toString()
==================
class Employee {
	private int salary; // default, int is zero

	public String toString() {
		return this.salary + "";
	}
}

class Main {

	public static void main(String[] args) {
		Employee e1 = new Employee();
		System.out.println(e1);
	}
}


== vs .equals
==============
public class Main
{
	public static void main(String[] args)
	{
		String s1 = "testing";
		
		String s2 = "test";
		s2 = s2 + "ing";

		System.out.println(s1==s2); //false
		System.out.println(s1.equals(s2)); //true
		
		String s3 = s1;
		System.out.println(s1==s3);  //true
		System.out.println(s1.equals(s3));  //true
	}
}


.length or array
================
public class Main
{
	public static void main(String[] args)
	{
		int[] arr = new int[3];
		System.out.println(arr.length);
		arr.length=2;
	}
}


static method, non-static field
===============================
class Day {
	private int year, month, day;

	static public boolean isLeapYear(int y) {
		if (y % 400 == 0)
			return true;
		else if (y % 100 == 0)
			return false;
		else if (y % 4 == 0)
			return true;
		else
			return false;
	}
}

public class Main
{
	public static void main(String[] args)
	{
		boolean result = Day.isLeapYear(2023);
		System.out.println(result);
	}
}

//System.out.println(this.year);
//System.out.println(year);


exchange salary/object
================
class Employee {
	private int salary; 

	public Employee(int s) { salary = s; }

	public String toString() { return this.salary+""; }

	public static void swapSalary(Employee e1, Employee e2) {
		int temp = e1.salary;
		e1.salary = e2.salary;
		e2.salary = temp;
	}

	public static void swapEmployee(Employee e1, Employee e2) {
		Employee temp = e1;
		e1 = e2;
		e2 = temp;
	}
}

public class Main {

	public static void main(String[] args) {
		Employee a = new Employee(100);
		Employee b = new Employee(200);

		Employee.swapEmployee(a, b);
		System.out.println(a + ", " + b); // Output: 100,200

		Employee.swapSalary(a, b);
		System.out.println(a + ", " + b); // Output: 200,100

		// swapEmployee's e1 copies Main's a.  
		// => Changing e1 doesn't mean change a.

		// swapSalary's e1 and Main's a SEES the same object.  
		// => So, salary swapped.  ^_^
	}
}