Lesson 01 · I/O Streams & Readers

Objectives

After this lesson you will be able to:

• Tell byte streams from character streams and pick the right one.
• Wrap streams for buffering and use try-with-resources.
• Read user input from the console.

Don't confuse these "streams"

The I/O InputStream/OutputStream here are unrelated to the java.util.stream.Stream of Module 06. Same word, different worlds.

Byte vs character streams

	Byte streams	Character streams
Base types	InputStream / OutputStream	Reader / Writer
Unit	raw bytes	char (text, with an encoding)
Use for	images, binary, any bytes	text

// bytes
try (InputStream in = new FileInputStream("a.bin")) { int b = in.read(); }
// text
try (Reader r = new FileReader("a.txt")) { int c = r.read(); }

A bridge converts bytes↔chars with an explicit charset: InputStreamReader / OutputStreamWriter.

Buffering

Wrap a raw stream in a buffered decorator to read/write in chunks — far fewer system calls. BufferedReader adds the handy readLine().

try (BufferedReader br = new BufferedReader(new FileReader("a.txt"))) {
    String line;
    while ((line = br.readLine()) != null) { process(line); }
}

Exam trap

read() returns an int (the byte/char value, or -1 at end of stream), not a byte/char — this lets -1 signal EOF unambiguously. Storing it in a char/byte and comparing to -1loops forever. The decorator pattern wraps streams: new BufferedReader(new FileReader(...)).

Console input

// Modern, simple line input:
var br = new BufferedReader(new InputStreamReader(System.in));
String name = br.readLine();

// Console (null when not attached to a terminal, e.g. in an IDE/test):
Console c = System.console();
char[] pw = c.readPassword("Password: ");   // doesn't echo

SDET note

Don't hard-code System.in/System.out deep in logic — inject a Reader/Writer (or Appendable) so a test can supply a StringReader and capture a StringWriter. That makes I/O code unit-testable without touching the real console or filesystem.

Key Takeaways

• Byte streams (InputStream/OutputStream) carry raw bytes; character streams (Reader/Writer) carry text with an encoding. Bridge with InputStreamReader/ OutputStreamWriter.
• Wrap in buffered decorators for efficiency; BufferedReader.readLine() reads a line.
• read() returns an int and -1 at EOF — never store it in char/byte for the EOF test.
• Always use try-with-resources so streams close.

Lesson Quiz

Lesson Quiz · I/O Streams & Readers0 / 5

1. What does InputStream.read() return at end of stream?

   • A0
   • B-1
   • Cnull
   • Dthrows EOFException

2. Which is a CHARACTER stream?

   • AFileInputStream
   • BFileReader
   • CFileOutputStream
   • DByteArrayInputStream

3. Why store read()'s result in an int, not a char?

   • APerformance
   • BSo -1 (EOF) is distinguishable from a valid value
   • Cchar can't hold letters
   • DIt's required syntax

4. What does BufferedReader add over a plain Reader?

   • AEncoding
   • BreadLine() and buffering
   • CCompression
   • DRandom access

5. Which bridges bytes to characters with a charset?

   • ABufferedReader
   • BInputStreamReader
   • CFileWriter
   • DPrintStream

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Next: NIO.2 Path & Files. Run the matching code in labs/src/main/java/com/jse21/m08_io/.