Saving ArrayList to File

[nwboy74]

Is it possible to write an ArrayList holding an unknown type to a file and read it back later? Say I have an ArrayList of apples and and ArrayList of thumbtacks. I know it's either one or the other. Apples and thumbtacks don't have a common ancestor besides Object. Rather than write a method to handle each type, I'd like to just have one method that will handle any type. I've read about generics, but nothing I do is working. Apple and ThumbTack are both Serializable.

import java.util.*;
import java.io.*;
...
   ArrayList<Apple> apples = new ArrayList<Apple>();
   //[I]populate apples here[/I]
   handleList(apples, "somefile", true); //[I]no warning[/I]
   ArrayList<ThumbTack> thumbtacks = new ArrayList<ThumbTack>();
   //[I]populate thumbtacks here[/I]
   handleList(thumbtacks, "someotherfile", true); //[I]no warning[/I]
...
   ArrayList<Apple> otherApples = new ArrayList<Apple>();
   handleList(otherApples, "somefile", false); //[I]unchecked cast warning[/I]
...
private <T> void handleList(ArrayList<T> list, String filename, boolean save) {
   File f = new File(filename);
   if (f.exists()) {
      if (save) {
         try {
            FileOutputStream fos = new FileOutputStream(f);
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(list);
            fos.close();
         } catch (IOException ioe) { }
      } else {
         try {
            FileInputStream fis = new FileInputStream(f);
            ObjectInputStream ois = new ObjectInputStream(fis);
            try {
               [B]list = (ArrayList<T>) ois.readObject();[/B] //[I]bad monkey[/I]
            } catch (ClassNotFoundException cnfe) { }
            fis.close();
         } catch (IOException ioe) { }
      }
   }
}

Code is just an example to hopefully show you what I'm trying to do. I understand that there's no way to know when I read a file that it will be of the type I'm looking for and could be an unsafe cast. But how do I fix it? Or is it just a warning and I can believe that my application will only be dealing with known types and ignore the warning?

[Eranga]

What unchecked cast warning you get? Seems to me there is no any warnings.

[nwboy74]

Here is the warning I get from the compiler:

warning: [unchecked] unchecked cast
found : java.lang.Object
required: java.util.ArrayList<T>

It points to the bold line in my code example.

[Eranga]

I'm still not find any error there, I've test that code on my IDE and compiled fine. No warning either.

[w3user]

remove all instances of <T> from your code

[Fubarable]

It's a warning, not an error. You can either just ignore it or use the proper annotation so that the compiler won't complain. This works for me:

@SuppressWarnings("unchecked")
ArrayList myList = new ArrayList();

But having said that, I don't like your idea. An ArrayList should know what it is holding be that a class type or an interface type else you're program can have nasty surprises down the line.

[Eranga]

Ya, I think you should read little more about annotations in Java. So you can have clear idea about warnings.