Pequena alternativa para responder como input stream uma coleção de elementos

ServeInputStream.java

import java.io.IOException;
import java.io.InputStream;
import java.util.Iterator;
import java.util.function.Function;

public class ServeInputStream<T> extends InputStream {

    private final Iterator<T> iterator;
    private final Function<T, byte[]> serializer;

    private byte[] lastElementSerialized;
    private int lastElementSize = -1;
    private int readHead = -1;
    private boolean valid = true;

    public ServeInputStream(Iterable<T> iterable, Function<T, byte[]> serializer) {
        this.iterator = iterable.iterator();
        this.serializer = serializer;
    }

    @Override
    public int read() throws IOException {
        ensureFill();
        if (!valid) {
            return -1;
        }
        int r = lastElementSerialized[readHead];
        readHead++;
        return r;
    }

    @Override
    public int read(byte[] b, int off, int len) throws IOException {
        ensureFill();
        if (!valid) {
            return -1;
        }
        final int maxPossibleBytesRead = Math.min(len, lastElementSize - readHead);
        System.arraycopy(lastElementSerialized, readHead, b, off, maxPossibleBytesRead);
        readHead += maxPossibleBytesRead;
        return maxPossibleBytesRead;
    }

    private void ensureFill() {
        if (!valid) {
            return;
        }
        if (readHead < lastElementSize) {
            return;
        }
        if (!iterator.hasNext()) {
            valid = false;
            readHead = lastElementSize = -1;
            return;
        }
        lastElementSerialized = serializer.apply(iterator.next());
        readHead = 0;
        lastElementSize = lastElementSerialized.length;
    }
}