💻AP Computer Science A Unit 5 – Writing Classes in AP Computer Science A

What Are Classes?

• Classes serve as blueprints or templates for creating objects in object-oriented programming
• Define the structure, behavior, and attributes of objects
• Encapsulate related data and methods into a single unit
• Provide a way to organize and modularize code
• Enable the creation of multiple instances (objects) based on the class definition
• Facilitate code reuse and maintainability by allowing the creation of reusable components
• Promote the principles of object-oriented programming (encapsulation, inheritance, polymorphism)

Anatomy of a Class

• A class consists of a class declaration, which includes the

  class

  keyword followed by the class name
• Inside the class, there are instance variables (fields) that represent the state or attributes of objects
  • Instance variables are typically declared as private to ensure data encapsulation
• Methods are defined within the class to define the behavior and operations that objects can perform
  • Methods can have parameters and return values
  • Constructors are special methods used to initialize objects when they are created
• Access modifiers (public, private, protected) are used to control the visibility and accessibility of class members
• Classes can also contain static variables and methods that belong to the class itself rather than individual objects

Creating Objects

• Objects are instances of a class created using the

  new

  keyword followed by a constructor call
• The

  new

  keyword allocates memory for the object and invokes the constructor to initialize its state
• Multiple objects can be created from the same class, each with its own unique set of instance variable values
• Objects are stored in memory and can be assigned to variables or passed as arguments to methods
• The dot notation (

  .

  ) is used to access the instance variables and methods of an object
• Objects can interact with each other by invoking methods and exchanging data

Instance Variables and Methods

• Instance variables are non-static variables declared within a class but outside any method
• Each object created from the class has its own copy of the instance variables
• Instance variables represent the state or attributes of an object (color, size, name)
• Instance methods are non-static methods defined within a class
• Instance methods can access and manipulate the instance variables of the object on which they are invoked
• Instance methods define the behavior or actions that objects can perform (move, calculate, display)
• Instance methods can have parameters to receive input and can return values as output

Constructors and Initialization

• Constructors are special methods used to initialize objects when they are created using the

  new

  keyword
• Constructors have the same name as the class and do not have a return type
• The purpose of a constructor is to set the initial state of an object by assigning values to its instance variables
• If no constructor is explicitly defined, Java provides a default constructor with no parameters
• Constructors can be overloaded, meaning multiple constructors with different parameter lists can be defined
• Constructors can call other constructors using the

  this

  keyword to avoid code duplication
• The

  this

  keyword refers to the current object instance and can be used to differentiate between instance variables and local variables with the same name

Encapsulation and Access Modifiers

• Encapsulation is the principle of bundling data (instance variables) and methods that operate on that data within a class
• Access modifiers are used to control the visibility and accessibility of class members (variables and methods)

• public

  members are accessible from anywhere, both within the class and from outside the class

• private

  members are only accessible within the class itself and cannot be accessed from outside the class

• protected

  members are accessible within the same package and by subclasses in other packages
• Encapsulation helps in achieving data hiding and protects the internal state of an object from unauthorized access
• Getter and setter methods can be used to provide controlled access to private instance variables
  • Getter methods (accessors) allow reading the value of an instance variable
  • Setter methods (mutators) allow modifying the value of an instance variable

Static Members

• Static members (variables and methods) belong to the class itself rather than individual objects
• Static variables are shared among all instances of a class and can be accessed using the class name
• Static methods can be invoked without creating an instance of the class
• Static methods can only directly access static members of the class
• The

  static

  keyword is used to declare static variables and methods
• Common use cases for static members include utility methods, constants, and counters
• Static variables are initialized when the class is loaded into memory
• Static methods cannot use the

  this

  keyword as they do not have access to instance-specific data

Practical Applications in Java

• Classes are fundamental building blocks in Java and are used extensively in object-oriented programming
• Java's standard library provides numerous built-in classes for various purposes (String, ArrayList, Math)
• Custom classes can be created to model real-world entities or concepts (Person, Car, BankAccount)
• Classes can be used to organize related functionality and promote code modularity and reusability
• Inheritance allows the creation of specialized classes (subclasses) based on existing classes (superclasses)
• Polymorphism enables the use of inheritance to treat objects of different subclasses as objects of a common superclass
• Classes can be used to implement abstract data types (ADTs) and data structures (Stack, Queue, LinkedList)
• Java's collections framework heavily relies on classes to provide efficient and reusable data structures and algorithms