Using Text Files: Revision Notes (Computer Science A 4.6)

Full explanation, traces and examples: Topic 4.6 study guide.

Recap

A file stores data that persists after the program ends. A program reads it by creating a File object from the file name and a Scanner from that File. The Scanner returns data forward only, a token or a line at a time.

Scanner in = new Scanner(new File(fileName));
while (in.hasNext())
{
    // read with in.next(), in.nextInt(), in.nextDouble(), in.nextBoolean() or in.nextLine()
}
in.close();

The method that contains this code needs throws IOException in its header. File and IOException come from java.io; Scanner comes from java.util. Both need import statements.

Key relationships

Item	Meaning	Remember
File(String str)	opens the file named str for reading	only creates the link; no reading yet
Scanner(File f)	Scanner that reads from f	needs throws IOException
hasNext()	is there another token?	looks only; does not move the cursor
next()	next token as String	stops at whitespace
nextInt(), nextDouble(), nextBoolean()	next token as that type	InputMismatchException if the token is the wrong kind
nextLine()	rest of the line	may return "" straight after another reading method
close()	finished with the file	call it after the loop
split(String del)	String[] of pieces around del	trailing empty pieces are dropped

Assumptions the standard loop makes

• The file exists at the stated path (otherwise the program terminates).
• Every token has the kind the method expects.
• Records are complete: a line has all of its fields before it is split.
• Mixing nextLine with other Scanner methods on one source is outside the exam scope.

Mistakes to avoid

1. Leaving out throws IOException or an import.
2. Treating nextInt() as reading a whole line.
3. Calling nextInt() twice when you meant to reuse one value.
4. Using >= instead of > when the first of several tied records must win.
5. Using hasNext() as if it advanced the cursor.
6. Assuming "a,b,,".split(",") has four elements. Trailing empty strings are removed, so it has two (checked by running it).
7. Forgetting that an empty file means the loop body never runs.

A mini-example to rehearse

A file rain.txt holds 12 0 7 / 3 25 / 0 8 1. The loop while (in.hasNext()) { total += in.nextInt(); } runs eight times (one per token, not one per line) and leaves total at 56. If one token were the word two, the third nextInt() would throw InputMismatchException and total would never be returned.

For records such as Bilal,88 read with nextLine(), the pattern is: String[] parts = in.nextLine().split(","); then parts[0] is the name and Integer.parseInt(parts[1]) is the score. Keep the best-so-far in two variables and use strict > so that the first of two tied records stays.

How this topic is usually tested

• Trace a loop over a file described in words or as a table, and give the final value of a variable.
• Spot the missing piece: a missing throws IOException, a missing import, no close(), or a loop that reads two tokens per pass.
• Predict an exception: wrong-kind token, no data left, or a missing file.
• Write a method that reads numbers or records and returns a total, a count, a maximum or an average.

When you write code, state the loop condition first (hasNext()), then decide whether each pass reads a token or a line, and only then write the body.

Quick self-check

1. A file holds 4 9 2. A loop adds each nextInt() to total while hasNext(). What is total? 15
2. Which header is needed for a method that builds a Scanner from a File? throws IOException (or code that handles it).
3. What does "Bilal,88".split(",")[1] return? "88", a String.
4. A file holds 5 8 two 3. What is thrown by a nextInt() loop? InputMismatchException when it reaches two.
5. How many times does a next() loop run for a file with the cat sat and on the mat on two lines? 6, one per token.