Files and I/O

Java reads and writes files with Path and Files (modern) or stream classes (older).

🔎 Unravel this example Prerequisites (1)

Learn these first

try-with-resources

At a glance

Mental model

Using a library book: open it, read or write, and return it so others can use it, in a known language.

What happens, in order

1Files.readString(path), Files.readAllLines(path) and Files.writeString(path, text) handle small filesFiles.lines(path) streams a large file and must be closed.
2Path.of("data", "in.txt") builds paths portablyPrefer Path over the older File.
3Streams are bytes; readers and writers are charactersWrap a stream in a reader with an explicit charset: new InputStreamReader(in, StandardCharsets.UTF_8).

Remember

Reading a whole large file into memory causes OutOfMemoryError; stream it
Streams are decorators layered on a native file handle; BufferedReader reads big chunks and serves lines from memory

Watch out

Platform-default encodingsNot closing streamsLoading huge files in one goTrusting user-supplied paths

Example (highlighted lines explain it)

Path file = Path.of("data", "input.txt");List<String> lines = Files.readAllLines(file, StandardCharsets.UTF_8);try (Stream<String> big = Files.lines(Path.of("big.log"))) {    long errors = big.filter(l -> l.contains("ERROR")).count();}

line 1 uses Path, the core of Files and I/O
line 2 uses Files, the core of Files and I/O
line 3 uses Files, the core of Files and I/O

Read the full explanation (about 1 min)

Java reads and writes files with Path and Files (modern) or stream classes (older). Always say which character encoding you mean and close what you open.

A mental model

Using a library book: open it, read or write, and return it so others can use it, in a known language.

How it works

Files.readString(path), Files.readAllLines(path) and Files.writeString(path, text) handle small files. Files.lines(path) streams a large file and must be closed.

Path.of("data", "in.txt") builds paths portably. Prefer Path over the older File.

Streams are bytes; readers and writers are characters. Wrap a stream in a reader with an explicit charset: new InputStreamReader(in, StandardCharsets.UTF_8).

Going deeper

Reading a whole large file into memory causes OutOfMemoryError; stream it. User-supplied file names can contain .. and escape your directory: normalise and check the resolved path (path traversal). Buffer reads and writes; unbuffered byte-by-byte I/O is very slow.

Under the hood

Streams are decorators layered on a native file handle; BufferedReader reads big chunks and serves lines from memory. Files methods use NIO channels.

Common mistakes

Platform-default encodings.
Not closing streams.
Loading huge files in one go.
Trusting user-supplied paths.

How Unravel recognises it

When you paste code, Unravel looks for these signs of files and i/o. Spotting them yourself is a good habit when you read unfamiliar code:

Types: Files Path Paths BufferedReader BufferedWriter FileReader FileWriter InputStream OutputStream FileInputStream FileOutputStream Scanner File InputStreamReader and 1 more

Related concepts

try-with-resourcesErrors and logging

Builds on this

CSV and Excel file processingI/O and data Properties files, Locale and resource bundlesI/O and data Byte streams, readers and writersI/O and data NIO.2: Path and Files in practiceI/O and data XML parsing: DOM, SAX, StAXI/O and data

Frequently asked questions

What is Files and I/O?

Java reads and writes files with Path and Files (modern) or stream classes (older). Always say which character encoding you mean and close what you open.

What are the most common mistakes with Files and I/O?

Platform-default encodings. Not closing streams. Loading huge files in one go. Trusting user-supplied paths.

How can I see Files and I/O in real code?

Paste your own Java into Unravel and it will point out where files and i/o appears, explain how it works at the detail level you choose, and draw the flow of the code around it.