A class to validate, increment, decrement and compare dates(Java)

class ValidateDate 
{
 int day,month,year;

 ValidateDate()
 {
  this.day=1;
  this.month=1;
  this.year=1;
 }

 ValidateDate(int d,int m,int y)
 {
  this.day=d;
  this.month=m;
  this.year=y;
 }

 void incrementByOneDay()
 {
  this.day=this.day+1;
  if(this.day > 30 )
  {
   this.day=this.day-30;
   this.month=this.month+1;
   if(this.month > 12)
   {
    this.month=this.month-12;
    this.year=this.year+1;
   }
  }
 }

 void decrementByOneDay()
 {
  this.day=this.day-1;
  if(this.day == 0 )
  {
   this.day=30;
   this.month=this.month-1;
   if(this.month == 0)
   {
    this.month=12;
    this.year=this.year-1;
   }
  }
 }

 boolean compareDates(ValidateDate d)
 {
  if(this.day==d.day && this.month==d.month && this.year==d.year)
   return true;
  else
   return false;
 }

 void displayDate()
 {
  System.out.print("Date=>"+this.day);
  System.out.print(":"+this.month);
  System.out.print(":"+this.year);
 }


 public static void main(String[] args) 
 {
  ValidateDate d1=new ValidateDate(30,12,2001); 
  d1.displayDate();
  d1.incrementByOneDay(); 
  System.out.println("After incrementing:");
  d1.displayDate();
  d1.decrementByOneDay();
  System.out.println("After decrementing:");
  d1.displayDate();

  ValidateDate d2=new ValidateDate(30,12,2001);
   if(d1.compareDates(d2))
    System.out.println("EQUAL");
   else
    System.out.println("NOT EQUAL");
   }
  }