Creating new data types in Java

So far we've seen eight primitive data types. These have determined the type of data our programs can store and work with. We can work with integers, floating point numbers, Boolean values, Strings and others.

What if you have data you want to store that is more complex than numbers or letters? What if you want to work with a Date? You need to be able to compute the number of days in a month. And calculate if the year is a leap year. And other programmers should be able to use our code to do the same. But you want the data type to figure all that out for you. After all, we have more important things to program and don't want to start over each time.

Lucky for us Java enables us create our own Date data type. How? By creating a class. We can include fields for month, day and year. We can even include a method to display our date.

Our class would look like this:

public class DateProcessor {
    public int month = 1;
    public int day = 1;
    public int year = 1970;

    public String displayDate(){
        return month + "-" + day + "-" + year;

    }
}

And work like this:

public class StartHere {
    public static void main(String[] args){
        DateProcessor dp = new DateProcessor();
        dp.month = 5;
        dp.day = 23;
        dp.year = 1995;

        System.out.printf("Java was born on %s\n", dp.displayDate());            
    }
}

This is fabulous! Our own data type. All our problems are solved. Programmers can set the month, day and year. And return the date! Programmers everywhere can use our date type to save time displaying dates. What could go wrong?

What if ... a programmer tries to enter 360 for the month? And tries to set the date to 2/31/2015? How do we fix that?

Improving our class

A little more effort will allow us to guarantee that a date is actually a valid date. If, we include methods to check if values are valid. We can also include methods to return the number of days in the month. And whether the year is a leap year. Those methods may come in handy later.

The first thing we do is prevent the programmer from changing the month, day and year to unrealistic values. We start by setting those member variables to private. Then limit access to them by creating methods called getters and setters. These methods will validate the incoming values. Or set defaults.

After a little rework this is our new, even-more-fabulous than the last fabulous DateProcessor.

public class DateProcessor {

    private int month = 1;
    private int day = 1;
    private int year = 1970;

    public int getMonth() {
        return month;
    }

    public void setMonth(int month) {
        if (month >0 && month<=12){
            this.month = month;
        }
    }

    public int getNumberDaysInMonth() {
        // 30 days hath September, April, June and November

        // since we're defaulting to 31, we could actually skip 
        // the first 7 cases 
        int lastDay = 31;

        switch (month){
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                lastDay = 31;
                break;
            case 11:
            case 4:
            case 9:
            case 6:    
                lastDay = 30;
                break;
            case 2:    
                if (isLeapYear()){
                    lastDay = 29;
                }else{
                    lastDay = 28;
                }
                break;
        }

        return lastDay;
    }

    public boolean isLeapYear(){
        // returns t/f if the year is a leap year
        // leap years are all years divisible by 4 but not 100
        // years that are divisible by 400 are leap years, too
        // is it divisible by 4?

        // Note that the year is not passed as an argument. It could be but 
        // I choose to use the class level private variable for my method.
        boolean bLeapYear = false;
        bLeapYear = (year % 4 == 0);

        // is it divisible by 4 and not 100
        bLeapYear = bLeapYear && (year % 100 != 0);

        // is it divisible by 4 and not 100 unless it's divisible by 400
        bLeapYear = bLeapYear || (year % 400 == 0);

        return bLeapYear;
    }

    public void setDay(int day) {
        if (day <= getNumberDaysInMonth() && day >=1){
            this.day = day;
        }else{
            day = 1;
        }
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        if (year > 1 && year <=4000){
            this.year = year;
        }else{
            this.year = 1970;
        }
    }



    public String displayDate(){
        return month + "-" + day + "-" + year;

    }

}

Your assignment

Now that you've seen how it's done, create a class that has private member variables for first name, last name and age. Then create getters and setters for each.

Once you have created your classs. use it in the main the main routine of your program.
The following line should print the line below it:

System.out.printf("My full name is %s and I am %d years old.",MyClass.fullName,MyClass.age);

My full name is Joe Programmer and I am 22 years old.

Creating new data types in Java