Piped streams

Example.java

import java.io.*;

public class Example {

    private InputStream reportProducer(int lines) throws IOException {
        var pin = new PipedInputStream();
        var pout = new PipedOutputStream(pin);
        var out = new PrintStream(pout);
        Runnable generator = () -> {
            for (int i = 0; i < lines; i++) {
                out.println("Generating line " + i + "...");
            }
            out.println("Done.");
            out.close();
        };
        new Thread(generator, "writer").start();
        return pin;
    }

    private void writeReport(int lines) throws IOException {
        String line;
        try (var file = new PrintStream(new FileOutputStream("/tmp/test.txt"));
            var ins = reportProducer(lines);
            var reader = new BufferedReader(new InputStreamReader(ins))) {
            while ((line = reader.readLine()) != null) {
                file.println(line);
            }
        }
    }

    public static void main(String... args) throws IOException {
        new Example().writeReport(10_000);
    }
}