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    A NullPointerException occurs when you try to use a reference that points to null as if it were an actual object. With Scanner, this often happens if you declare a variable but never assign it a value before using it.

    Example:

    public class Example {
    public static String userInput;

    public static void main(String[] args) {
    // This will throw NullPointerException because userInput is null
    System.out.println(userInput.length());
    }
    }
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    In this case, userInput is declared but never initialized, so calling .length() on it causes the exception.

    When working with Scanner, a common mistake is shadowing variables or forgetting to assign the scanned value to the intended field. For example:

    public static String user_input;

    public static void input() {
    Scanner scanner = new Scanner(System.in);
    // Wrong: reading input but not assigning to the class variable
    String temp = scanner.nextLine();
    }
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    Later, if you loop over user_input.length(), it will throw an NPE because user_input is still null.

    Corrected version:

    import java.util.Scanner;

    public class Example {
    public static String user_input;

    public static void input() {
    Scanner scanner = new Scanner(System.in);
    System.out.print("Enter text: ");
    user_input = scanner.nextLine(); // Assign directly to the static variable
    }

    public static void main(String[] args) {
    input();
    for (int i = 0; i < user_input.length(); i++) {
    System.out.println(user_input.charAt(i));
    }
    }
    }
    Copied!
    What is NullPointerException (NPE) in Java?
    If you’ve been programming in Java, you’ve likely encountered the infamous NullPointerException (NPE). This runtime error occurs when your code tries to use an object reference that has not been initialized. Left unchecked, it can cause your application to crash unexpectedly.
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  1. Stack Overflow
    https://stackoverflow.com › questions

    java - NullPointerException using Scanner - Stack Overflow

    Dec 15, 2013 · I am new to programming and I'm starting to create a simple calculator in Java, but I keep getting an error on the line for (int i = 0; i < user_input.length (); i++) The error says: Except...

    • Reviews: 3
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      The Null Pointer Exception Trap: How to Escape

      Sep 15, 2024 · Discover the hidden dangers of Null Pointer Exceptions (NPEs) in Java development. Learn how to identify, prevent, and debug these common errors to avoid costly mistakes and improve …

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      Handling NullPointerException in Java: An Expert Guide

      Aug 25, 2024 · Java developers will undoubtedly encounter NullPointerExceptions (NPEs) during projects. This comprehensive 4-part guide dives deep into the root causes, prevention, and mitigation …

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      Jan 24, 2024 · In this article, we will see how to fix and avoid NullPointerException in Java with examples. The NullPointerException is a runtime exception in Java that occurs when a variable or …

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    • Deep dive into Java NullPointerException Using Scanner

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