//same Date BUT with added equals() and hashcode() methods (***see below)
//so Date can be used in a HashSet

public class Date {
    private int dMonth;		//variable to store the month
    private int dDay;		//variable to store the day
    private int dYear;		//variable to store the year
    
    //Default constructor
    //Data members dMonth, dDay, and dYear are set to
    //the default values
    //Postcondition: dMonth = 1; dDay = 1; dYear = 1900;
    public Date() {
	dMonth = 1;
	dDay = 1;
	dYear = 1900;
    }

    //Constructor to set the date
    //Data members dMonth, dDay, and dYear are set
    //according to the parameters
    //Postcondition: dMonth = month; dDay = day;
    //               dYear = year;
    public Date(int month, int day, int year) {
	dMonth = month;
	dDay = day;
	dYear = year;
    }
    
    //Method to set the date
    //Data members dMonth, dDay, and dYear are set
    //according to the parameters
    //Postcondition: dMonth = month; dDay = day;
    // 			 dYear = year;
    public void setDate(int month, int day, int year) {
	dMonth = month;
	dDay = day;
	dYear = year;
    }
    
    //Method to return the month
    //Postcondition: The value of dMonth is returned
    public int getMonth() {
	return dMonth;
    }
    
    //Method to return the day
    //Postcondition: The value of dDay is returned
    public int getDay() {
	return dDay;
    }
    
    //Method to return the year
    //Postcondition: The value of dYear is returned
    public int getYear() {
	return dYear;
    }
    
    //Method to return the date in the form mm-dd-yyyy@hashcode
    //show the overridden hashcode value
    public String toString() {
	return (dMonth + "-" + dDay + "-" + dYear + super.toString());
    }

    //***override equals() of Object.
    //***used to determine if two objects (of Date class) are "equal".
    //***MUST be overridden to use Date objects (uniquely, as keys) in a HashSet. 
    //***equality is defined by this method. 
    //***without this method, two objects are equal only if their references are the same.
    //***must be this signature:  parameter MUST be type Object.
    public boolean equals( Object other ) {   //parameter will be the other Date
	//***check that other really is Date, 
	//**then (safely) downcast to Date to access instance variables
	if ((other instanceof Date) && 
	    ((((Date)other).dYear==dYear) &&
	     (((Date)other).dMonth==dMonth) &&
	     (((Date)other).dDay==dDay)))
	    return true;
	else
	    return false;    //if other is null, or not a Date, or doesn't equal
    }

    //equals must be 
    //1. reflexive: x.equals(x) is true
    //2. symmetric: x.equals(y) == y.equals(x)
    //3. transitive: if x.equals(y) and y.equals(z) then x.equals(z)
    //also: x.equals(null) is false



    //***override hashcode() of Object
    //***if two Dates are equal, they must have same hashcode.
    //***must be this signature:
    public int hashCode() {
	//return 1492;  //would work but inefficient   
	//?? how to make good hash??
	//some function of the instance variables modded by some prime...
	return (dYear + dMonth + dDay) % 101;  
    }
}