Trying out different I/O streams in Java.

IOStream.java

import java.io.*;

import static java.lang.System.out;

public class Main {
    public static void main(String[] args) {
        try (var in = new FileInputStream("hello.txt")) {
            var buf = new byte[4];
            int n = in.read(buf);
            while (n != -1) {
                out.write(buf, 0, n);
                n = in.read(buf);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

        out.println();
        final var m = new MyData(new int[]{'a', '1', 'P', 'z', 'S', '2', '9', 'b'});
        try (var in = m.toInputStream()) {
            var buf = new byte[4];
            int n = in.read(buf);
            while (n != -1) {
                out.write(buf, 0, n);
                n = in.read(buf);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

        out.println();

        try (var ignored = new MyResource()) {
            out.println("created resource");
        }

        try (var fout = new FileOutputStream("abc.txt");
             var out = new CustomOutputFilter(fout);
             var in = m.toInputStream()) {

            var buf = new byte[4];
            int n = in.read(buf);
            while (n != -1) {
                out.write(buf, 0, n);
                n = in.read(buf);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

class CustomOutputFilter extends FilterOutputStream {
    public CustomOutputFilter(OutputStream out) {
        super(out);
    }

    @Override
    public void write(int b) throws IOException {
        if (b >= '0' && b <= '9') {
            super.write(b);
        } else {
            super.write('_');
        }
    }
}

class MyData {
    int[] data;

    public MyData(int[] data) {
        this.data = data;
    }

    public InputStream toInputStream() {
        return new MyDataInputStream(this);
    }

    private static class MyDataInputStream extends InputStream {
        MyData myData;
        int cur = 0;

        public MyDataInputStream(MyData myData) {
            this.myData = myData;
        }

        @Override
        public int read() {
            if (cur < myData.data.length) {
                return myData.data[cur++];
            }
            return -1;
        }
    }
}

class MyResource implements Closeable, AutoCloseable {
    @Override
    public void close() {
        out.println("I'm closing!");
    }
}