Store HashMap on SD card

I want to store the HashMap object in a file on SD card. So, that the same object is used again on application restart. I have no idea how to do this, please provide me a brief summary.

For example, I want the LazyList of images that is saved in HashMap to be stored in a file on SD card. So, that images are downloaded once and never need to be downloaded again.

I am including the LazyList code where I need to implement such scenario:

package com.gogozing.util;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.OutputStream;
import java.net.URL;
import java.util.HashMap;
import java.util.Stack;

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.os.Environment;
import android.widget.ImageView;

import com.abc.app.R;

public class ImageLoader {

    // the simplest in-memory cache implementation. This should be replaced with
    // something like SoftReference or BitmapOptions.inPurgeable(since 1.6)
    private HashMap<String, Bitmap> cache = new HashMap<String, Bitmap>();

    private File cacheDir;

    public ImageLoader(Context context) {
        // Make the background thead low priority. This way it will not affect
        // the UI performance
        photoLoaderThread.setPriority(Thread.NORM_PRIORITY - 1);

        //String path = Environment.getExternalStorageDirectory().toString();
        //OutputStream fOut = null;
        //File cacheDir = new File(path, "LazyList");

        // try {
        // fOut = new FileOutputStream(cacheDir);
        // } catch (FileNotFoundException e) {
        // // TODO Auto-generated catch block
    开发者_运维知识库    // e.printStackTrace();
        // }
        // Find the dir to save cached images

        if (android.os.Environment.getExternalStorageState().equals(
                android.os.Environment.MEDIA_MOUNTED))
            cacheDir = new File(
                    android.os.Environment.getExternalStorageDirectory(),
                    "LazyList");
        else
            cacheDir = context.getCacheDir();
        if (!cacheDir.exists())
            cacheDir.mkdirs();
    }

    final int stub_id = R.drawable.no_image;

    public void DisplayImage(String url, Activity activity, ImageView imageView) {

        if (cache.containsKey(url))
            imageView.setImageBitmap(cache.get(url));
        else {
            queuePhoto(url, activity, imageView);
            imageView.setImageResource(stub_id);
        }
    }

    private void queuePhoto(String url, Activity activity, ImageView imageView) {
        // This ImageView may be used for other images before. So there may be
        // some old tasks in the queue. We need to discard them.
        photosQueue.Clean(imageView);
        PhotoToLoad p = new PhotoToLoad(url, imageView);
        synchronized (photosQueue.photosToLoad) {
            photosQueue.photosToLoad.push(p);
            photosQueue.photosToLoad.notifyAll();
        }

        // start thread if it's not started yet
        if (photoLoaderThread.getState() == Thread.State.NEW)
            photoLoaderThread.start();
    }

    private Bitmap getBitmap(String url) {
        // // I identify images by hashcode. Not a perfect solution, good for
        // the
        // // demo.
        // String filename = String.valueOf(url.hashCode());
        // File f = new File(cacheDir, filename);
        //
        // // from SD cache
        // Bitmap b = decodeFile(f);
        // if (b != null)
        // return b;

        // from web
        try {
            return new BitmapDrawable(new URL(url).openStream()).getBitmap();
        } catch (Exception ex) {
            ex.printStackTrace();
            cache.clear();
            return null;
        }
    }

    // decodes image and scales it to reduce memory consumption
    private Bitmap decodeFile(File f) {
        try {
            // decode image size
            BitmapFactory.Options o = new BitmapFactory.Options();
            o.inJustDecodeBounds = true;
            BitmapFactory.decodeStream(new FileInputStream(f), null, o);

            // Find the correct scale value. It should be the power of 2.
            final int REQUIRED_SIZE = 70;
            int width_tmp = o.outWidth, height_tmp = o.outHeight;
            int scale = 1;
            while (true) {
                if (width_tmp / 2 < REQUIRED_SIZE
                        || height_tmp / 2 < REQUIRED_SIZE)
                    break;
                width_tmp /= 2;
                height_tmp /= 2;
                scale++;
            }

            // decode with inSampleSize
            BitmapFactory.Options o2 = new BitmapFactory.Options();
            o2.inSampleSize = scale;
            return BitmapFactory.decodeStream(new FileInputStream(f), null, o2);
        } catch (FileNotFoundException e) {

        }

        return null;
    }

    // Task for the queue
    private class PhotoToLoad {
        public String url;
        public ImageView imageView;

        public PhotoToLoad(String u, ImageView i) {
            url = u;
            imageView = i;
        }
    }

    PhotosQueue photosQueue = new PhotosQueue();

    public void stopThread() {
        photoLoaderThread.interrupt();
    }

    // stores list of photos to download
    class PhotosQueue {
        private Stack<PhotoToLoad> photosToLoad = new Stack<PhotoToLoad>();

        // removes all instances of this ImageView
        public void Clean(ImageView image) {
            for (int j = 0; j < photosToLoad.size();) {
                if (photosToLoad.get(j).imageView == image)
                    photosToLoad.remove(j);
                else
                    ++j;
            }
        }
    }

    class PhotosLoader extends Thread {
        public void run() {
            try {
                while (true) {
                    // thread waits until there are any images to load in the
                    // queue
                    if (photosQueue.photosToLoad.size() == 0)
                        synchronized (photosQueue.photosToLoad) {
                            photosQueue.photosToLoad.wait();
                        }
                    if (photosQueue.photosToLoad.size() != 0) {
                        PhotoToLoad photoToLoad;
                        synchronized (photosQueue.photosToLoad) {
                            photoToLoad = photosQueue.photosToLoad.pop();
                        }
                        Bitmap bmp = getBitmap(photoToLoad.url);
                        cache.put(photoToLoad.url, bmp);

                        if (photoToLoad.url == null) {

                        } else {
                            if (((String) photoToLoad.imageView.getTag())
                                    .equals(photoToLoad.url)) {
                                BitmapDisplayer bd = new BitmapDisplayer(bmp,
                                        photoToLoad.imageView);
                                Activity a = (Activity) photoToLoad.imageView
                                        .getContext();
                                a.runOnUiThread(bd);
                            }

                        }
                    }
                    if (Thread.interrupted())
                        break;
                }
            } catch (InterruptedException e) {
                // allow thread to exit
            }
        }
    }

    PhotosLoader photoLoaderThread = new PhotosLoader();

    // Used to display bitmap in the UI thread
    class BitmapDisplayer implements Runnable {
        Bitmap bitmap;
        ImageView imageView;

        public BitmapDisplayer(Bitmap b, ImageView i) {
            bitmap = b;
            imageView = i;
        }

        public void run() {
            if (bitmap != null)
                imageView.setImageBitmap(bitmap);
            else
                imageView.setImageResource(stub_id);
        }
    }

    public void clearCache() {
        // clear memory cache
        cache.clear();

        // clear SD cache
        File[] files = cacheDir.listFiles();
        for (File f : files)
            f.delete();
    }

}

All objects in the HashMap must be Serializable, then you should be able to just serialize the map with something like

try {
  ObjectOutputStream out = new ObjectOutputStream(...);
  out.writeObject(yourMap);
  out.close();
} catch(IOException ex) {
  ...
}

However you have bitmaps in your map, and bitmaps makes little sense to serialize like that. You should rather just save your jpg/png/etc files to the phone memory or sd-card, and read them from there on the next application start.