demos.apex

// Creating a new date with some starting value
Date newDate = Date.newInstance(2022, 10, 27);
System.debug(newDate);

Date newDate2 = Date.newInstance(2023, 10, 27);
System.debug(newDate);

// Calling some non-static methods of the date instance
Integer year = newDate.year();
System.debug('year :' + year);

Integer year2 = newDate2.year();
System.debug('year :' + year2);

// Calling static methods of the date class
Boolean isLeapYear = Date.isLeapYear(2022);
System.debug('isLeapYear :' + isLeapYear);

// Calling a custom class instance methods
Integer result = MyCalculator.sum(1,6);
System.debug('result :' + result);

MyCounter birdCounter = new MyCounter();
birdCounter.incrementCounter();

MyCounter dogCounter = new MyCounter();
dogCounter.decrementCounter();

birdCounter.incrementCounter();

// Instantiate a custom class after adding a constructor with params
MyCounter catCounter = new MyCounter('Cat');
catCounter.incrementCounter();

MyCalculator.cls

public with sharing class MyCalculator {
   // Static methods are used to perform instant calculations,
   // method starts, runs, and returns something, but nothing is stored in memory!
   public static Integer sum (Integer a, Integer b) {
        return a + b;
   }
}

MyCounter.cls

public with sharing class MyCounter {
    private Integer counter = 0; 
    private String counterName = 'Default counter name';

    /* A default constructor with no parameters exists for each class, 
    if you need to define a constructor with parameters, do it this way:
    public MyCounter(String counterNameParam) {
        // Do whatever
        this.counterName = counterNameParam;
    }*/

    // When you create a instance of a class, its state is stored
    // in its instance variables (counter in this case) --> instance methods can see and modify those vars
    public void incrementCounter() {
        this.counter++;
        System.debug(this.counterName + ' counter value is: ' + this.counter);
    }

    public void decrementCounter() {
        this.counter--;
        System.debug(this.counterName + ' counter value is: ' + this.counter);
    }
}