AP Computer Science A Series: Unit 5 - Writing Classes

Introduction

• Focus on writing classes and using objects.
• Object-Oriented Programming (OOP) languages: Java, C++, C#, Python.

Key Concepts

Objects

• States: Represented by fields in Java.
• Behaviors: Represented by methods.
• Example: A dog with states (color, awake status, age) and behaviors (bark, run, eat).

Classes

• A blueprint or template for an object.
• Static Methods: Accessible without creating an object; identified by static keyword.
  • Example: rotateWheel or openHood in a car blueprint.
• Non-static Methods: Require an object to access.
  • Example: turnOnEngine or drive.

Class Variables

• Also known as static fields.
• Changing a class variable affects all instances.
  • Example: Changing wheelSize changes it for all cars.

Instance Variables

• Known as non-static fields.
• Only have values when an instance is created.
• Changing instance variables affects only that instance.
  • Example: color or licensePlate.

Methods

Static vs. Non-static

• Static Methods: Can access class variables.
• Non-static Methods: Can access both class and instance variables.

Access Modifiers

• Public: Accessible from inside or outside the class.
• Private: Accessible only within the class.

Method Declaration

• Return Type: Can be void or any data type.
• Method Name: Should be descriptive and in lower camel case.
• Parameters: Optional; used to pass data to methods.

Getter and Setter Methods

• Getter: Returns the value of a field.
• Setter: Sets or updates the value of a field.
• Example: getStudentId, setStudentId for instance variables.
• Static getters and setters are used for static fields.

Overloading Methods

• Same name in the same class, different parameters.
• Rules:
  • Must have different number or types of parameters.
  • Can have different return types.
  • Can be public or private.
  • Can have different static or non-static values.

Constructors

• Special type of non-static method to set up a new object.
• Overloaded Constructors: Different parameter lists.
• this keyword: Used to refer to current object or call another constructor.

Scope and Lifetime of Variables

• Scope: Where the variable can be accessed.
• Lifetime: When the variable is created and destroyed.
• Local Variables: Exist within methods or blocks.
• Parameters: Created with method call, scope within method.
• Class Variables: Static fields, shared across all instances.
• Instance Variables: Non-static fields, unique to each instance.

Conclusion

• Understanding static vs. non-static, object behaviors, and variable scope is crucial.
• Practice with examples to solidify understanding.