calendar. Pit point of getactualmaximum (calendar. Day_of_month)

The problem code is as follows

public static Date monthLastDate(Integer year, Integer month) throws ParseException {
    
        if (year == null || month == null || month > 12 || month < 1) {
    
            return null;
        }
        Calendar calendar = Calendar.getInstance();
        calendar.set(Calendar.YEAR, year);
        calendar.set(Calendar.MONTH, month - 1);
        int i = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
        calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
        return calendar.getTime();
    }

Regardless of the method passed in month Why is it worth ,i Value is always the maximum value of the current month .

because Calendar The system time of class initialization has been set Calendar.DAY_OF_MONTH The maximum of , When we change the values of year and month ,Calendar It is not necessary to modify and set the corresponding fields at the same time ActualMaximum Value .

terms of settlement :

If you want to modify the created calendar Value , Empty first calendar All initial defaults , Then modify the value .
Can be understood as using clear() Method to remove calendar The cache of .

public static Date monthLastDate(Integer year, Integer month) throws ParseException {
    
        if (year == null || month == null || month > 12 || month < 1) {
    
            return null;
        }
        Calendar calendar = Calendar.getInstance();
        calendar.clear();
        calendar.set(Calendar.YEAR, year);
        calendar.set(Calendar.MONTH, month - 1);
        int i = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
        calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
        return calendar.getTime();
    }

Question inquiry

Let's take a look at his set Method , When we set Field A The value of , Not all with A The relevant values are modified at the same time , Just set a state , Other fields have not been set in the table .

 public void set(int field, int value)
    {
    
        // If the fields are partially normalized, calculate all the
        // fields before changing any fields.
        if (areFieldsSet && !areAllFieldsSet) {
    
            computeFields();
        }
        internalSet(field, value);
        isTimeSet = false;
        areFieldsSet = false;
        isSet[field] = true;
        stamp[field] = nextStamp++;
        if (nextStamp == Integer.MAX_VALUE) {
    
            adjustStamp();
        }
    }

When we get A field B The value of , It will call the method of calculating all fields , After the calculation, go to get this field B Value .

public int get(int field)
    {
    
        complete();
        return internalGet(field);
    }

Here's how to calculate all fields

/** * Fills in any unset fields in the calendar fields. First, the {@link * #computeTime()} method is called if the time value (millisecond offset * from the <a href="#Epoch">Epoch</a>) has not been calculated from * calendar field values. Then, the {@link #computeFields()} method is * called to calculate all calendar field values. */
    protected void complete()
    {
    
        if (!isTimeSet) {
    
            updateTime();
        }
        if (!areFieldsSet || !areAllFieldsSet) {
    
            computeFields(); // fills in unset fields
            areAllFieldsSet = areFieldsSet = true;
        }
    }
    ```