调用其他方法和不可变实例

Question

This is a question from an exam past paper. I have completed the question and it works. However, i feel my implementation may be weak eg my use of static throughout the Gregorian class.

I was given three methods to write in any way i saw fit (in the Gregorian class) given a scenario for each. Was I right in using static on the three methods in the Gregorian class.

Also the day, month and year fields are meant to be immutable, is setting them as private enough? (once they are created the field values cannot be changed)

public class Date {

private int day;// needs to be immutable?
private String month;// needs to be immutable?
private int year;// needs to be immutable?

public Date(int theDay, String theMonth, int theYear) {
    this.day = theDay;
    this.month = theMonth;
    this.year = theYear;
}

public int getDay() {
    return day;
}

public String getMonth() {
    return month;
}

public int getYear() {
    return year;
}

}

public class Gregorian {

public static Date d;

public static boolean leapYear(){

    if(d.getYear() %400==0 || (d.getYear()%4==0 && d.getYear()%100!=0)){
        return true;
    }else{
        return false;
    }
}

public static int getGregorianDateNumber(){
    int a = (d.getYear()*384)*(32+d.getDay());
    return a;
}

public static int getISO8601Date(){
    int b = (d.getYear()*367)+d.getDay();
    return b;
}

public static void main (String[] args){
    d = new Date(9, "June", 8);
    System.out.println(getGregorianDateNumber());
    System.out.println(getISO8601Date());
    System.out.println(leapYear());
}

}

Answer 1

Instead of the static methods and the static field d make them all non-static.

public class Gregorian {
    private final Date d;

    public Gregorian(Date d_) {
        this.d = d_;
    }

    public boolean isLeapyear() {
        ... // implemented as above
    }

    ... // Other methods as above, but all non-static.
}

And main as follows:

public static void main (String[] args){
    Date d = new Date(9, "June", 8);
    Gregorian g = new Gregorian(d);
    System.out.println(g.getGregorianDateNumber());
    System.out.println(g.getISO8601Date());
    System.out.println(g.leapYear());
}

Answer 2

Strings are by default immutable.

private int day;// needs to be immutable?
private int year;// needs to

are not immutable fields as you defined. Their state can change. Make them final.

NOTE: Making a reference final doesn't mean object state can't be changed (In your case this note is irrelevant, because you are not referencing objects).

Answer 3

I'll agree with thinksteep - adding "final" to your fields will help keep them from being changed. Not having setters reinforces this.

In addition, I want to point out that

private String month;// needs to be immutable?

can be created as anything, from "January" to "Pie". If I may suggest, change it to an enum and establish the allowed values for months.

public enum MonthName {
    JAN,FEB,MAR,APR,MAY,JUN,JUL,AUG,SEP,OCT,NOV,DEC;
}

Change your Date class to the following:

private final int day;
private final MonthName month;
private final int year;

public Date(int theDay, MonthName theMonth, int theYear) {
    this.day = theDay;
    this.month = theMonth;
    this.year = theYear;
}

Answer 4

both day and year are primitives and There is already immutable version of int available which is Integer you can make use of that.

Second thing is, rather having a static reference to Date in Gregorian , pass the Date as an argument to each of the static methods. You can assure the thread safety then.