ArrayList passing by reference with dates object?

I pass dates to an ArrayList 开发者_如何学Gobut when I change the Date object, all the Dates inside the ArrayList will change. This is an example:

Date currentDate = new Date("6/10/2011");
ArrayList<Date> datesList = new ArrayList();

currentDate.setDate(currentDate.getDate() + 1);
datesList.add(currentDate);

currentDate.setDate(currentDate.getDate() + 1);
datesList.add(currentDate);

currentDate.setDate(currentDate.getDate() + 1);
datesList.add(currentDate);

System.out.println(datesList.toString());

This will print:

[Mon Jun 13 00:00:00 EDT 2011, Mon Jun 13 00:00:00 EDT 2011, Mon Jun 13 00:00:00 EDT 2011]

Any idea of why this happening and how could I solve it?

This is because the variable currentDate references a single instance of Date, which you have added to the list many times. When you call currentDate.setDate(currentDate.getDate() + 1) you're simply updating that same object's state, and every time you call datesList.add(currentDate) that same object is added to the ArrayList.

Also note that setDate() and getDate() are deprecated methods. You should look into using a Calendar for date manipulation:

Calendar cal = Calendar.getInstance();
cal.set(2011, 5, 10);

ArrayList<Date> datesList = new ArrayList<Date>();

datesList.add(cal.getTime());

cal.add(Calendar.DATE, 1);
datesList.add(cal.getTime());

cal.add(Calendar.DATE, 1);
datesList.add(cal.getTime());

System.out.println( datesList.toString());

or better yet, Joda Time.

You're repeatedly adding references to the same object, so you end up with a list that contains multiple references to that same object.

Try adding copies instead:

currentDate.setDate(currentDate.getDate()+1)
datesList.add(currentDate.clone());          // note the .clone()

List datesList has the same instance in all positions. It's the same Date instance instantiated at Date currentDate = new Date("6/10/2011");. Try cloning it with clone(), and modify & add each clone to the list.

   ArrayList<Date> datesList = new ArrayList();

   Date currentDate = new Date("6/10/2011"); ;
   currentDate.setDate(currentDate.getDate()+1);
   datesList.add(currentDate);

   currentDate = currentDate.clone();
   currentDate.setDate(currentDate.getDate()+1);
   datesList.add(currentDate);

   currentDate = currentDate.clone();
   currentDate.setDate(currentDate.getDate()+1);
   datesList.add(currentDate);

   System.out.println( datesList.toString());

By doing this, each time you do currentDate = currentDate.clone(), you're assigning a new instance of Date with the same properties to the variable currentDate, while not modifying the original instances already added to the list.

int dateNum = 3;
ArrayList<Date> datesList = new ArrayList();

for(int x = 0; x < dateNum; x++){
   Date currentDate = new Date("6/10/2011"); //Creating a new object so your not pointing to the same one in memory
   currentDate.setDate(currentDate.getDate()+1);
   datesList.add(currentDate);
}

   System.out.println( datesList.toString());