I'm learning java, and would really appreciate any tips on how to improve this code.

It's just a simple date validation class.

import java.util.Map;
import java.util.HashMap;

class DateValidator {
    private int day;
    private int month;
    private int year;
    
    private enum Months {
        JANUARY(1, 31), FEBRUARY(2, 28), MARCH(3, 31), APRIL(4, 30), MAY(5, 31), JUNE(6, 30), 
        JULY(7, 31), AUGUST(8, 31), SEPTEMBER(9, 30), OCTOBER(10, 31), NOVEMBER(11, 30), DECEMBER(12, 31);
        
        private final int MonthNumber;
        private final int numberOfDaysInMonth;
        
        private static Map<Integer, Integer> map;
        
        private Months(int monthMnumber, int numberOfDays) {
            this.MonthNumber = monthMnumber;
            this.numberOfDaysInMonth = numberOfDays;
        }
        
        static {
            map = new HashMap<Integer, Integer>();
            
            for (Months m : Months.values()) {
                map.put(m.MonthNumber, m.numberOfDaysInMonth);
            }
        }
        
        public static Map<Integer, Integer> getMap() {
            return map;
        }
        
    }   
    
    DateValidator(int d, int m, int y) {
            this.day = d;
            this.month = m;
            this.year = y;
    }
    
    private static boolean leapYear(int y) {
        return ((y % 4 == 0) && (y % 100 != 0) || (y % 400 == 0));
    }
    
    public boolean checkDay() {
        if (this.month == 2 && this.leapYear(this.year)) {
            Months.getMap().put(this.month, 29);
        }
        
        return (this.day <= Months.getMap().get(this.month));
    }
}

class Main {
    public static void main(String[] args) {
        DateValidator d = new DateValidator(31, 10, 90);
        
        System.out.print(d.checkDay());
    }
}