Question

I want a nice way to get the current unix timestamp from a java Date object, this is my solution:

public class Date extends java.util.Date {

    public int getUnixTimeStamp() {
        int unixtimestamp = (int) (this.getTime() * .001);
        return unixtimestamp;
    }
}

That works fine, but the problem is when I try to cast a java Date object to my custom date class, i.e:

Calendar foo = Calendar.getInstance();
foo.set(0, 0, 0, 12, 30);
myapp.Date foo2 = (myapp.Date)foo.getTime();

This generates: Exception in thread “main” java.lang.ClassCastException: java.util.Date cannot be cast to myapp.Date

I understand why this exception is made but is there any way to cast a superclass to a subclass? If not, how would you implement the unixtimestamp method? I use it quite often in my application.

Answer 1

You can’t cast a superclass in a subclass because the subclass may have a bigger interface than the superclass means the subclass can have functions like getUnixTimeStamp() that the superclass doesn’t have. In your example that would mean you trying to call getUnixTimeStamp on a java.util.Date.

You can use a Delegate or Composition instead of inheritance here.

That means you have a Class

public class Date {

    private java.util.date date;

    public Date(java.util.Date date) {
        this.date = date
    }

    public int getUnixTimeStamp() {
        int unixtimestamp = (int) ((this.date.getTime()) * .001);
        return unixtimestamp;
    }

}

In this way you don’t cast the date into your date you create a class arround the java.util.date.

If you use only this function it could be an easier solution to just create a static util function getUnixTime(Date date) that returns the changed date.