Reference

File I/O & Serialization (`pkg9io`)

Runnable demos: `pkg9io`. Run any with java pkg9io/io1FileBasics.java.

The two stream families

1Bytes  : InputStream  / OutputStream   -> binary data (images, any file)2Chars  : Reader       / Writer         -> text (charset-aware)

Wrap a base stream in a Buffered variant to batch syscalls and speed up I/O.

Need	Legacy (java.io)	Modern (java.nio.file)
Path handle	`File`	`Path` / `Paths.get` / `Path.of`
Read bytes	`FileInputStream`	`Files.readAllBytes`
Write bytes	`FileOutputStream`	`Files.write`
Read text	`FileReader` + `BufferedReader`	`Files.readString` / `readAllLines`
Write text	`FileWriter` / `PrintWriter`	`Files.writeString`
Stream lines	`BufferedReader.lines()`	`Files.lines()` (lazy)

Rule: prefer java.nio.file (NIO.2) for new code — it is shorter, UTF-8 by default, and richer.

#	File	Topic
1	`io1FileBasics`	`File`: create, inspect, list, delete
2	`io2ByteStreams`	byte streams, buffering, `transferTo`, `readAllBytes`
3	`io3CharacterStreams`	`Reader`/`Writer`, line-by-line, `PrintWriter`
4	`io4NioFilesAndPaths`	`Path` + `Files` (the recommended API)
5	`io5Serialization`	`Serializable`, `transient`, object <-> bytes
6	`io6TryWithResourcesAndScanner`	`AutoCloseable`, `Scanner` parsing

Always use try-with-resources — it closes and flushes, even on exceptions.
Resources close in reverse declaration order; close errors become suppressed exceptions.
transient fields and static fields are not serialized.
Security: never deserialize untrusted bytes (RCE risk) — use JSON for external data.
Specify a charset (StandardCharsets.UTF_8) to avoid platform-default surprises.