Filter (search and replace) array of bytes in an InputStream

Not sure you have chosen the best approach to solve your problem.

That said, I don’t like to (and have as policy not to) answer questions with “don’t” so here goes…

Have a look at FilterInputStream.

From the documentation:

A FilterInputStream contains some other input stream, which it uses as its basic source of data, possibly transforming the data along the way or providing additional functionality.

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It was a fun exercise to write it up. Here’s a complete example for you:

import java.io.*;
import java.util.*;

class ReplacingInputStream extends FilterInputStream {

    LinkedList<Integer> inQueue = new LinkedList<Integer>();
    LinkedList<Integer> outQueue = new LinkedList<Integer>();
    final byte[] search, replacement;

    protected ReplacingInputStream(InputStream in,
                                   byte[] search,
                                   byte[] replacement) {
        super(in);
        this.search = search;
        this.replacement = replacement;
    }

    private boolean isMatchFound() {
        Iterator<Integer> inIter = inQueue.iterator();
        for (int i = 0; i < search.length; i++)
            if (!inIter.hasNext() || search[i] != inIter.next())
                return false;
        return true;
    }

    private void readAhead() throws IOException {
        // Work up some look-ahead.
        while (inQueue.size() < search.length) {
            int next = super.read();
            inQueue.offer(next);
            if (next == -1)
                break;
        }
    }

    @Override
    public int read() throws IOException {    
        // Next byte already determined.
        if (outQueue.isEmpty()) {
            readAhead();

            if (isMatchFound()) {
                for (int i = 0; i < search.length; i++)
                    inQueue.remove();

                for (byte b : replacement)
                    outQueue.offer((int) b);
            } else
                outQueue.add(inQueue.remove());
        }

        return outQueue.remove();
    }

    // TODO: Override the other read methods.
}

Example Usage

class Test {
    public static void main(String[] args) throws Exception {

        byte[] bytes = "hello xyz world.".getBytes("UTF-8");

        ByteArrayInputStream bis = new ByteArrayInputStream(bytes);

        byte[] search = "xyz".getBytes("UTF-8");
        byte[] replacement = "abc".getBytes("UTF-8");

        InputStream ris = new ReplacingInputStream(bis, search, replacement);

        ByteArrayOutputStream bos = new ByteArrayOutputStream();

        int b;
        while (-1 != (b = ris.read()))
            bos.write(b);

        System.out.println(new String(bos.toByteArray()));

    }
}

Given the bytes for the string "Hello xyz world" it prints:

Hello abc world