Tokens, Lines, and the hasNext Cascade

Where you are: Week 0 review > Tokens, lines, and the hasNext cascade

Try This First

A program calls sc.nextInt() to read an age, then sc.nextLine() to read a full name on the next line, but the name comes back empty. Decide why before reading on.

Before You Start

Check each box you can do from memory. A box you cannot check yet is not a problem; it points you to a quick refresher, not a grade.

• I can open a file with Scanner and read from it token by token or line by line.
• I can write a while loop that stops when a hasNext-style guard returns false.
• I can explain why a guard must match its corresponding read method.

See Reading a File with Scanner if any of the above felt unfamiliar.

What You Need To Walk In With

Walk into the next class able to state these:

• Match the guard to the read: hasNextLine goes with nextLine, hasNextInt goes with nextInt, hasNext goes with next. A mismatch is the usual cause of a file-reading crash.
• Line mode reads one record per line (hasNextLine / nextLine). Token mode reads whitespace- separated pieces (hasNext / next, or hasNextInt / nextInt).
• hasNextInt() returns false at the end of the file and at the first token that is not an integer, so it doubles as a sentinel value detector.
• After nextInt(), an immediate nextLine() returns the empty remainder of that line.

You should be able to: pick line or token mode for a described file, and avoid the nextInt-then- nextLine trap.

How It Works

The file’s shape decides the mode:

Line mode:

while (sc.hasNextLine()) {
    String line = sc.nextLine();
    System.out.println(line);
}

Token mode for integers:

int sum = 0;
while (sc.hasNextInt()) {
    sum = sum + sc.nextInt();
}

hasNextInt() returns false at the end of the file and at the first token that is not a valid integer. A file that ends with the word STOP makes hasNextInt() return false at STOP, so the loop stops on its own with no if inside it.

The nextInt then nextLine trap

nextInt() reads the number and leaves the cursor just before the newline. An immediate nextLine() reads from there to that newline and returns an empty string. Two standard fixes:

• Call an extra sc.nextLine() right after nextInt() to consume the leftover newline, then read the real line.
• Read every line with nextLine() and convert with Integer.parseInt(line.trim()), so the modes never mix.

Worked Example: Predict, Then Check

A file holds the integers 10 20 30 on one line. The token loop above sums them. Predict the result.

A Common Mistake

The deepest file-reading bug is a guard that does not match its read, such as hasNextInt() guarding a nextLine(), or reading with nextLine() right after nextInt() without consuming the newline. Keep the guard and the read in the same family, and remember that nextInt() does not move past the end of its line. (Source: BJP (Reges and Stepp), Ch 6.)

Go Deeper (optional)

For the curious: a Scanner splits input into tokens on whitespace and also tracks line boundaries, so the two modes share one cursor. Mixing them is what causes the trap. Many programs avoid the issue entirely by reading whole lines and parsing each line themselves, which keeps one consistent mode and makes the cursor easy to reason about.

Check Yourself

Close the notes and answer each one from memory, then reveal it. Pulling an idea back from memory is one of the strongest ways to make it stick.