Java compiler warning caused by generics when saving ArrayLists [duplicate]

Question

This question already has answers here:

How do I address unchecked cast warnings? (23 answers)

Closed 8 years ago.

This seems like a pretty standard problem, but it's just so simple I don't know what to do about it. From what I've read elsewhere on SO, this is caused by Generics when ArrayLists are initialized without specifying parameters. I also know that the warning does not come up when I comment out ArrayList<Integer> p = (ArrayList<Integer>)is.readObject();, so I'm pretty sure there is a generics safety issue with the compiler by me trying to cast the Object to an ArrayList<Integer>. How can I save the ArrayList without causing this?

Here's the code:

import java.util.*;
import java.io.*;

public class FileTest
{
    public static void main(String[] args)
    {
        String fileName = "fred.txt";
        ArrayList<Integer> a = new ArrayList<Integer>();
        a.add(2);
        a.add(3);
        a.add(4);
        try
        {
            ObjectOutputStream os = new ObjectOutputStream(new FileOutputStream(fileName));
            os.writeObject(a);
            os.close();
        }
        catch(FileNotFoundException e){e.printStackTrace();}
        catch(IOException e){e.printStackTrace();}
        try
        {
            ObjectInputStream is = new ObjectInputStream(new FileInputStream(fileName));
            ArrayList<Integer> p = (ArrayList<Integer>)is.readObject();
            //for(int i : p) System.out.println(i);
        }
        catch(FileNotFoundException e){e.printStackTrace();}
        catch(IOException e){e.printStackTrace();}
        catch(ClassNotFoundException e){e.printStackTrace();}
    }
}

Answer 1

I'm pretty sure there is a generics safety issue with the compiler by me trying to cast the Object to an ArrayList.

Not just a Generics safety issue, a type safety issue. You are assuming that the next thing is an ArrayList and that you can safely use it as an ArrayList<Integer>.

How can I save the ArrayList without causing this?

Your problem isn't with saving it, it is with getting it back. Add @SuppressWarnings("unchecked") or whatever it is before the declaration.