关于Java常数的困惑

Question

I'm in the middle of studying java programming and I'm a bit confused about constants. From what I've read so far constants are final and cannot be reassigned a new value unlike variables. However, when I'm using the Calender class there is the set method which seems to change a constant field. For example:

Calendar cal = Calendar.getInstance();
System.out.println("The year is " + cal.get(Calendar.YEAR));

System.out.println("The year is " + cal.get(Calendar.YEAR));

If the Calendar.YEAR field is declared final in the Calendar class then why am I able to change it to another value using the set method?

Answer 1

Calendar.YEAR is just a constant saying which logical field you want to set within the calendar.

The aim was to avoid having an API with

setYear
setDay
setMonth
...

In retrospect, I'd say this was a spectacularly bad idea - along with most of the rest of the design of java.util.Calendar and java.util.Date .

So this call:

cal.set(Calendar.YEAR, 2001)

doesn't change the value of Calendar.YEAR ... it changes some other (private) field within the Calendar object.

Answer 2

Calendar.set(Calendar.YEAR, xx) changes the value of the year (for the calendar instance), not the value of Calendar.YEAR ( which is 1 ), see the javadoc

NB: it's easy to make a short program to verify that.

Answer 3

Actually calendar.set(blah, blahVal) is not updating the first params (Calendar.YEAR) , it update the value which is internally maintain by the calendar, for example if you give cal.set(Calendar.YEAR, 2001) Calendar.YEAR is a constant which is index of an array. that array contain the specified value. that value is update dramatically.

Answer 4

Calendar.YEAR is just an int which is defined in Constant Field Values , see the docs :

public static final int YEAR

It's not changing constant field, this int defines what field should be changed, see the source code :

1196    public void set(int field, int value)
1197    {
1198        if (isLenient() && areFieldsSet && !areAllFieldsSet) {
1199            computeFields();
1200        }
1201        internalSet(field, value);
1202        isTimeSet = false;
1203        areFieldsSet = false;
1204        isSet[field] = true;
1205        stamp[field] = nextStamp++;
1206        if (nextStamp == Integer.MAX_VALUE) {
1207            adjustStamp();
1208        }
1209    }

Take a look at the docs Calendar#set to better understand what it does.