How do you merge two input streams in Java?

Question

As commented, it's not clear what you mean by merge.

Taking available input "randomly" from either is complicated by InputStream.available not necessarily giving you a useful answer and blocking behaviour of streams. You would need two threads to be reading from the streams and then passing back data through, say, java.io.Piped(In|Out)putStream (although those classes have issues). Alternatively for some types of stream it may be possible to use a different interface, for instance java.nio non-blocking channels.

If you want the full contents of the first input stream followed by the second: new java.io.SequenceInputStream(s1, s2).

java.io.SequenceInputStream might be what you need. It accepts an enumeration of streams, and will output the contents of the first stream, then the second, and so on until all streams are empty.

You can write a custom InputStream implementation that does this. Example:

import java.io.IOException;
import java.io.InputStream;
import java.util.Collections;
import java.util.Deque;
import java.util.LinkedList;

public class CatInputStream extends InputStream {
    private final Deque<InputStream> streams;

    public CatInputStream(InputStream... streams) {
        this.streams = new LinkedList<InputStream>();
        Collections.addAll(this.streams, streams);
    }

    private void nextStream() throws IOException {
        streams.removeFirst().close();
    }

    @Override
    public int read() throws IOException {
        int result = -1;
        while (!streams.isEmpty()
                && (result = streams.getFirst().read()) == -1) {
            nextStream();
        }
        return result;
    }

    @Override
    public int read(byte b[], int off, int len) throws IOException {
        int result = -1;
        while (!streams.isEmpty()
                && (result = streams.getFirst().read(b, off, len)) == -1) {
            nextStream();
        }
        return result;
    }

    @Override
    public long skip(long n) throws IOException {
        long skipped = 0L;
        while (skipped < n && !streams.isEmpty()) {
            int thisSkip = streams.getFirst().skip(n - skipped);
            if (thisSkip > 0)
                skipped += thisSkip;
            else
                nextStream();
        }
        return skipped;
    }

    @Override
    public int available() throws IOException {
        return streams.isEmpty() ? 0 : streams.getFirst().available();
    }

    @Override
    public void close() throws IOException {
        while (!streams.isEmpty())
            nextStream();
    }
}

This code isn't tested, so your mileage may vary.