Using Text Files: Practice Questions (Computer Science A 4.6)

These are original Marlbridge practice questions, not past exam questions. Mark points are a suggested Marlbridge rubric, not an official scoring guideline. Every code fragment was compiled and run. Assume the imports java.io.File, java.io.IOException and java.util.Scanner are present and that each file named exists in the current folder unless the question says otherwise.

Question 1 (multiple choice · foundation)

A method opens a file with Scanner in = new Scanner(new File(name));. Which change is required for the method to compile?

(A) Add throws IOException to the method header
(B) Declare name as a File
(C) Call in.close() before the first read
(D) Replace Scanner with String

Answer and explanation

Answer: (A). Building a Scanner from a File can fail, so the code must state what happens. The course’s way is a throws IOException clause in the header (the method also needs the imports).

(B) new File(name) expects a String file name; name is correctly a String.
(C) Closing before any read would make later reads fail; close() goes after the loop.
(D) A Scanner cannot be replaced by a String; they are unrelated types.

Question 2 (multiple choice · core)

The file nums.txt contains one line: 4 9 2. What is printed?

Scanner in = new Scanner(new File("nums.txt"));
int total = 0;
while (in.hasNext())
{
    total += in.nextInt();
}
in.close();
System.out.println(total);

(A) 3
(B) 15
(C) 429
(D) 2

Answer and explanation

Answer: (B). Three tokens are read: 4, 9 and 2, so total goes 4, 13, 15. Output: 15.

(A) counts the tokens instead of adding them.
(C) joins the digits as text; += on an int adds numbers.
(D) is just the last value read.

Question 3 (multiple choice · core)

The file bad.txt contains 5 8 two 3. The loop in Question 2 is run on it. What happens?

(A) total becomes 16 and two is ignored
(B) total becomes 13 and the loop ends normally
(C) An InputMismatchException is thrown after 5 and 8 have been added
(D) A NoSuchElementException is thrown immediately

Answer and explanation

Answer: (C). 5 and 8 are read (total 13). The third nextInt() sees the token two, which is not an int, so InputMismatchException is thrown. Running this, the exception was java.util.InputMismatchException.

(A) nextInt() never skips a bad token.
(B) The loop does not end normally; the exception stops the program.
(D) NoSuchElementException is for running out of data, and there is data left.

Question 4 (multiple choice · core)

What is the length of the array returned by "a,b,,".split(",")?

(A) 2
(B) 3
(C) 4
(D) 5

Answer and explanation

Answer: (A). split removes empty strings from the end of the result, so only "a" and "b" remain. Running "a,b,,".split(",").length printed 2. For comparison, "ab,cd,,ef".split(",") has length 4, because the empty piece is in the middle and is kept.

(B) would count one empty piece.
(C) counts the delimiters plus one, assuming nothing is dropped.
(D) counts every comma plus one with an extra piece; no rule gives this.

Question 5 (multiple choice · core)

The file words.txt contains the cat sat on the first line and on the mat on the second. How many times does the loop body run?

Scanner in = new Scanner(new File("words.txt"));
int n = 0;
while (in.hasNext())
{
    in.next();
    n++;
}
in.close();

(A) 2
(B) 3
(C) 6
(D) 18

Answer and explanation

Answer: (C). next() returns one token per call and whitespace (including the line break) separates tokens. There are six words, so n is 6 (confirmed by running).

(A) counts lines; that would need nextLine().
(B) counts the words in one line only.
(D) counts the characters including spaces; nothing in the code reads characters.

Question 6 (multiple choice · core)

After a Scanner has returned the last token of the file, a program calls in.nextInt() once more. What happens?

(A) It returns 0
(B) It returns -1
(C) It throws a NoSuchElementException
(D) The program waits for keyboard input

Answer and explanation

Answer: (C). When there is nothing left to read, a next call throws NoSuchElementException (confirmed on a file containing three numbers). That is why loops test hasNext() first.

(A) and (B) are made-up default values; Java returns nothing here.
(D) The scanner reads a file, not the keyboard, and keyboard input is not part of this course.

Question 7 (multiple choice · core)

Which statement about hasNext() is true?

(A) It returns the next token and moves past it
(B) It returns true if another item can be read, without moving the cursor
(C) It closes the file when the data is exhausted
(D) It returns the number of tokens left

Answer and explanation

Answer: (B). hasNext() is a test. The following next... call returns the item.

(A) describes next().
(C) only close() closes a Scanner.
(D) hasNext() returns a boolean, not a count.

Question 8 (multiple choice · core)

The file vals.txt holds the six numbers 3 14 / 20 9 / 11 2 on three lines. A student wants the number of values greater than 10. Which loop body is correct?

(A) if (in.nextInt() > 10) count++;
(B) if (in.nextInt() > 10 && in.nextInt() > 10) count++;
(C) if (in.hasNext() > 10) count++;
(D) if (in.next() > 10) count++;

Answer and explanation

Answer: (A). It reads one int per pass and tests it. For this file the values above 10 are 14, 20 and 11, so count is 3 (run and confirmed).

(B) reads two numbers per pass, and the second is skipped when the first is not above 10 because of short-circuit evaluation, so tokens are consumed unevenly.
(C) hasNext() returns a boolean, which cannot be compared with 10; it does not compile.
(D) next() returns a String, which cannot be compared with >; it does not compile.

Question 9 (constructed response · core)

Write the method countAbove(String fileName, int limit). The file holds only whole numbers separated by whitespace, in any layout. The method returns how many of them are strictly greater than limit. The method must compile as written and must release the file.

Answer and explanation

public static int countAbove(String fileName, int limit) throws IOException
{
    Scanner in = new Scanner(new File(fileName));
    int count = 0;
    while (in.hasNext())
    {
        if (in.nextInt() > limit)
        {
            count++;
        }
    }
    in.close();
    return count;
}

For vals.txt with limit 10 the method returns 3 (run and confirmed).

Suggested Marlbridge rubric (4 points)

Point	Award for
1	Header includes `throws IOException` and the method returns an `int`
2	Creates the `Scanner` from a `File` built from `fileName`
3	A `while (in.hasNext())` loop that reads exactly one `int` per pass
4	Counts only values strictly greater than `limit`, closes the Scanner and returns the count

Alternative: reading with next() and Integer.parseInt is acceptable. Do not award point 3 for reading two numbers per pass or for a loop that can read past the end.

Question 10 (constructed response · stretch)

Each line of a file holds a name, a comma and a whole-number score, such as Ana,70. Write averageScore(String fileName), which returns the mean score as a double, or 0.0 if the file has no lines. Do not use integer division for the mean.

Answer and explanation

public static double averageScore(String fileName) throws IOException
{
    Scanner in = new Scanner(new File(fileName));
    int sum = 0;
    int n = 0;
    while (in.hasNext())
    {
        String[] parts = in.nextLine().split(",");
        sum += Integer.parseInt(parts[1]);
        n++;
    }
    in.close();
    if (n == 0)
    {
        return 0.0;
    }
    return (double) sum / n;
}

With Ana,70, Ben,85, Cy,90 the sum is 245 and n is 3, so the result is 81.66666666666667 (run and confirmed). An empty file returns 0.0 without dividing by zero.

Suggested Marlbridge rubric (6 points)

Point	Award for
1	`throws IOException`, `Scanner` built from a `File`
2	Loop controlled by `hasNext()`
3	Reads one line per pass with `nextLine()`
4	Uses `split(",")` and `Integer.parseInt` on the second piece
5	Accumulates a sum and a count
6	Returns 0.0 for no data, otherwise a `double` mean (cast before dividing), and closes the Scanner

Do not award point 6 if sum / n is computed with two int values before conversion (the decimal part would be lost, giving 81.0 here).

How did you do?

Questions 1 to 8 are worth one mark each. Count the slips: a missing throws, a mixed-up next method, or a miscounted token each point to the Topic 4.6 study guide. Then tick off the checklist.