Easy way to write contents of a Java InputStream to an OutputStream

As WMR mentioned, org.apache.commons.io.IOUtils from Apache has a method called copy(InputStream,OutputStream) which does exactly what you're looking for.

So, you have:

InputStream in;
OutputStream out;
IOUtils.copy(in,out);
in.close();
out.close();

...in your code.

Is there a reason you're avoiding IOUtils?

I think this will work, but make sure to test it... minor "improvement", but it might be a bit of a cost at readability.

byte[] buffer = new byte[1024];
int len;
while ((len = in.read(buffer)) != -1) {
    out.write(buffer, 0, len);
}

The JDK uses the same code so it seems like there is no "easier" way without clunky third party libraries (which probably don't do anything different anyway). The following is directly copied from java.nio.file.Files.java:

// buffer size used for reading and writing
private static final int BUFFER_SIZE = 8192;

/**
  * Reads all bytes from an input stream and writes them to an output stream.
  */
private static long copy(InputStream source, OutputStream sink) throws IOException {
    long nread = 0L;
    byte[] buf = new byte[BUFFER_SIZE];
    int n;
    while ((n = source.read(buf)) > 0) {
        sink.write(buf, 0, n);
        nread += n;
    }
    return nread;
}

PipedInputStream and PipedOutputStream should only be used when you have multiple threads, as noted by the Javadoc.

Also, note that input streams and output streams do not wrap any thread interruptions with IOExceptions... So, you should consider incorporating an interruption policy to your code:

byte[] buffer = new byte[1024];
int len = in.read(buffer);
while (len != -1) {
    out.write(buffer, 0, len);
    len = in.read(buffer);
    if (Thread.interrupted()) {
        throw new InterruptedException();
    }
}

This would be an useful addition if you expect to use this API for copying large volumes of data, or data from streams that get stuck for an intolerably long time.

There's no way to do this a lot easier with JDK methods, but as Apocalisp has already noted, you're not the only one with this idea: You could use IOUtils from Jakarta Commons IO, it also has a lot of other useful things, that IMO should actually be part of the JDK...

Use Commons Net's Util class:

import org.apache.commons.net.io.Util;
...
Util.copyStream(in, out);

I use BufferedInputStream and BufferedOutputStream to remove the buffering semantics from the code

try (OutputStream out = new BufferedOutputStream(...);
     InputStream in   = new BufferedInputStream(...))) {
  int ch;
  while ((ch = in.read()) != -1) {
    out.write(ch);
  }
}

This is my best shot!!

And do not use inputStream.transferTo(...) because is too generic. Your code performance will be better if you control your buffer memory.

public static void transfer(InputStream in, OutputStream out, int buffer) throws IOException {
    byte[] read = new byte[buffer]; // Your buffer size.
    while (0 < (buffer = in.read(read)))
        out.write(read, 0, buffer);
}

I use it with this (improvable) method when I know in advance the size of the stream.

public static void transfer(int size, InputStream in, OutputStream out) throws IOException {
    transfer(in, out,
            size > 0xFFFF ? 0xFFFF // 16bits 65,536
                    : size > 0xFFF ? 0xFFF// 12bits 4096
                            : size < 0xFF ? 0xFF // 8bits 256
                                    : size
    );
}

Not very readable, but effective, has no dependencies and runs with any java version

byte[] buffer=new byte[1024];
for(int n; (n=inputStream.read(buffer))!=-1; outputStream.write(buffer,0,n));

PipedInputStream and PipedOutputStream may be of some use, as you can connect one to the other.

I used ByteStreamKt.copyTo(src, dst, buffer.length) method

Here is my code

public static void replaceCurrentDb(Context context, Uri newDbUri) {
    try {
        File currentDb = context.getDatabasePath(DATABASE_NAME);
        if (currentDb.exists()) {
            InputStream src = context.getContentResolver().openInputStream(newDbUri);
            FileOutputStream dst = new FileOutputStream(currentDb);
            final byte[] buffer = new byte[8 * 1024];
            ByteStreamsKt.copyTo(src, dst, buffer.length);
            src.close();
            dst.close();
            Toast.makeText(context, "SUCCESS! Your selected file is set as current menu.", Toast.LENGTH_LONG).show();
        }
        else
            Log.e("DOWNLOAD:::: Database", " fail, database not found");
    }
    catch (IOException e) {
        Toast.makeText(context, "Data Download FAIL.", Toast.LENGTH_LONG).show();
        Log.e("DOWNLOAD FAIL!!!", "fail, reason:", e);
    }
}

Try Cactoos:

new LengthOf(new TeeInput(input, output)).value();

More details here: http://www.yegor256.com/2017/06/22/object-oriented-input-output-in-cactoos.html