Series 3: File I/O

Reading and writing files with Scanner and PrintStream, the count-allocate-fill array pattern, the JDK I/O class zoo, and the APE four-step pattern.

Lessons

7a: From File to Array (and Back)

~14 min · Week 7 · Prereq: lessons 6a-6f

The mental model for the unit: read a file into an array, do Week 6 array work in the middle, write the result. Two new APIs, one new exception keyword, the rest is unchanged.

7b: Reading a File with Scanner

~16 min · Week 7 · Prereq: 7a

The File class as a path (not the contents). new Scanner(new File(path)). throws FileNotFoundException as the course route. The canonical EOF loop.

7c: The next* Cascade and the nextLine Trap

~18 min · Week 7 · Prereq: 7b

Four readers (next, nextLine, nextInt, nextDouble). The hasNextInt before hasNextDouble rule. The leftover-newline trap with the throwaway-nextLine() fix.

7d: Writing Files with PrintStream

~13 min · Week 7 · Prereq: 7b

System.out is already a PrintStream. Pointing one at a file is the only new step. Closing matters; the empty-output-file bug is almost always a missing close().

7e: From File to Array: Count-Allocate-Fill

~18 min · Week 7 · Prereq: 7d

The dominant array-from-file pattern. Walk the file twice: once to count, once to fill. End-to-end worked example. First taste of the four-step APE pattern.

7f: The I/O Class Recognition Guide

~18 min · Week 7 · Prereq: 7e

Why the JDK has so many reader and writer classes. BufferedReader, FileReader, PrintWriter, BufferedWriter, FileWriter, Files.readString, Files.writeString. Byte streams vs character streams at recognition level.

7g: File-Class Power Tools

~16 min · Week 7 · Prereq: 7d

Append mode, parent-directory creation with getParentFile().mkdirs(), file inspection (isFile, isDirectory, length, lastModified), and management (delete, renameTo, listFiles).

7h: APE File I/O Walkthrough

~24 min · Week 7 · Prereq: 7e, 7g

The capstone. All six APE Practice File I/O exercises walked sentence-by-sentence with the four-step pattern. Construct-recognition decoder ring, the close-before-null rule, specificity ordering, and the JUnit-over-Javadoc rule.

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