IllegalStateException

Why This Happens

IllegalStateException indicates that a method is called when the object is not in an appropriate state. Examples include calling next() on an iterator before hasNext(), writing to a closed stream, or starting an already-started thread.

The Problem

Scanner scanner = new Scanner(System.in);
scanner.close();
String line = scanner.nextLine(); // IllegalStateException: Scanner closed

The Fix

Scanner scanner = new Scanner(System.in);
String line = scanner.nextLine(); // Use before closing
scanner.close();

Step-by-Step Fix

1. 1

   Identify the state violation

   Read the exception message to understand what state the object was in versus what state was required.

2. 2

   Check the object lifecycle

   Trace the object usage to find where its state changes (close, start, initialize) relative to the failing call.

3. 3

   Reorder operations

   Ensure methods are called in the correct order according to the object's lifecycle contract.