File Util's. Set of basic static final methods for working with the file system on Android OS.

FileUtils.java

import java.io.File;

import android.annotation.TargetApi;
import android.content.Context;
import android.os.Build;
import android.os.Environment;
import android.util.Log;

/**
 * File Util's.
 * 
 * @author Evgeny Shishkin
 * 
 */
public class FileUtils {
    private static final String LOG_TAG = FileUtils.class.getSimpleName();

    /**
     * Invisible constructor.
     */
    private FileUtils() {
    }

    /**
     * Returns application files directory. Files directory will be created on SD card 
     * <i>("/Android/data/[app_package_name]/files")</i> if card is mounted.
     * Else - Android defines files directory on device's file system.
     * 
     * @param context Application context
     * @return Files {@link File directory}
     */
    public static File getFilesDir(Context context) {
        File filesDir = null;
        if (isExternalStorageWritable()) {
            filesDir = getExternalFilesDir(context);
        }
        if (filesDir == null) {
            filesDir = getInternalFilesDir(context);
        }
        return filesDir;
    }

    /**
     * Returns application cache directory. Cache directory will be created on SD card 
     * <i>("/Android/data/[app_package_name]/cache")</i> if card is mounted.
     * Else - Android defines cache directory on device's file system.
     * 
     * @param context Application context
     * @return Cache {@link File directory}
     */
    public static File getCacheDir(Context context) {
        File cacheDir = null;
        if (isExternalStorageWritable()) {
            cacheDir = getExternalCacheDir(context);
        }
        if (cacheDir == null) {
            cacheDir = getInternalCacheDir(context);
        }
        return cacheDir;
    }

    /**
     * Returns the absolute path to the directory on the external filesystem 
     * (that is somewhere on Environment.getExternalStorageDirectory()) where the
     * application can place persistent files it owns.
     * 
     * @param context
     * @return Returns the path of the directory holding application files on external storage. 
     * Returns null if external storage is not currently mounted so it
     * could not ensure the path exists; you will need to call this method again when it is available.
     */
    @TargetApi(Build.VERSION_CODES.FROYO)
    private static File getExternalFilesDir(Context context) {
        File filesDir;
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.FROYO) {
            filesDir = new File(Environment.getExternalStorageDirectory().getPath() + "/Android/data/" + context.getPackageName() + "/files/");
            if (!filesDir.exists() && !filesDir.mkdirs()) {
                Log.w(LOG_TAG, "Unable to create external files directory");
                return null;
            }
        } else {
            filesDir = context.getExternalFilesDir(null);
        }
        return filesDir;
    }

    /**
     * Returns the absolute path to the directory on the external filesystem 
     * (that is somewhere on Environment.getExternalStorageDirectory() where the
     * application can place cache files it owns.
     * 
     * @param context
     * @return Returns the path of the directory holding application cache files on external storage. 
     * Returns null if external storage is not currently mounted
     * so it could not ensure the path exists; you will need to call this method again when it is available.
     */
    @TargetApi(Build.VERSION_CODES.FROYO)
    private static File getExternalCacheDir(Context context) {
        File cacheDir;
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.FROYO) {
            cacheDir = new File(Environment.getExternalStorageDirectory().getPath() + "/Android/data/" + context.getPackageName() + "/cache/");
            if (!cacheDir.exists() && !cacheDir.mkdirs()) {
                Log.w(LOG_TAG, "Unable to create external cache directory");
                return null;
            }
        } else {
            cacheDir = context.getExternalCacheDir();
        }
        return cacheDir;
    }

    /**
     * Returns the absolute path to the directory on the filesystem where
     * files created with {@link #openFileOutput} are stored.
     *
     * @param context
     * @return Returns the path of the directory holding application files on internal storage.
     */
    private static File getInternalFilesDir(Context context) {
        File filesDir = context.getFilesDir();
        return filesDir;
    }

    /**
     * Returns the absolute path to the application specific cache directory
     * on the internal filesystem.
     *
     * @param context
     * @return Returns the path of the directory holding application cache files on internal storage.
     */
    private static File getInternalCacheDir(Context context) {
        File cacheDir = context.getCacheDir();
        return cacheDir;
    }

    /**
     * Checks if external storage is available for read and write.
     * 
     * @return true if external storage is available for read and write else false
     */
    public static boolean isExternalStorageWritable() {
        String state = Environment.getExternalStorageState();
        if (Environment.MEDIA_MOUNTED.equals(state)) {
            return true;
        }
        return false;
    }

    /**
     * Checks if external storage is available to at least read.
     * 
     * @return true if external storage is available to at least read else false
     */
    public static boolean isExternalStorageReadable() {
        String state = Environment.getExternalStorageState();
        if (Environment.MEDIA_MOUNTED.equals(state) || Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
            return true;
        }
        return false;
    }

}

I would recommend:

• Using @TargetApi(NN) (with an appropriate value of NN) instead of @SuppressLint("NewApi")
• Not mixing your code that requires Apache Commons IO with the rest of the code (as people will grab your gist and get a bunch of compile errors, then perhaps lose trust in the rest of your code)

Thanks for very useful comments!