Java I/O and Files

1A stream is bytes or characters in a line

Java I/O is built on streams: a source or sink you read or write in order. You do not load a whole disk file as one string unless you choose to.

Byte streams (InputStream, OutputStream) move raw bytes. Character streams (Reader, Writer) move text with a charset. Pick characters when the file is a booking list; pick bytes when it is a picture.

No diagram — the idea is carried by the prose or the Java listing.

2Scanner cuts text into tokens

Scanner reads System.in or a file and hands you the next token: next() for a word, nextInt() for an int, nextLine() for the rest of the line.

It is the typed-input tool from Unit I, now seen as an I/O class. Close a Scanner that owns a file. Do not close one that wraps System.in if the rest of the program still needs the keyboard.

Figure. Scanner cuts "A12 200" on whitespace. next() is the seat word A12; nextInt() is the price 200.

Tokens from the keyboard

import java.util.Scanner;
Scanner sc = new Scanner(System.in);
String seat = sc.next();
int price = sc.nextInt();

Coding lab. Tokenise a booking line runs in the app, with checks on your output.

3A file is a stream with a path

To read bookings.txt you open a character stream on that path and walk lines. Files.newBufferedReader (Path) is the modern form; FileReader is the older one.

The path is not the file's contents. If the path is wrong, you get an IOException — a checked exception the last lesson named.

Figure. Path.of("bookings.txt") names the file; Files.readAllLines walks its lines. A wrong path throws IOException, the checked type the exceptions lesson named.

How it works

1. A file is a stream with a pathA path names the file. Reading can throw a checked IOException.

Read every booking line

import java.nio.file.Files;
import java.nio.file.Path;
for (String line : Files.readAllLines(
        Path.of("bookings.txt"))) {
    System.out.println(line);
}

4Writing a file is a stream the other way

Files.writeString or a BufferedWriter sends text to a path. You choose whether to replace the file or append.

A write that succeeds on disk is still your data — check the path you passed. Writing "A12 200" is not the same as writing a Ticket object; you choose the text format.

Figure. Files.writeString sends the characters A12 200 to the path. That is not the same as writing a Ticket object; you choose the text format.

How it works

1. Writing a file is a stream the other wayWriting stores the characters you send. It does not store a Java object unless you choose a format.

Replace bookings.txt with one line

import java.nio.file.Files;
import java.nio.file.Path;
Files.writeString(Path.of("bookings.txt"),
        "A12 200\n");

5try-with-resources closes the stream

try (BufferedReader br = Files.newBufferedReader(path)) opens the reader and guarantees close, even if the body throws.

That is why it exists: a catch or a return used to skip br.close(). The resource must implement AutoCloseable. Scanner and the file streams do.

Figure. try (var br = Files.newBufferedReader(path)) opens the reader and closes it on the way out, even if readLine throws. BufferedReader implements AutoCloseable.

How it works

1. try-with-resources closes the streamOpen the stream in try (...). Close is then not your finally draft.

br.close runs on the way out

try (var br = Files.newBufferedReader(
        Path.of("bookings.txt"))) {
    String line = br.readLine();
    System.out.println(line);
}