Why is “while (rs.next())” necessary here?

Question

I want to select the maximum line number from my database \"Logs\" and store it in a variable m.

Here\'s my code:

ResultSet rs          


        

Answer 1

Why?

The cursor is initially placed before the first element. You need to advance it once to access the first element.

This was obviously done because traversing the results using a loop is very convenient then, as you see. From the official documentation:

Moves the cursor forward one row from its current position. A ResultSet cursor is initially positioned before the first row; the first call to the method next makes the first row the current row; the second call makes the second row the current row, and so on.

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Solution

So, while you don't need any loop, you need to advance the cursor once. A single rs.next(); would technically be enough:

rs.next();
// Access the result

However, you probably want to account for the case where there was no match at all, since:

When a call to the next method returns false, the cursor is positioned after the last row. Any invocation of a ResultSet method which requires a current row will result in a SQLException being thrown.

So the code would fail in this case.

Because of that, you should account for the case and use the returned boolean to guard your access:

if (rs.next()) {
    // Access result ...
} else {
    // No match ...
}