gistfile1.txt

import android.content.Context;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.FileChannel;
import java.nio.channels.ReadableByteChannel;
import java.nio.channels.WritableByteChannel;

public class test {

    /*
    * Для работы в пределах песочницы приложения можно использовать директории, получаемые из Context'a (контекст это наша активити к примеру)
    *
    * Смотри  https://developer.android.com/reference/android/content/Context.html#getCacheDir() и ниже
    *
    *
    * для доступа к памяти устройства (вне пределов песочницы) - нужно
    * 1) Заделкарировать в манифесте приложения пермишины
    * <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    * <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    * 2) Реализовать РантаймПермишн https://developer.android.com/training/permissions/requesting
    * */
    public static void writeToFile(Context context, byte[] data) {

        try {

            File file = createFile(context.getCacheDir(), "somefile.bin");

            FileOutputStream fileOutputStream = new FileOutputStream(file);

            ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(data);

            fastCopy(byteArrayInputStream, fileOutputStream);

            byteArrayInputStream.close();

            fileOutputStream.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static byte[] readFromFile(Context context) {
        try {
            File file = createFile(context.getCacheDir(), "somefile.bin");
            if (!file.exists()) {
                return null;
            }

            FileInputStream fileInputStream = new FileInputStream(file);
            ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
            fastCopy(fileInputStream, byteArrayOutputStream);

            fileInputStream.close();
            return byteArrayOutputStream.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null;
    }

    public static File createFile(File dir, String filename) {
        if (!dir.exists())
            dir.mkdirs();
        return new File(dir, filename);
    }

    public static void copy(File src, File dst) throws IOException {
        if (dst.exists()) {
            dst.delete();
            dst.createNewFile();
        }
        FileInputStream inStream = new FileInputStream(src);
        FileOutputStream outStream = new FileOutputStream(dst);
        FileChannel inChannel = inStream.getChannel();
        FileChannel outChannel = outStream.getChannel();
        inChannel.transferTo(0, inChannel.size(), outChannel);
        inStream.close();
        outStream.close();
    }

    public static void fastCopy(final InputStream src, final OutputStream dest) throws IOException {
        final ReadableByteChannel inputChannel = Channels.newChannel(src);
        final WritableByteChannel outputChannel = Channels.newChannel(dest);
        fastCopy(inputChannel, outputChannel);
        inputChannel.close();
        outputChannel.close();
    }

    public static void fastCopy(final ReadableByteChannel src, final WritableByteChannel dest) throws IOException {
        final ByteBuffer buffer = ByteBuffer.allocateDirect(16 * 1024);

        while (src.read(buffer) != -1) {
            buffer.flip();
            dest.write(buffer);
            buffer.compact();
        }

        buffer.flip();

        while (buffer.hasRemaining()) {
            dest.write(buffer);
        }
    }
}