OutputStream that writes to a direct ByteBuffer

ByteBufferOutputStream.java

package io.github.ncruces.utils;

import java.io.OutputStream;
import java.nio.ByteBuffer;

public final class ByteBufferOutputStream extends OutputStream {
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    private ByteBuffer buf;

    public ByteBufferOutputStream() { buf = ByteBuffer.allocate(32); }

    public ByteBufferOutputStream(int size) {
        if (size < 0) throw new IllegalArgumentException("size < 0");
        buf = ByteBuffer.allocateDirect(size);
    }

    public synchronized int size() { return buf.position(); }

    public synchronized void reset() { buf.clear(); }

    public synchronized ByteBuffer toByteBuffer() {
        ByteBuffer result;

        if (buf.isDirect()) {
            result = buf.asReadOnlyBuffer();
        } else {
            result = ByteBuffer.allocateDirect(buf.flip().limit()).put(buf);
            buf.limit(buf.capacity());
        }

        result.flip();
        return result;
    }

    @Override
    public synchronized void write(int b) {
        if (buf.remaining() == 0) expand(1);
        buf.put((byte)b);
    }

    @Override
    public synchronized void write(byte[] b) {
        if (b.length == 0) return;
        if (b.length > buf.remaining()) expand(b.length);
        buf.put(b);
    }

    @Override
    public synchronized void write(byte[] b, int off, int len) {
        if ((off | len | off + len | b.length - (off + len)) < 0) {
            throw new IndexOutOfBoundsException();
        }

        if (len == 0) return;
        if (len > buf.remaining()) expand(len);
        buf.put(b, off, len);
    }

    @Override
    public void close() {}

    private void expand(int len) {
        ByteBuffer old = buf;

        int oldCapacity = old.capacity();
        int minCapacity = old.position() + len;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0) {
            if (minCapacity < 0) throw new OutOfMemoryError();
            newCapacity = minCapacity > MAX_ARRAY_SIZE ? minCapacity : MAX_ARRAY_SIZE;
        }

        buf = ByteBuffer.allocate(newCapacity);
        old.flip();
        buf.put(old);
    }
}