Old notes from 2023

Agenda

• What is Java

  • A JIT (compiled “Just-In-Time”) language

    • Compiled into Java bytecode

    • Then bytecode is translated into machine code on target platform

    • Makes code more cross-platform than C/C++, which is directly compiled

  • Statically typed

    • Every variable has a “type”

    • E.g. “int” == integer number, “double” == decimal number, “string” == text, etc

    • Lower case type is primitive type: basic type included with java

    • Upper case type is class type: complex type, see next

• Object oriented programming

  • Everything is an “object” of a certain “class”

  • Each class can have “variables” and “functions”

  • There’s also “constructors”, which are methods that run when you create an object

  • There’s also modifiers: “public”, “private”, “readonly”, “static”

  • Private things can only be accessed inside the object

  • Static are the same for all object instances of a class

  • Readonly cant be modified after creation, all readonly are static

  • Btw, “variable” == “member” == “attribute” and “function” == “method”

  • Python and C++ also have classes

  • Example class:

Car.java

class Car {  
	public Engine engine = new Engine();  
public float fuel = 0; // Unit is gallons  
public float mileage = 0; // Unit is miles  
	  
// The constructor  
public Car(float fuel, float mileage) {  
	this.fuel = fuel;  
	this.mileage = mileage;  
}

// This method drives the car by X miles  
public void driveCar(float miles) {  
	this.mileage += miles;  
	this.fuel -= miles / 33.0; // 33 miles per gallon  
}

// Adds fuel to car  
	public void addFuel(float fuel) {  
		this.fuel += fuel;  
	}  
}	  

Main.java

// ...  
Car hondaCivic = new Car(0, 0); // Brand new car with zero mileage  
hondaCivic.addFuel(10.0f); // Filled it up  
hondaCivic.driveCar(30.0f); // Drove the car to IMSA  
// ...  

• Libraries

  • Other code you import, typically written by other people

  • Consists of classes

  • Standard library is included with java

  • WPILib is the library we use for FRC

  • It has stuff like classes for motors and sensors

  • We also have a library for our gyroscope written by the manufacturer

  • And also our own library for reusable code: Titan Algorithms

• Walkthrough practice: inventory manager

Item.java

class Item {  
	public string Name;  
	public int Quantity;

	public Item(string name, int quantity) {  
		this.Name = name;  
		this.Quantity = quantity;  
	}  
}  

Inventory.java

class Inventory {  
private ArrayList<Item> items = new ArrayList<Item>();

public Inventory() {}

public void addItem(Item item) {  
items.add(item);  
}

public String search(String query) {  
	String result = "";  
	for (int i = 0; i < items.size(); i++) {  
		If (items.get(i).Name.contains(query)) {  
			// Yes you can add strings  
			// "\n" or "\r\n" on Windows is special character for new line  
			result += items.get(i).Name + "\r\n";  
		}  
}   
}	  
}  

Main.java

import java.util.Scanner; // Import standard library for print, this is done automatically by VSCode

// Name of our program  
class InventoryManager {  
	public static void main(String[] args) {  
 		// Java’s print function  
    		System.out.println("Welcome to inventory manager.");  
    		System.out.println("Type ‘/add [item name] [quantity]’ to add an item. No spaces in name.");  
		System.out.println("Type ‘/search [item name]’ to find an item.");  
		System.out.println("Type ‘/exit’ to leave the program.");

		// Scanner object used to record user input  
Scanner inputScanner = new Scanner(System.in);

// Our inventory  
Inventory inventory = new Inventory();

		// While loops run if the condition is true  
		boolean isRunning = true;  
		while (isRunning) {  
String userInput = inputScanner.nextLine(); // Get command input  
String[] splittedCommand = userInput.split("\\s+"); // Array with each word

			If (userInput.startsWith("/exit")) {  
				isRunning = false;  
System.out.println("Exiting.");  
			} else if (userInput.startsWith("/add")) {  
				// Item count is a string but has to be converted to an integer type  
				Item newItem = new Item(splittedCommand[1], Integer.parseInt(  
splittedCommand[2]));  
				inventory.addItem(newItem);  
System.out.println("Item added.");  
			} else if (userInput.startsWith("/search")) {  
				String query = splittedCommand[1];  
				System.out.println(inventory.search(query));  
			} else {  
				System.out.println("Unknown command.");  
			}  
		}  
	}  
}