Reflecting our own class as a Field

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tchou - 07 Nov 2006 13:06 GMT

Hello everyone,

I want to be able to dynamically load the attributes of a class Turtle.
That Turtle class have some Book attributes, Book being a class I made.

I want to load a list of those attributes but :

Field f = getClass().getField(book); // won't work.

because it is said that the getField method can't take this kind of
arguments.

The cause seems to be that Book is a class I made. How can I make
reflection work also on my own class?

I tried to implements Type and Member in my Book class, but it did not
work.

What am I doing wrong?

Thanks,
Tchouye.

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Andrew Thompson - 07 Nov 2006 13:41 GMT

...

> That Turtle class have some Book attributes, Book being a class I made.

Out of curiosity, is there supposed to be a
connection between Turtles and Books?
Do the turtles read the books? Loan them to frogs?
Or are the books merely turtle-food?

Andrew T.

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tchou - 07 Nov 2006 13:46 GMT

My application is a financial one and I found it more understandable to
replace the attributes with words that I like :-))

But it my imaginary world, turtles live vey long and have learnt to
read and are leaders in book publishing.

Any ideas to solve the problem?

Andrew Thompson a ?crit :

> ...
> > That Turtle class have some Book attributes, Book being a class I made.
[quoted text clipped - 5 lines]
>
> Andrew T.

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voorth - 07 Nov 2006 13:48 GMT

> Hello everyone,
>
[quoted text clipped - 15 lines]
>
> What am I doing wrong?

getField() expects the fields _name_. See
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Class.html#getField(java.lang.
String)

after retrieving a Field object, you can then get your Book object with
the Field.get() method.
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/reflect/Field.html#get(java.la
ng.Object)

In your case, this would be:
Field f = getClass().getField("whateverYourBookFieldisCalled");
Book b = (Book) f.get(this);

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tchou - 07 Nov 2006 14:00 GMT

As I said, that does not work for Book but it's fine for the other
attributes.

class Turtle {
Book book;
String name;
}

Field f = getClass().getField(name); //ok
Field f = getClass().getField(book); //error :

Mmm my mistake.
I gave the attribute to the getField method, instead of a STRING...
Apparently the cast was implicit for String, as it's a String but not
for Book...

Thank you very much !

voorth a ?crit :

> > What am I doing wrong?
> getField() expects the fields _name_. See
[quoted text clipped - 7 lines]
> Field f = getClass().getField("whateverYourBookFieldisCalled");
> Book b = (Book) f.get(this);

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Patricia Shanahan - 07 Nov 2006 14:24 GMT

> As I said, that does not work for Book but it's fine for the other
> attributes.
[quoted text clipped - 11 lines]
> Apparently the cast was implicit for String, as it's a String but not
> for Book...

There was no cast involved, for the first call, because getField takes a
String parameter, and name is a String. However, I'm not 100% sure you
are expecting what will happen at run time. getField will look for a
field whose identifier matches the value of the name field at the time
of the call.

Patricia

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tchou - 07 Nov 2006 14:44 GMT

Indeed, you're right. The field is not found as it takes the value of
the string.

Thank you Patricia,

Florence.

Patricia Shanahan a ?crit :

> There was no cast involved, for the first call, because getField takes a
> String parameter, and name is a String. However, I'm not 100% sure you
[quoted text clipped - 3 lines]
>
> Patricia

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Daniel Pitts - 07 Nov 2006 19:50 GMT

> Patricia Shanahan a écrit :
>
[quoted text clipped - 10 lines]
>
> Florence.

No one has thought to point out that Reflection is a dangerous path to
go down.

There is probably a better solution to what you are trying to do. Why
do you need access to the Field through reflection?

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Reflecting our own class as a Field

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