Creating instances of abstract class DateFormat?

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KAR120C - 22 Jan 2006 18:31 GMT

I am trying to learn the requirements for the new Java Certification exam and
came across something that doesn't make sense to me. According to the JDK 1.
5 docs, java.text.DateFormat is an abstract class, yet it has a constructor,
DateFormat(). It also has a method, getDateInstance(int style), that returns
a static reference that can be used to call a non-static method as in theses
four lines from the same documentation:

To format a date for a different Locale, specify it in the call to
getDateInstance().

DateFormat df = DateFormat.getDateInstance(DateFormat.LONG, Locale.FRANCE);

You can use a DateFormat to parse also.
myDate = df.parse(myString);

How can an abstract class have a constructor and how can a static reference
be used to call a non-static method?

Thanks

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Ian Mills - 22 Jan 2006 19:14 GMT

> I am trying to learn the requirements for the new Java Certification exam and
> came across something that doesn't make sense to me. According to the JDK 1.
[quoted text clipped - 15 lines]
>
> Thanks

It is the method getDateInstance that is static and not the reference it
returns. Also why shouldn't it have a constructor. This would just be
the default constructor called by any class extending DateFormat (if you
look at Format, the supercalss of DateFormat, you will see that this
also has a constructor).

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Hendrik Maryns - 22 Jan 2006 22:19 GMT

Ian Mills uitte de volgende tekst op 01/22/2006 08:14 PM:

>> I am trying to learn the requirements for the new Java Certification
>> exam and
[quoted text clipped - 26 lines]
> look at Format, the supercalss of DateFormat, you will see that this
> also has a constructor).

I for my point would make that constructor protected. Absolutely no use
for a public one and just confusing, as illustrated by this thread.

H.

- --
Hendrik Maryns

==================
www.lieverleven.be
http://aouw.org

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KAR120C - 22 Jan 2006 19:34 GMT

I see what your saying about the the abstract class, DateFormat, having a
constructor as does its superclass, Format, but I'm still fuzzy about how
this abstract class can be instantiated. Any more thoughts?

Thanks

>I am trying to learn the requirements for the new Java Certification exam and
>came across something that doesn't make sense to me. According to the JDK 1.
[quoted text clipped - 15 lines]
>
>Thanks

Reply to this Message

Stefan Schulz - 22 Jan 2006 19:43 GMT

> I see what your saying about the the abstract class, DateFormat, having a
> constructor as does its superclass, Format, but I'm still fuzzy about how
> this abstract class can be instantiated. Any more thoughts?

Well, since you want to prepare for certification, i am a bit surprised
this point came up, but...

abstract class A {
public abstract void foo();

public static A newInstance(){
return new B();
}
}

class B extends A {
public void foo(){
System.out.println("Foo called");
}
}

See the pattern?

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KAR120C - 22 Jan 2006 20:21 GMT

In other words, the getDateInstance method is returning an unspecified
subtype of DateFormat. That makes sense. Thanks for your help.

>> I see what your saying about the the abstract class, DateFormat, having a
>> constructor as does its superclass, Format, but I'm still fuzzy about how
[quoted text clipped - 18 lines]
>
>See the pattern?

Reply to this Message

Creating instances of abstract class DateFormat?

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