. Other classes must use methods to access them.

Question 10

What does the following code do?

java
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public class Rectangle {
public int length;
public int width;
}
Rectangle rect = new Rectangle();
rect.length = 10;
rect.width = 5;
System.out.println(rect.length * rect.width);
a. Prints 50.
b. Throws a compilation error because fields are not private.
c. Prints 15.
d. Causes a runtime error.

Answer 10

a
Explanation: The length and width fields are public, so they can be directly accessed and manipulated. The program prints 10 * 5 = 50.

Question 11

TRUE OR FALSE: Access specifiers only apply to methods.

Answer 11

False
Explanation: Access specifiers can apply to fields, methods, and constructors.

Question 12

What happens when an object is created from a class?
a. The object is stored in memory and can be used to call methods or access fields.
b. A new class file is generated for the object.
c. Only fields are available, not methods.
d. The object replaces the class definition.

Answer 12

a
Explanation: Objects are stored in memory and can use the fields and methods defined by their class.

Question 13

What will the following code print?

java
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public class Circle {
private double radius;
public Circle(double r) {
radius = r;
}
public double getArea() {
return Math.PI * radius * radius;
}
}
Circle c = new Circle(5);
System.out.println(c.getArea());
a. Prints the area of a circle with radius 5.
b. Compilation error due to the private radius field.
c. Prints 0 because the radius was not initialized.
d. Runtime error because Math.PI is undefined.

Answer 13

a
Explanation: The radius is initialized via the constructor, and Math.PI is a constant. The code calculates and prints the area of the circle.

Question 14

TRUE OR FALSE: Every instance of a class shares the same fields and methods.

Answer 14

Answer: False
Explanation: Each object (instance) of a class has its own copy of the fields, but all objects share the same methods.

Question 15

What is the term for the process of bundling fields and methods into a single unit?
a. Polymorphism
b. Inheritance
c. Encapsulation
d. Abstraction

Answer 15

c
Explanation: Encapsulation bundles fields and methods together and restricts access to the data through access specifiers.

Question 16

Which of the following is true about data hiding in Java?
a. Fields in a class are public.
b. Fields in a class are private and accessed via methods.
c. Fields in a class can only be accessed by constructors.
d. Fields in a class are automatically protected.

Answer 16

b
Explanation: Data hiding ensures that fields in a class are private and accessed via accessors (getters) and mutators (setters).

Question 17

What is the purpose of an accessor method?
a. Modifies the value of a private field.
b. Retrieves the value of a private field.
c. Deletes the private field.
d. Hides the value of a field from other classes.

Answer 17

b
Explanation: Accessor methods (getters) retrieve the value of a private field.

Question 18

TRUE OR FALSE: A method with the same name but a different parameter list is an example of method overloading.

Answer 18

TRUE
Explanation: Method overloading occurs when multiple methods in the same class have the same name but different parameter lists.

Question 19

Consider the following code snippet. What will happen?

java
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Rectangle box;
System.out.println(box.getArea());
a. It will print the area of the rectangle.
b. It will throw a compilation error because the box variable is not initialized.
c. It will throw a runtime error because box is null.
d. It will compile but not run.

Answer 19

b
Explanation: The box variable is not initialized and does not reference any object, causing a compilation error.

Question 20

What is the default constructor in Java?
a. A constructor provided by the programmer.
b. A constructor with arguments.
c. A no-arg constructor provided by Java if no constructor is written.
d. A constructor that initializes all fields to custom values.

Answer 20

c
Explanation: Java provides a default constructor with no arguments if no other constructor is defined in the class.

Question 21

What will the following code do?

java
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String name = new String(“John Doe”);
a. Creates a reference variable name and points it to a String object with “John Doe”.
b. Throws an error because the constructor is incorrect.
c. Does nothing.
d. Creates a String object without initializing it.

Answer 21

a
Explanation: The code creates a new String object with the value “John Doe” and assigns it to the name reference variable.

Question 22

TRUE OR FALSE: Wildcard imports allow importing all classes in a package.

Answer 22

TRUE
Explanation: A wildcard import (e.g., import java.util.*) imports all classes in a package.

Question 23

What is shadowing in Java?
a. Declaring a variable with the same name in two classes.
b. Declaring a local variable with the same name as an instance field.
c. Overloading a method with the same name but different return types.
d. Using an object as a method parameter.

Answer 23

b
Explanation: Shadowing occurs when a local variable has the same name as an instance field, hiding the instance field within the method.

Question 24

What happens when you pass an object as an argument to a method?
a. The object’s memory address is passed to the parameter.
b. A copy of the object is passed to the parameter.
c. The object cannot be passed as an argument.
d. A runtime error occurs.

Answer 24

a
Explanation: Java passes objects by reference, meaning the method receives the memory address of the object.

Question 25

TRUE OR FALSE: Method signatures include the return type.

Answer 25

FALSE
Explanation: Method signatures include the method name and parameter types, but not the return type.

Question 26

Identify the error in the following code:

java
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public Rectangle(double len, double wid)
{
length = len;
}
a. Missing a return type.
b. The constructor is missing the assignment for the width field.
c. Constructors cannot accept parameters.
d. There is no error.

Answer 26

Answer: b
Explanation: The wid parameter is passed but not assigned to the width field.

Question 27

What does this code do?

java
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Rectangle room = new Rectangle();
a. Creates a reference variable room without initializing it.
b. Calls the default constructor and creates a Rectangle object.
c. Calls the no-arg constructor written by the programmer.
d. Throws a runtime error.

Answer 27

b
Explanation: If no constructor is written, Java provides a default constructor that initializes the Rectangle object.

Question 28

TRUE OR FALSE: An object’s fields must be accessed using public methods when they are private.

Answer 28

TRUE
Explanation: Private fields can only be accessed via public accessor and mutator methods.

Question 29

What will happen when the following code executes?

java
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public double getArea()
{
return length * width;
}
a. It will dynamically calculate the area of the rectangle.
b. It will throw an error if length or width is not initialized.
c. It will store the result in a variable called area.
d. It will return the value 0 by default.

Answer 29

a
Explanation: The method calculates and returns the area dynamically based on the current values of length and width.

Question 30

Which of the following is NOT true about a constructor?
a. It has the same name as the class.
b. It has no return type.
c. It can be overloaded.
d. It must always be private.

Answer 30

d
Explanation: Constructors are typically public to allow objects to be instantiated.