使用Scanner触发NoSuchElementException，求解释第19行报错原因

Hey there! Let's break down why that NoSuchElementException is hitting your code at line 19, and how to fix it.

What's Causing the Exception? #

There are a couple of key issues in your input handling logic that lead to this error:

1. Mishandling the newline after nextInt()
   When you use scanner.nextInt() to read the number of contacts n, it only grabs the integer value—the newline character after that number stays in the input buffer. Your attempt to fix this with scanner.next() is wrong: next() reads the next non-whitespace token (which would be the first contact's name), leaving the rest of the input sequence out of sync. This misalignment eventually causes your code to try reading input that doesn't exist, triggering the NoSuchElementException when it hits scanner.nextLine() in the while loop.

2. Input reading logic that doesn't match typical test case formats
   Most HackerRank test cases for this problem format each contact as a single line with the name and phone number separated by a space (e.g., Alice 12345). Your code uses two separate nextLine() calls per contact, which would only work if each name and number are on their own lines. When the input doesn't match this, your code reads data incorrectly, leading to missing or extra entries in your contact book, and eventually running out of input to read.

3. Unsafe while loop condition
   Using n-- > 0 for your query loop assumes exactly n queries exist in the input. But if your earlier input handling messed up the sequence, you might exhaust the input stream before the loop finishes, causing scanner.nextLine() to throw the exception.

Fixed Code #

Here's a revised version of your code that addresses all these issues:

import java.util.*;
import java.io.*;

class Solution{
    public static void main(String []args) {
        Scanner scanner = new Scanner(System.in);
        Map<String, String> contactBook = new HashMap<>();
        
        int n = scanner.nextInt();
        scanner.nextLine(); // Properly consume the newline after the integer
        
        // Read n contacts (assuming name and number are on the same line)
        for(int i = 0; i < n; i++) {
            String[] contactDetails = scanner.nextLine().split(" ");
            String name = contactDetails[0];
            String phoneNumber = contactDetails[1];
            contactBook.put(name, phoneNumber);
        }
        
        // Read queries until there's no more input (more robust than relying on n)
        while(scanner.hasNextLine()) {
            String search = scanner.nextLine();
            if(contactBook.containsKey(search)) {
                System.out.println(search + "=" + contactBook.get(search));
            } else {
                System.out.println("Not found");
            }
        }
        
        scanner.close(); // Good practice to close the scanner
    }
}

Key Fixes Explained: #

• Correct newline handling: scanner.nextLine() after nextInt() clears the leftover newline without consuming actual contact data.
• Matching input format: Using split() to read each contact line into name and number aligns with standard HackerRank test case formatting.
• Robust query reading: scanner.hasNextLine() checks if there's still input to read before calling nextLine(), preventing the NoSuchElementException entirely.

内容的提问来源于stack exchange，提问作者Hegyi Levente