How do I save and multiple objects from a single file?

问题

I'm trying to create a system where a user can choose a set of ID's(Federation IDs) to retrieve data from an external API. After the data is retrieved from the web service I wish to store it locally for later use. Now, my problem is that I know how to load an object from an ObjectInputStream by taking the entire file(federations.dat). Is there a way for me to load say "object WHERE id = N" from "federations.dat" ? Or do I have to create separate files for each object?

This is my load method:

public static Object load(Context ctx, String filename) throws FileNotFoundException 
{
    Object loadedObj = null;
    InputStream instream = null;

    instream = ctx.openFileInput(filename);

    try {
        ObjectInputStream ois = new ObjectInputStream(instream);
        loadedObj = ois.readObject();
        return loadedObj;

    } catch (StreamCorruptedException e) {
        e.printStackTrace();
        return null;
    } catch (IOException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
    return null;
}

Any suggestions come to mind ?

回答1:

An extention to @Jan 's code, fixing the problem of keeping ois open if an exception is thrown, along with some minor issues.

public static ArrayList<Object> load(Context ctx, String filename) throws FileNotFoundException {
    InputStream instream = ctx.openFileInput(filename);

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

    try {
        ObjectInputStream ois = new ObjectInputStream(instream);
        try{
            Object loadedObj = null;
            while ((loadedObj = ois.readObject()) != null) {
                objects.add(loadedObj);
            }

            return objects;
        }finally{
            ois.close();
        }

    } catch (StreamCorruptedException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
    return null;
}

回答2:

You can use it like this..

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

Object obj = null;

while ((obj = ois.readObject()) != null) {
    arrayList.add(obj);
}

You can return an ArrayList on your method.

return arrayList;

Edit: Full code would be like this..

public static ArrayList<Object> load(Context ctx, String filename) 
{
    InputStream fis = null;
    ObjectInputStream ois = null;

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

    Object loadedObj = null;
    try {
        fis = ctx.openFileInput(filename);
        ois = new ObjectInputStream(fis);

        while ((loadedObj = ois.readObject()) != null) {
             arrayList.add(loadedObj);
        }
    } catch (Exception e) {
        e.printStackTrace();
        return null;
    } finally {
        if (null != ois) ois.close();
        if (null != fis) fis.close();
    }

    return arrayList;
}

Hope it helps..

来源：https://stackoverflow.com/questions/11257667/how-do-i-save-and-multiple-objects-from-a-single-file

标签

java

android

objectinputstream