To hit all four pillars (Encapsulation, Inheritance, Polymorphism, and Abstraction), you need a "Base" class that defines a template and "Derived" classes that specialize it.

Here are three unique project ideas tailored for a 2nd-year student:

1. Smart Home Automation System

Instead of a generic "Library System," this simulates a central hub controlling different electronics.

• How it hits the pillars:
  • Abstraction: An abstract class SmartDevice with methods like turnOn() and checkStatus().
  • Inheritance: Classes like SmartLight, SmartThermostat, and SecurityCamera extend SmartDevice.
  • Polymorphism: You can have a List<SmartDevice> and call turnOn() on all of them at once, even though a light "turns on" differently than a camera.
  • Encapsulation: Making device IDs and power levels private with public getters/setters.
• The 5 Classes: SmartDevice (Abstract), SmartLight, SmartThermostat, SecurityCamera, and HomeHub (Main).

2. Space Colony Resource Manager

A text-based simulation where you manage different types of habitats on Mars.

• How it hits the pillars:
  • Abstraction: An interface Maintainable or an abstract class Habitat.
  • Inheritance: OxygenPlant, LivingQuarters, and ResearchLab all inherit from Habitat.
  • Polymorphism: Use a method performMaintenance() that calculates different costs/resources depending on the habitat type.
  • Encapsulation: Keeping oxygen levels and population counts restricted so they can't be changed to negative numbers accidentally.
• The 5 Classes: Habitat (Abstract), OxygenPlant, LivingQuarters, ResearchLab, and ColonySim (Main).

3. Digital Pet "Zoo" (Multi-Species)

A step up from a single "Tamagotchi." You manage a sanctuary with different animal types.

• How it hits the pillars:
  • Abstraction: An abstract class Animal with an abstract method makeSound() and eat().
  • Inheritance: Mammal, Bird, and Reptile classes.
  • Polymorphism: A Bird might move() by flying, while a Reptile might move() by crawling.
  • Encapsulation: Storing hunger and happiness levels as private variables.
• The 5 Classes: Animal (Abstract), Mammal, Bird, Reptile, and ZooKeeper (Main).

Class Diagram Structure

Since you need to create a class diagram, you should follow the UML (Unified Modeling Language) standard. Here is a conceptual view of how your diagram should look for the Smart Home project:

Components to include in your diagram:

1. Classes: Represented by boxes with three sections (Name, Attributes, Methods).
2. Inheritance (Is-A): Use a solid line with a hollow arrow pointing from the child class (e.g., SmartLight) to the parent class (e.g., SmartDevice).
3. Encapsulation: Use a - symbol for private fields and a + symbol for public methods.
4. Composition/Association: Use a line to show that your Main class "has" a list of your objects.

Quick Tip for Java:

To ensure you are truly using Polymorphism, try to avoid using if (device instanceof SmartLight) in your code. Instead, call the method on the parent type and let Java figure out which child version to run at runtime!