Java从另一个构造函数调用构造函数而不立即拥有参数

Question

Is there is any way to call a constructor from another constructor without immediately having the parameters for it?

I ran into this problem while trying to create a constructor for my SimpleDate class that took in a millis time parameter and used another constructor to create the class (code below). The problem that I ran into was that the constructor call has to be on the first line but I don't really see anyway to get a Calendar instance with the right time without first setting the time in milliseconds on a previous line. I don't see how do that on one line, because setTimeInMillis is a void method and I don't think it possible to return the value after invoking a method (if it is than i would very much like to know how as well). I realize this whole thing is not really completely necessary but i do want to know if it is possible and if it is, how I would do it.

        public SimpleDate(long timeMillis) {
            this(Calendar.getInstance().setTimeInMillis(timeMillis));//Obviously this doesn't work because setTimeInMillis is a void method
        }

        public SimpleDate(Calendar calendar) {
            this.year = calendar.get(Calendar.YEAR);
            this.month = calendar.get(Calendar.MONTH) + 1;
            this.day = calendar.get(Calendar.DAY_OF_MONTH);

            this.hour = calendar.get(Calendar.HOUR_OF_DAY);
            this.minute = calendar.get(Calendar.MINUTE);
            this.second = calendar.get(Calendar.SECOND);
        }

Answer 1

You could do this by calling a static method on the class to perform the necessary conversion for you.

protected static Calendar getCalendarForMillis(long millis) {
    Calender ret = Calendar.getInstance();
    ret.setTimeInMillis(millis);
    return ret;
}

public SimpleDate(long millis) {
    this(getCalendarForMillis(millis));
}

The answer to your question is no: you can't call another constructor later in the initialization if the parameters aren't ready right at the top of the constructor. That's why you have to do some static method workaround.

Answer 2

What you want is just reuse of the code which creates your fields from a Calendar instance, with the ability to convert one constructor's long millis argument to a Calendar instance first.

If you put that Calendar -> your fields code directly into one constructor, it means that to reuse it you have to call that constructor on the first line of the other constructor. This is a bit clumsy because it forces you to use a separate static millis -> Calendar method, which probably won't be used elsewhere.

A more flexible, arguably cleaner solution is to just put that Calendar -> your fields code in a separate method, and then call it from both constructors:

public SimpleDate(Calendar calendar) {
  setDateFields(calendar);        
}

public SimpleDate(long millis) {
  Calendar calendar = Calendar.getInstance();
  calendar.setTimeInMillis(millis);
  setDateFields(calendar);      
}

private void setDateFields(Calendar calendar) {
  this.year = calendar.get(Calendar.YEAR);
  this.month = calendar.get(Calendar.MONTH);
  this.day = calendar.get(Calendar.DAY_OF_MONTH);
  this.hour = calendar.get(Calendar.HOUR_OF_DAY);
  this.minute = calendar.get(Calendar.MINUTE);
  this.second = calendar.get(Calendar.SECOND);
}

This way, you can do setup, or anything else you like, before calling the method in either constructor, which I believe is what you really wanted.