error with arrayList

[Bumsys]

I add file to arraylist
File file1 = new File("a.txt");
ArrayList arr = new ArrayList();
arr.add(file1);
then I want to get this file and do the following:
File filenew = (File)arr.get(0);
But I have error java.util.Arrays$ArrayList cannot be cast to
java.io.File.
Why?

 

[Donkey Hot]

(e-mail address removed) wrote in (e-mail address removed):

I add file to arraylist
File file1 = new File("a.txt");
ArrayList arr = new ArrayList();
arr.add(file1);
then I want to get this file and do the following:
File filenew = (File)arr.get(0);
But I have error java.util.Arrays$ArrayList cannot be cast to
java.io.File.
Why?

No idea, I tried it and it of course worked fine.

Are you sure you posted all the code? It looks like your arr.get(0) is not
a File, but an another ArrayList...

 

[Jensie]

I add file to arraylist
File file1 = new File("a.txt");
ArrayList arr = new ArrayList();
arr.add(file1);
then I want to get this file and do the following:
File filenew = (File)arr.get(0);
But I have error java.util.Arrays$ArrayList cannot be cast to
java.io.File.
Why?

The ClassCastException involves the (private) Arrays.ArrayList class,
which means you probably used Arrays.asList() somewhere.
Maybe you used ArrayList.add(Object) instead of
ArrayList.addAll(Collection).

If
File[] files = new File[] {new File("a.txt"), new
File("b.txt"), ...}

then

arr.add(Arrays.asList(files)) ==> adds Arrays.ArrayList to 'arr'
and raises ClassCastException in your code

arr.addAll(Arrays.asList(files)) ==> adds objects of type File
which is the intended behaviour

Define arr as List<File> to avoid this, the compiler will not allow
you to insert a collection (Arrays.ArrayList) into arr.

 

[Donkey Hot]

[...]
I'm with Donkey Hot, the syntax without pareentheses works just fine.
I think Jensie is on to the right answer.

Yeah, yeah...serves me right for making the idiotic assumption that
the code posted is the code that's actually not working.

Still, using ArrayList<File> would improve things. So I got that much
right.

Well yes, if the OP is using Java 5 or later. He did not mention. Generics
is great, and I'm waiting until my company gets it.

 

[Donkey Hot]

Is there a pre-Java 5, non-generic ArrayList class? I'd just assumed
that the only class named "ArrayList" was the actual generic
ArrayList<E> class. I don't see any non-generic version in the docs.

Sure there is. ArrayList works fine in 1.4.2

 

[Roedy Green]

I add file to arraylist
File file1 = new File("a.txt");
ArrayList arr = new ArrayList();
arr.add(file1);
then I want to get this file and do the following:
File filenew = (File)arr.get(0);
But I have error java.util.Arrays$ArrayList cannot be cast to
java.io.File.
Why?

The grown up way to handle this is to use generics. Then the casting
is automatic.

ArrayList<File> thefiles = new ArrayList<File>(20);
theFiles.add( new File( "a.txt" ) );

File got = theFiles.get(0);

If you do it the old way, you must you extra () to make it clear what
you are casting [ arr ] or [ arr.get() ].

 

[Arne Vajhøj]

Where did it go in the docs? I can't find the non-generic. Or is there
some aspect of the way classes are organized in the docs that I just
don't understand?

It did not go anywhere.

It is the same class.

In Java generics is a compile thing only - it is not in the
byte code.

Java pre-1.5:

ArrayList lst = new ArrayList();

is equivalent to Java 1.5+:

ArrayList<Object> lst = new ArrayList<Object>();

and the pre-1.5 syntax is still valid in 1.5+ (just
with a warning).

And:

ArrayList lst = new ArrayList();
ArrayList<Object> lst = new ArrayList<Object>();
ArrayList<Foobar> lst = new ArrayList<Foobar>();

all ends up as the same in the byte code.

Arne