PART 1

Java Class Example: Student

1. Class Definition and Purpose:
This introduces the Student class with a comment explaining its purpose, which is to model a student in terms of properties (like name and ID) and behaviors (such as adding grades and calculating averages).

2. Instance Variables (Fields):
• name and studentID are strings representing the student's name and ID.
• grades is an array of doubles to store the student's grades.
• numGrades is an integer to track how many grades have actually been entered into the grades array.

3. Constructor:
The constructor initializes a new instance of Student with the provided name and studentID, sets up an array to hold up to 10 grades, and initializes numGrades to 0.

4. Accessor Methods (Getters):
These methods allow external code to get the values of the name and studentID fields.

5. Mutator Methods (Setters):
These methods allow external code to set new values for the name and studentID.

6. Adding Grades:
Adds a new grade to the grades array if there is space. If the array is full, it outputs a message indicating no space is available.

7. Calculating Average:
Computes the average of the grades stored. It first checks if there are any grades to avoid division by zero.

8. Displaying Information:
Outputs the student's name, ID, and average grade to the console.

9. Main Method:
This method is used to run the class, demonstrating its functionality by creating a Student instance, adding some grades, and then displaying the student's details and average grade.

Java Class Example: Book

Class Declaration

• public class Book: Declares a public class named Book.

Instance Variables (Fields)

• Instance Variables: These are properties of a Book object. They are declared as private to ensure encapsulation, which means the data can only be accessed through methods of the class.
• title: Stores the title of the book.
• author: Stores the name of the author.
• pageCount: Stores the number of pages in the book.
• isAvailable: A boolean indicating the availability of the book.

Constructor

• Constructor: This method initializes a new instance of the Book class with specific values for title, author, and page count. It sets isAvailable to true by default.
• this keyword is used to refer to the current object instance.

Accessor Methods (Getters)

• Getters: These methods provide read access to the instance variables. They return the value of each field.

Mutator Methods (Setters)

• Setters: These methods allow the modification of the instance variables. They perform updates to the fields, like changing the title or the number of pages. The setPageCount method includes a check to ensure that the page count is positive.

Display Method

• displayInfo: This method prints the details of the book to the console.

Main Method

• main method: This is the entry point of the program where an instance of Book is created with the title "1984" by George Orwell, containing 328 pages. It displays the book details, updates the availability, and then displays the updated information.

• title：存储图书的标题。

• author：存储作者的姓名。

• pageCount：存储图书的页数。

• isAvailable：一个布尔值，指示图书的可用性。

PART 2

Key Points

Anatomy of a Class

1. Class Declaration:
• A class is defined using the syntax public class ClassName { ... }.

2. Instance Variables (Fields):
• These variables represent the state (attributes) of an object and are declared inside the class but outside any methods.

3. Constructors:
• Special methods that have the same name as the class and no return type; they initialize new objects.

4. Methods:
• Functions within the class that define behaviors. They may be instance methods (operating on objects) or static methods (belonging to the class).

5. Main Method:
• The entry point of a Java program, defined as public static void main(String[] args).

6. Access Modifiers:
• Keywords such as public, private, and protected control the visibility of class members.

7. The this Keyword:
• Used inside constructors and methods to reference the current object.

8. Encapsulation:
• The principle of hiding an object’s internal state by making fields private and providing public methods (getters/setters) for access.

9. Static Members:
• Fields and methods declared with static belong to the class itself rather than any individual instance.

10. Method Overloading:
• Defining multiple methods with the same name but different parameter lists to perform similar tasks.

Creating a Class

1. Blueprint for Objects:
• A class serves as a template from which objects are created.

2. Defining Fields and Methods:
• Clearly separate the data (fields) from the behaviors (methods).

3. Constructor Design:
• Constructors initialize object states. Consider overloading them to allow different ways to create an object.

4. Code Readability:
• Use clear, descriptive names and comments to enhance maintainability.

5. Access Control:
• Apply appropriate access modifiers to safeguard and control access to class members.

Sample Code

Sample Code 1: Simple Class with a Constructor and a Method

Sample Code 2: Class with Multiple Constructors and a Static Member

AP-Style Multiple Choice Questions

1. Which of the following best describes the role of a constructor in a Java class?
A. It is used to create a new object and initialize its state.
B. It defines the behavior of a class's methods.
C. It is used to declare instance variables.
D. It is a method that returns a value.

2. In the context of a Java class, what is an instance variable?
A. A variable that is declared inside a method.
B. A variable that holds data common to all objects of the class.
C. A variable that holds unique data for each object instance.
D. A variable that is automatically initialized to null.

3. Which keyword is used to refer to the current object within an instance method?
A. current
B. self
C. this
D. object

4. What is the correct syntax for declaring a public class in Java?
A. class public MyClass {}
B. public class MyClass {}
C. public MyClass class {}
D. class MyClass public {}

5. Which access modifier restricts access to the members of a class to only within that class?
A. public
B. private
C. protected
D. default

6. What is the purpose of the main method in a Java class?
A. To declare instance variables.
B. To provide the entry point for program execution.
C. To initialize class fields.
D. To overload the constructor.

7. Which of the following is true about a constructor?
A. It can have a return type of void.
B. It must have the same name as the class.
C. It can be declared static.
D. It can be called explicitly like any other method.

8. How does a static method differ from an instance method?
A. A static method can access instance variables directly.
B. A static method belongs to the class rather than any specific instance.
C. A static method is only available within the constructor.
D. A static method requires an object to be invoked.

9. Which of the following best represents encapsulation in object-oriented programming?
A. Inheriting properties from a parent class.
B. Hiding the internal state of an object and requiring all interaction to be performed through methods.
C. Overloading methods with the same name.
D. Using static methods to perform calculations.

10. In a class, which component is used to perform actions or behaviors?
A. Constructors
B. Fields
C. Methods
D. Objects

11. What is the output of the following code segment?
A. Nothing
B. Greater
C. 5
D. Error

12. Consider the following class:
What is the value of value when a new object is created with new Sample(10)?
A. 0
B. 10
C. null
D. undefined

13. Which statement is correct about method overloading?
A. Methods with the same name must have the same parameter list.
B. Methods with the same name can have different return types only.
C. Methods with the same name must have different parameter lists.
D. Method overloading is not allowed in Java.

14. What happens if you do not provide any constructor in a Java class?
A. The class will not compile.
B. The compiler will generate a default constructor.
C. You must create a constructor in a subclass.
D. An error occurs at runtime.

15. In Java, which of the following is a correct way to create an object of class Rectangle defined as:
A. Rectangle rect = new Rectangle();
B. Rectangle rect = new Rectangle(5, 10);
C. Rectangle rect = Rectangle(5, 10);
D. Rectangle rect = new Rectangle(width, height);

16. Which of the following best describes a class's field?
A. A method that returns the object's state.
B. A variable that holds data for each object created from the class.
C. A static method that applies to all objects.
D. A constructor that initializes the object.

17. Which of the following is not a valid reason for declaring a variable as private?
A. To encapsulate and hide internal data.
B. To prevent unauthorized access.
C. To allow unrestricted access across the program.
D. To control how the variable is accessed and modified.

18. What is the role of a static variable in a class?
A. To store a value that is unique for each object instance.
B. To store a value that is shared among all instances of the class.
C. To initialize the object state in the constructor.
D. To override instance methods.

19. Which of the following code snippets correctly uses the this keyword?
A. this.name = name;
B. Person.name = name;
C. name = this.name;
D. name = Person.name;

20. Given the following code:
What is the output when the program is executed?
A. MyClass obj = new MyClass();
B. Hello, world!
C. display();
D. Error due to multiple classes in one file.