FileWriter

// ── Pre-Java-11 constructors: implicit platform default charset ────────
FileWriter legacy1 = new FileWriter("output.txt");              // overwrite, default charset
FileWriter legacy2 = new FileWriter("output.txt", true);        // append,    default charset
FileWriter legacy3 = new FileWriter(new File("output.txt"));    // overwrite, default charset
FileWriter legacy4 = new FileWriter(new File("output.txt"), true); // append, default charset

// ── Java 11+: explicit charset ────────────────────────────────────────
FileWriter utf8Overwrite = new FileWriter("output.txt",  StandardCharsets.UTF_8);
FileWriter utf8Append    = new FileWriter("output.txt",  StandardCharsets.UTF_8, true);
FileWriter isoOverwrite  = new FileWriter(new File("output.txt"), StandardCharsets.ISO_8859_1);
FileWriter isoAppend     = new FileWriter(new File("output.txt"), StandardCharsets.ISO_8859_1, true);

// ── Overwrite (truncate) default — a common source of data loss ────────
File important = new File("important_data.txt");
// File has 1MB of data

try (FileWriter fw = new FileWriter(important, StandardCharsets.UTF_8)) {
    // File is NOW EMPTY — truncated on open, BEFORE any write
    // If fw.write() throws, the 1MB of data is gone permanently
    fw.write("replacement content");
}

// ── Safe overwrite pattern: write to temp, then rename ────────────────
File target = new File("important_data.txt");
File temp   = new File("important_data.txt.tmp");
try (FileWriter fw = new FileWriter(temp, StandardCharsets.UTF_8)) {
    fw.write("new content");
}   // temp file fully written and closed
temp.renameTo(target);   // atomic on most OS/filesystems — target replaced atomically

// ── Append mode: add to existing file ────────────────────────────────
// Log file: accumulate entries without truncating
try (FileWriter logger = new FileWriter("app.log", StandardCharsets.UTF_8, true)) {
    logger.write(LocalDateTime.now() + " INFO Application started
");
}   // existing log content preserved, new line added at end

// Multiple appends accumulate:
for (int i = 0; i < 5; i++) {
    try (FileWriter fw = new FileWriter("count.txt", StandardCharsets.UTF_8, true)) {
        fw.write("Line " + i + "
");
    }
}
// count.txt contains all 5 lines after 5 separate opens

// ── What charset to use? ──────────────────────────────────────────────
// UTF-8:          best default — encodes all Unicode, most common on modern systems
// ISO-8859-1:     Western European legacy files, 1 byte per char
// UTF-16:         fixed-width for BMP chars, rarely needed
// US-ASCII:       7-bit only, throws on any non-ASCII char
// windows-1252:   Windows legacy Western European files
System.out.println(StandardCharsets.UTF_8);   // always available, no checked exception

// ── FileWriter write methods ──────────────────────────────────────────
try (FileWriter fw = new FileWriter("chars.txt", StandardCharsets.UTF_8)) {
    fw.write('A');                     // write single char (via int)
    fw.write(65);                      // same: 'A' (Unicode code point 65)
    fw.write("Hello, World!");         // write entire string
    fw.write("Hello, World!", 7, 5);   // write "World" (offset=7, count=5)

    char[] letters = {'J', 'a', 'v', 'a'};
    fw.write(letters);                 // write entire char array
    fw.write(letters, 0, 2);          // write "Ja" (offset=0, count=2)
}

// ── ALWAYS wrap FileWriter in BufferedWriter ───────────────────────────
// Unbuffered: one system call per write — extremely slow for many writes
try (FileWriter fw = new FileWriter("slow.txt", StandardCharsets.UTF_8)) {
    for (int i = 0; i < 100_000; i++) {
        fw.write("Line " + i + "
");  // each call goes to OS — 100,000 system calls
    }
}

// Buffered: system calls reduced to ~12 (100,000 lines × ~50 chars / 8192 buffer)
try (BufferedWriter bw = new BufferedWriter(
        new FileWriter("fast.txt", StandardCharsets.UTF_8))) {
    for (int i = 0; i < 100_000; i++) {
        bw.write("Line " + i);
        bw.newLine();   // platform-appropriate line ending
    }
}   // close() flushes remaining buffer — all lines written

// ── newLine() vs hardcoded 
 ─────────────────────────────────────────
try (BufferedWriter bw = new BufferedWriter(
        new FileWriter("platform.txt", StandardCharsets.UTF_8))) {
    bw.write("First line");
    bw.newLine();   // 

 on Windows, 
 on Unix — correct for platform tools

    bw.write("Second line");
    bw.write("
");   // always 
 — correct for cross-platform text/JSON/CSV/HTTP
}

// ── PrintWriter: adds printf and println over FileWriter ───────────────
try (PrintWriter pw = new PrintWriter(new BufferedWriter(
        new FileWriter("report.txt", StandardCharsets.UTF_8)))) {
    pw.println("Report generated: " + LocalDate.now());   // platform newLine
    pw.printf("Total: %d items, $%.2f%n", 42, 1234.56);  // formatted output
    pw.println();   // blank line
}

// ── Modern alternative: Files.writeString and Files.write ─────────────
// Write entire string at once (no intermediate FileWriter needed):
Files.writeString(Path.of("output.txt"), "Hello, UTF-8 World!",
    StandardCharsets.UTF_8,
    StandardOpenOption.CREATE,
    StandardOpenOption.TRUNCATE_EXISTING);

// Write lines (adds platform line separator after each):
List<String> lines = List.of("Line 1", "Line 2", "Line 3");
Files.write(Path.of("lines.txt"), lines, StandardCharsets.UTF_8);

// Append with Files.writeString:
Files.writeString(Path.of("log.txt"), "New entry
",
    StandardCharsets.UTF_8,
    StandardOpenOption.CREATE,
    StandardOpenOption.APPEND);