IO & NIO

Every real program reads and writes data — config files, logs, uploads, exports. By the end of this lesson you'll read and write files the clean, modern way, stream huge files without running out of memory, and know exactly when to drop down to channels and buffers.

Learn IO & NIO in our free Java course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Java course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

You should already know Exception Handling (especially try-with-resources) and the Streams API , since the modern file methods return a Stream . A basic feel for the difference between bytes (raw data) and characters (text) helps too.

Keep that picture in mind: streams = the straw, buffers + channels = the brigade, and the Files class = the button that hides the brigade behind a one-liner.

A stream is a one-way flow of data. Java splits them in two: InputStream / OutputStream move raw bytes (images, zip files), while Reader / Writer move characters (text). For files you'd use FileReader / FileWriter .

Reading one character at a time is slow, so you wrap a reader in a BufferedReader . "Buffered" means it grabs a big chunk into memory and hands it to you piece by piece — and it adds the handy readLine() method. The try (...) block below is try-with-resources : anything you open in those parentheses is closed automatically when the block ends.

Read the fully-commented example, then run it. Note how readLine() returns null at the end of the stream — that's your signal to stop the loop.

Since Java 7, java.nio.file replaced most of that ceremony. A Path is simply a location on disk (the modern stand-in for the legacy File class). The Files class is a toolbox of static helpers that turn whole tasks into one line.

Files.readString(path) — read an entire small file into a String (Java 11+)

Files.readAllLines(path) — read every line into a List String

Files.writeString(path, s) — write a String, creating or overwriting the file

Files.lines(path) — a lazy Stream String , safe for huge files

The example below writes a file, reads it three different ways, then deletes it. The key habit: always pass StandardCharsets.UTF_8 so the same code reads the same text on every machine.

Finish the program: write three lines, read them back as a List , and print the count. Replace each ___ using the // 👆 hints. Check your work against the expected output in the comments.

Count the WARN lines in a log without loading the whole file into memory. Fill in the streaming method and the text to match, then compare with the expected output.

The convenient Files helpers are built on lower-level pieces you'll occasionally meet. A channel ( FileChannel ) is a two-way pipe to a file or socket. A buffer ( ByteBuffer ) is a fixed-size box of bytes that the channel fills from or drains into.

The one trick people forget is buffer.flip() : after you fill a buffer you must flip it to switch from "write mode" to "read mode" so the channel knows where your data ends. You rarely need this layer directly — but it's what powers fast, large-file work like FileChannel.transferTo() (zero-copy) and memory-mapped files.

No blanks this time — just an outline. Write a sentence to a temp file, read it back with Files.readString , split it on spaces, and print the word count. The starter has only comments; you write the code.

Great work! You can now read and write files the classic way with buffered Reader / Writer , close them safely with try-with-resources, and do the same far more cleanly with the modern Path + Files API. You also know to stream huge files with Files.lines instead of loading them whole, and what channels, buffers, and non-blocking IO are for.

Next up: JDBC — connecting your Java programs to databases so your data outlives a single run.

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