Chapter 92: Writing File in Java

Reading (Chapter 91) pulled bytes in. Writing pushes bytes out: create, replace, or append depending on how you open the file.

1. Topic title

Writing a file: choose truncate vs append, then flush mental model of “disk commits”

2. What it means

Files.writeString(path, text, charset, OPEN_OPTIONS…) — common options:

• CREATE — make the file if missing.
• TRUNCATE_EXISTING — start from empty when combined with WRITE.
• APPEND — add bytes after existing content.

Combine with StandardOpenOption varargs. When unsure, read the enum Javadoc once—names are honest.

3. Why it is used

Export CSV, append audit lines, dump debug snapshots—any output that must survive past main.

4. Mental sketch

Truncate is erasing the whiteboard before lecture. Append is adding bullet points to yesterday’s notes.

5. Java code example

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;
 
public class WriteFileDemo {
    public static void main(String[] args) throws IOException {
        Path tmp = Files.createTempFile("write-demo", ".log");
        Files.writeString(tmp, "start\n", StandardCharsets.UTF_8, StandardOpenOption.TRUNCATE_EXISTING);
 
        Files.writeString(
                tmp,
                "more\n",
                StandardCharsets.UTF_8,
                StandardOpenOption.CREATE,
                StandardOpenOption.APPEND);
 
        System.out.print(Files.readString(tmp, StandardCharsets.UTF_8));
        Files.deleteIfExists(tmp);
    }
}

First write replaces content with start\n. Second call appends more\n.

6. Explanation of code

CREATE + APPEND is a common pair for log rotation handled elsewhere—here it simply guarantees the file exists before appending.

7. Common mistakes

Forgetting parent directories—Files.write does not mkdir a/b/file.txt automatically unless you createDirectories first.

Platform newlines—use System.lineSeparator() or \n consistently depending on consumer expectations.

8. Best practices

Write temp files, flush (implicit on close for buffered writers), then move atomically when publishing—reduces half-written visible files.

Test on read-only volumes occasionally—errors teach permission handling.

9. Small practice task

Write three JSON lines (fake objects) using three append calls, then readString and print—confirm order matches call order.

Beginner tip

If nothing appears on disk, check you flushed/closed writers—buffered data can sit in RAM until close.