Creating instances of abstract class DateFormat?

[KAR120C]

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

 

[Ian Mills]

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

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).

 

[KAR120C]

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

 

[Stefan Schulz]

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?

 

[KAR120C]

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

 

[Hendrik Maryns]

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

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).

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

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