Spotify 'best before' test program solved

BestBefore.java

import java.util.Date;
import java.util.Scanner;
import java.util.LinkedList;
import java.util.Iterator;
import java.text.SimpleDateFormat;

public class BestBefore {

	//Verify if it is a leap year
	public static boolean isaleapyear (int year) {
		boolean result=false;
		
		if ((year%4==0)&&((year%100!=0)||(year%400==0))) { result=true; }
		
		return result;
		
	}
	
	//Verify is generated date could be a valid date
	public static boolean correctDate(String s_year, String s_month, String s_day) {
		int year=Integer.parseInt(s_year),month=Integer.parseInt(s_month),day=Integer.parseInt(s_day);
		int[] monthDays={0,31,28,31,30,31,30,31,31,30,31,30,31};
		boolean result=true;
		
		if (isaleapyear(year)) { monthDays[2]=29; }
		if ((month<=0)||(month>12)) {
			result=false;
		} else {
			if ((day<=0)||(day>monthDays[month])) { result=false; }
		}
			
		return result;
	}
	
	//Transform input year in a four digit year
	public static String extendedYear (String s_year) {
		int year=0;
		
		year=Integer.parseInt(s_year);
		if (year<2000) { year=year+2000; }
		
		return String.valueOf(year);
	}
	
	//Verify input string is correctly formated
	public static boolean inputStringisCorrect(String line) {
		int k=0,x=0,y=0,fourDigitYear=0;
		String n_line;
		boolean result=true;
		
		n_line = new String(line.replaceAll("/", ""));
		if ((n_line.length()+2)!=line.length()) {
			result=false;
		} else {
			String[] number = line.split("/");
		
			for (int i=0;i<3;i++) {
				if (number[i].length()==4) { fourDigitYear++; k=i; }
			}
		
			if (fourDigitYear>1){
				result=false;
			} else {
				if (fourDigitYear==1) {
					switch(k) {
						case 0: x=1; y=2; break;
						case 1: x=0; y=2; break;
						case 2: x=0; y=1; break;
					}
					if ((Integer.parseInt(number[k])<2000)||(Integer.parseInt(number[k])>2999)) { result=false; }
					if ((number[x].length()<1)||(number[x].length()>2)) { result=false; }
					if ((number[y].length()<1)||(number[y].length()>2)) { result=false; }
				} else {
					for (int i=0; i<3; i++) {
						if ((number[i].length()<1)||(number[i].length()>2)) { result=false; }
					}
				}
			}
		}
		
		return result;
	}
	
	//Main
	public static void main(String[] args) {
		try {
			long value=0;
			boolean anyDate=false;
			int val1=0,val2=0,val3=0;
			
			Date currentDate;
			Date resultDate = new Date();
			LinkedList<Date> listofDates = new LinkedList<Date>();
			SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd");
			
			//Read initial date from the standard system input
			Scanner sca = new Scanner(System.in);
		    String line = sca.nextLine();
			
			//Verify input string is correct
			if (inputStringisCorrect(line)) {
				String[] number = line.split("/");
				//In each iteration we change the position of array elements to generate all possible date combinations (up to six)
				for (int i=0;i<6;i++) {					
					switch (i) {
						case 0: val1=0;val2=1;val3=2; break;
						case 1: val1=0;val2=2;val3=1; break;
						case 2: val1=1;val2=0;val3=2; break;
						case 3: val1=1;val2=2;val3=0; break;
						case 4: val1=2;val2=0;val3=1; break;
						case 5: val1=2;val2=1;val3=0; break;
					}
					
					if (correctDate(extendedYear(number[val1]), number[val2], number[val3])) {
						currentDate=dateFormat.parse(extendedYear(number[val1])+"/"+number[val2]+"/"+number[val3]);
						if (currentDate.getTime()>=value) { value=currentDate.getTime(); }
						listofDates.add(currentDate);
						
					}
				}
			
				//Now, we got a list of valid dates. We're going to search on it to find the earliest one
				Iterator<Date> iterator = listofDates.iterator();
				
				while(iterator.hasNext())
				{
			        currentDate = (Date) iterator.next();
			        if (currentDate.getTime()<=value) {
			        	anyDate=true;
			        	value=currentDate.getTime();
			        	resultDate = currentDate;
			        }
				}
			
				//Finally, if there is any valid date, we show it in the required output format
				if (anyDate) {
					java.text.SimpleDateFormat sdf=new java.text.SimpleDateFormat("yyyy-MM-dd");
					System.out.println(sdf.format(resultDate));
				} else {
					System.out.println(line+" is illegal");
				}
			} else {
				//Input string is incorrect so we write an illegal message on standard output
				System.out.println(line+" is illegal");
			}
		}catch (Exception e){
			System.err.println("Error: " + e.getMessage());
		}
	}
}