Date class

1. Date class and Unix time

From the very beginning of Java, the language has had a special class for working with times and dates — Date. Over time, several more classes for working with dates appeared, but programmers continue to use the Date class even today.

This is because it is very simple and convenient. And as a programmer, you will definitely encounter it in any real project. Whether to use it or not is your choice, but you must know it. So give it a few minutes of your precious time.

What makes the Date class so good? Its simplicity.

The Date class stores date and time information as the number of milliseconds that have elapsed since January 1, 1970. That's is a lot of milliseconds, so the long type is used to store them.

But you can very easily find out how much time has passed between two dates: just subtract one date from the other, and you get the time difference between the dates to the nearest millisecond.

Here are some useful examples of working with the Date class.

2. Getting the current date

To get the current time and date, simply create a Date object. Each new object stores the time when it was created. It looks very simple:

Date current = new Date();

After executing this command, the current variable will store a reference to the created Date object, which will internally store the time of its creation — the number of milliseconds that have elapsed since January 1, 1970.

Displaying the current date on the screen

To display the current date, simply: a) create a new Date object, b) print it to the screen using the System.out.println() method.

Example:

Date current = new Date();
System.out.println(current);

Thu Feb 21 14:01:34 EET 2019

Here's what the console output means:

3. Setting a specific date

We figured out how to get the current time, but how do we create a Date object that stores a different date or time?

Again, everything is simple here. To set a specific date, you need to write code like this:

Date birthday = new Date(year, month, day);

Everything is simple and obvious, but there are two nuances to be aware of:

1. The year must be counted from 1900.
2. Months are numbered from zero.

For example, I was born on March 21, 1989. March is the third month, so I need to write:

Date current = new Date(89, 2, 21);
System.out.println(current);

Tue Mar 21 00:00:00 EET 1989

Months are numbered from zero, but days are not That's a little strange, right?

We think true programmers should have prevailed and numbered the days of the month from zero too. Oh, these conformists 🙂

Setting a specific time

Setting a specific time is also quite simple: for this, you need to write a statement like this:

Date birthday = new Date(year, month, day, hour, minutes, seconds);

Hours, minutes and seconds are numbered from zero: let the programmer inside you breathe a sigh of relief.

Example:

Date current = new Date(105, 5, 4, 12, 15, 0);
System.out.println(current);

Sat Jun 04 12:15:00 EEST 2005

We set the time 12:15 and the date June 4, 2005. This is a little difficult for a non-programmer to read, but it works as it should.

4. Working with elements of a date

You can do more with a Date object than just display it. It has methods that let you get individual elements of the internally stored date:

int getYear()

int getMonth()

int getDate()

int getDay()

int getHours()

int getMinutes()

int getSeconds()

Example:

Date current = new Date(105, 5, 4, 12, 15, 0);
System.out.println(current.getYear());
System.out.println(current.getMonth());
System.out.println(current.getDate());
System.out.println(current.getDay());

105
5
4
6

By the way, a Date object not only lets you get individual elements of a date, but also change them:

5. Milliseconds

As we said previously, the Date object stores the number of milliseconds that have elapsed since January 1, 1970.

If we need that number, we can get it from the Date object:

long time = date.getTime();

The getTime() method returns the number of milliseconds stored inside the Date object.

Not only can you get the number of milliseconds, but you can also change that number in an existing object:

Date date = new Date();
date.setTime(1117876500000L);

By the way, you can write this even more concisely by passing the time to the Date object right when it is created:

Date date = new Date(1117876500000L);

6. Comparing dates

If you want to compare two dates and find out which one came first, you have three options

The first way is to simply compare the number of milliseconds they each store:

if (date1.getTime() < date2.getTime())

The second way is to use the before() method of a Date object:

if (date1.before(date2))

It reads like this: if date1 comes before date2, then...

The third way is to use the after() method of a Date object:

if (date2.after(date1))

It reads like this: if date2 is after date1, then...

7. DateFormat class

Remember, when we displayed the date on the screen, we saw something like this: Thu Feb 21 14:01:34 EET 2019. Everything seems correct, but the format is more how a date would be displayed for programmers rather than normal humans. We want to display the date more clearly for users. Something like Tuesday, February 21.

And without the year. Or with a year, if necessary. In general, we want to be able to display the date in different ways.

There is a special class for this: SimpleDateFormat.

Example:

Date current = new Date(105, 5, 4, 12, 15, 0);
SimpleDateFormat formatter = new SimpleDateFormat("MMM-dd-YYYY");
String message = formatter.format(current);
System.out.println(message);

Jun-04-2005

Look at what the program displayed: Jun-04-2005. That's not at all what it used to be.

The difference is that we displayed not the Date object itself, but a special string obtained by calling the format() method on a SimpleDateFormat object. But that's not even the main point here.

When the SimpleDateFormat object was created, we passed in the string "MMM-dd-YYYY" as a parameter. It is this string that conveys the date format that we ultimately saw in the console output.

• MMM indicates to display the name of the month, abbreviated using three letters
• dd indicates to display the day of the month
• YYYY indicates to display the year using four digits

If we want to output the month as numbers, then instead of MMM, we need to write MM, which yields the pattern "MM-dd-YYYY". The screen output will be 06-04-2005

We will discuss this class in more detail later.

8. Date.parse method

The Date class can do something else interesting and useful — it can get a date from a string. Or, as programmers say, it can parse a string.

It has the special parse() method for this. Parsing looks like this:

Date date = new Date();
date.setTime( Date.parse("Jul 06 12:15:00 2019") );

By the way, this code can be written more compactly:

Date date = new Date("Jul 06 12:15:00 2019");

We'll consider the parsing of strings in more detail in other lessons.