Using the Instantiated Date Object

• You can simply create a date object and pass it to a method, without ever assigning it to a variable.
• System.out.println(new Date()); will print the current date.
• Advanced topic:  Does this mean that the System.out object has a method println(Date date) that accepts Date?  Actually, no, it does not.  But all classes implement class Object at some point, and the Object class has the toString() method that converts the object to a String.  You can override this method to return whatever string you want.  In any case, println() accepts Object types, which is basically anything.  Then, it calls the toString() method on that object to get the String value and print it out.  Essentially, this is polymorphism.
• You can create an instance of the Date class and assign it to a variable for later use.

Date today = new Date();
String now = today.toString();
• You can create an instance of the date object, not assign it to a variable, but call a method on it.

String now = (new Date()).toString();
• The return type of the toString() method is a String object.
• You can declare an object variable and assign a Date object to it at a later time.  But, keep in mind, it is not an object until an object (an instance of a class) is assigned to it.  Even then, it is only really a pointer to an object, not the object itself.

Date today;
today = new Date();
String now = today.toString(); // this is fine.

Date today;
today.toString();    // This will not work.  It won't even compile. At this point,
                     // today is not an object and does not even refer to an object.

today = new Date();  // Ok, now today is an object.
 

• You can set an object variable to null to indicate that it currently does not hold an object.  Then you can test for that later.


Date today = null;

// more lines of code...

if (today == null) {
    System.out.println("No date provided.");
}
 

• You can copy one date object variable to another.
• But you really are not making a copy of that object.  Both object variables are pointers to the same object in memory.
• This can get confusing.  If you need to make a physical copy, use the clone() method.  This method is part of the Object class (like toString()), which all objects extend.  Thus, it is always available.  And, it can be overridden.


Date myDate = new Date();
Date yourDate;
yourDate = myDate;     // Though legal, this will just copy the reference.
                       // There is still one and only one Date object in existence.

yourDate = (Date) myDate.clone();
                        // Now we actually have a copy of the first date object in
                        // another date object.  We use (Date) because clone()
                        // actually returns an Object type, and we must cast it to
                        // Date type.

 Date Connundrum: Class GregorianCalendar

Created by:  Brandan Jones January 4, 2002