PrintWriter

// ── Writer-based constructors ─────────────────────────────────────────
// No buffering added — FileWriter is unbuffered, every write hits OS:
PrintWriter pw1 = new PrintWriter(new FileWriter("out.txt", StandardCharsets.UTF_8));

// Correct: wrap FileWriter in BufferedWriter first:
PrintWriter pw2 = new PrintWriter(
    new BufferedWriter(new FileWriter("out.txt", StandardCharsets.UTF_8)));

// With autoFlush — println/printf/format trigger flush automatically:
PrintWriter pw3 = new PrintWriter(
    new BufferedWriter(new FileWriter("out.txt", StandardCharsets.UTF_8)), true);

// ── OutputStream-based constructors ──────────────────────────────────
// Internally creates OutputStreamWriter(platform charset) + BufferedWriter:
PrintWriter pw4 = new PrintWriter(System.out);                      // stdout
PrintWriter pw5 = new PrintWriter(System.out, true);                // autoFlush
PrintWriter pw6 = new PrintWriter(new FileOutputStream("out.txt")); // file

// Java 10+: explicit charset:
PrintWriter pw7 = new PrintWriter(
    new FileOutputStream("out.txt"), true, StandardCharsets.UTF_8);

// ── File/String constructors ──────────────────────────────────────────
try {
    // Opens file, creates if absent, truncates if exists:
    PrintWriter pw8  = new PrintWriter("output.txt");                      // default charset
    PrintWriter pw9  = new PrintWriter(new File("output.txt"));            // default charset
    PrintWriter pw10 = new PrintWriter("output.txt", "UTF-8");             // explicit charset
    PrintWriter pw11 = new PrintWriter(new File("output.txt"), "UTF-8");   // explicit charset
    // Java 10+:
    PrintWriter pw12 = new PrintWriter("output.txt", StandardCharsets.UTF_8); // Charset object
} catch (FileNotFoundException e) {
    System.err.println("Cannot open file: " + e.getMessage());
}

// ── Preferred modern construction ─────────────────────────────────────
// Most explicit: BufferedWriter from Files, explicit charset, explicit options
try (PrintWriter pw = new PrintWriter(Files.newBufferedWriter(
        Path.of("output.txt"),
        StandardCharsets.UTF_8,
        StandardOpenOption.CREATE,
        StandardOpenOption.TRUNCATE_EXISTING))) {
    pw.println("UTF-8 with explicit options");
}

// ── autoFlush: only println/printf/format trigger it, NOT print/write ─
PrintWriter autoFlushing = new PrintWriter(System.out, true);
autoFlushing.print("not flushed");     // NO auto-flush — stays in buffer
autoFlushing.println("flushed now");   // println → auto-flush triggered
autoFlushing.printf("also flushed%n"); // printf → auto-flush triggered

// ── print() overloads for all types ──────────────────────────────────
try (PrintWriter pw = new PrintWriter(Files.newBufferedWriter(
        Path.of("out.txt"), StandardCharsets.UTF_8))) {

    pw.print(true);          // "true"
    pw.print('A');           // "A"
    pw.print(42);            // "42"
    pw.print(3.14f);         // "3.14"
    pw.print(3.14159265);    // "3.141592653589793"
    pw.print(9_876_543_210L);// "9876543210"
    pw.print(new char[]{'H','i'}); // "Hi"
    pw.print((Object)null);  // "null" — no NullPointerException
    pw.print("Hello");       // "Hello"
}

// ── println() adds line separator ─────────────────────────────────────
try (PrintWriter pw = new PrintWriter(Files.newBufferedWriter(
        Path.of("lines.txt"), StandardCharsets.UTF_8))) {

    pw.println("Line 1");    // "Line 1" + System.lineSeparator()
    pw.println(42);          // "42" + System.lineSeparator()
    pw.println();            // just System.lineSeparator() (blank line)
}

// ── printf/format: C-style formatted output ───────────────────────────
try (PrintWriter pw = new PrintWriter(Files.newBufferedWriter(
        Path.of("report.txt"), StandardCharsets.UTF_8))) {

    // Basic formatting:
    pw.printf("Name: %-20s Age: %3d%n", "Alice", 30);     // left-aligned name, right-aligned age
    pw.printf("Pi: %.5f%n", Math.PI);                      // 5 decimal places: 3.14159
    pw.printf("Hex: %08X%n", 255);                         // zero-padded hex: 000000FF
    pw.printf("Sci: %e%n", 1_234_567.89);                  // 1.234568e+06
    pw.printf("Bool: %b%n", null);                         // "false" (null → false for %b)

    // %n vs 
: use %n for portable platform newline in format strings:
    pw.printf("Line 1%nLine 2%n");    // 

 on Windows, 
 on Unix
    pw.printf("Line 1
Line 2
");    // always 
 — for cross-platform formats

    // Locale-specific number formatting:
    pw.printf(Locale.GERMANY, "Preis: %.2f EUR%n", 1234.56);  // "1234,56" (comma decimal)
    pw.printf(Locale.US,      "Price: $%.2f%n",    1234.56);  // "1234.56" (period decimal)

    // Date/time:
    pw.printf("Date: %tY-%tm-%td%n", LocalDate.now(), LocalDate.now(), LocalDate.now());
}

// ── Silent error model: checkError() ─────────────────────────────────
PrintWriter riskWriter = new PrintWriter(new FileOutputStream("risk.txt"));
riskWriter.println("Important data");
// Simulate: file system becomes unavailable after open (in practice this would happen
// on disk full, network share disconnect, etc.)
riskWriter.println("More important data");
riskWriter.println("Critical record");

// checkError() flushes, then reports accumulated errors:
if (riskWriter.checkError()) {
    System.err.println("WARNING: Write errors occurred — data may be incomplete");
    // But it's too late to know WHICH writes failed
}

// ── Compare: BufferedWriter throws immediately ────────────────────────
try (BufferedWriter bw = Files.newBufferedWriter(Path.of("safe.txt"))) {
    bw.write("Important data");
    bw.newLine();
    // If disk is full: IOException thrown HERE — caller handles it immediately
} catch (IOException e) {
    System.err.println("Write failed at known point: " + e.getMessage());
    // Recovery action: retry, alert, abort cleanly
}

// ── PrintWriter vs PrintStream ────────────────────────────────────────
// System.out is a PrintStream (byte-based, platform encoding):
System.out.println("Hello");    // encodes "Hello" to bytes using platform charset
System.out.printf("Pi=%.2f%n", Math.PI);

// PrintWriter wrapping System.out (character-based, explicit charset):
PrintWriter stdout = new PrintWriter(
    new BufferedWriter(new OutputStreamWriter(System.out, StandardCharsets.UTF_8)),
    true);   // autoFlush: println triggers immediate flush
stdout.println("Hello from PrintWriter");
stdout.printf("Pi=%.2f%n", Math.PI);
stdout.flush();   // explicit flush for non-println writes

// ── Servlet response pattern (javax.servlet / jakarta.servlet) ────────
// HttpServletResponse.getWriter() returns a PrintWriter:
@Override
public void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws IOException {
    resp.setContentType("text/html; charset=UTF-8");
    resp.setCharacterEncoding("UTF-8");
    try (PrintWriter out = resp.getWriter()) {
        out.println("<!DOCTYPE html>");
        out.println("<html><body>");
        out.printf("<h1>Hello, %s!</h1>%n", req.getParameter("name"));
        out.println("</body></html>");
    }
    // checkError() can detect write failures to the client (e.g., client disconnected):
    // if (out.checkError()) log.warn("Client disconnected during response");
}

// ── CLI tool: PrintWriter for formatted console output ────────────────
public static void printTable(List<DataRow> rows) {
    try (PrintWriter out = new PrintWriter(
            new BufferedWriter(new OutputStreamWriter(System.out, StandardCharsets.UTF_8)),
            false)) {  // no autoFlush — print header and all rows, then flush once

        out.printf("%-20s %10s %15s%n", "Name", "Count", "Percentage");
        out.printf("%-20s %10s %15s%n", "-".repeat(20), "-".repeat(10), "-".repeat(15));
        for (DataRow row : rows) {
            out.printf("%-20s %,10d %14.2f%%%n",
                row.name(), row.count(), row.percentage());
        }
        out.flush();   // one flush for all output
    }
}

// ── String.format() vs printf() performance trade-off ─────────────────
// String.format() creates intermediate String — useful when string is reused:
String formatted = String.format("User: %-20s ID: %08d", user.name(), user.id());
logger.info(formatted);     // reused for logging
pw.println(formatted);      // and for output

// printf() writes directly — no intermediate String — better for write-once output:
pw.printf("User: %-20s ID: %08d%n", user.name(), user.id());

// ── Wrapping PrintWriter in try-with-resources ────────────────────────
// PrintWriter.close() closes the underlying Writer — always use try-with-resources:
try (PrintWriter pw = new PrintWriter(Files.newBufferedWriter(
        Path.of("output.txt"), StandardCharsets.UTF_8))) {
    pw.println("Data written safely");
    pw.printf("Formatted: %d%n", 42);
}   // close() flushes BufferedWriter, closes FileOutputStream

// Check for errors after close (unusual pattern — only for critical files):
try (PrintWriter pw = new PrintWriter(Files.newBufferedWriter(
        Path.of("critical.txt"), StandardCharsets.UTF_8))) {
    pw.println("Critical data");
    pw.println("More critical data");
    boolean hadError = pw.checkError();   // checkError before close to detect write failures
    if (hadError) throw new IOException("PrintWriter reported write error");
}