Understanding IllegalStateException in Java

IllegalStateException is a runtime exception that typically indicates that the current state of the application is not suitable for the operation being attempted. This exception is often thrown when a method is called inappropriately or at an unexpected time. It’s part of the java.lang package and extends RuntimeException.

Common Causes of IllegalStateException:

1. Method Called at an Inappropriate Time: Certain methods may require that the object is in a specific state before they can be invoked. Calling such methods when the object is not in the expected state can result in an IllegalStateException.
2. Violation of Contracts: Some methods might have preconditions specified in their documentation. If these preconditions are not met when the method is called, an IllegalStateException may occur.
3. Concurrency Issues: In multi-threaded environments, improper synchronization or race conditions can lead to states that are unexpected, causing methods to throw IllegalStateException.

Examples:

Let’s look at some examples to understand when IllegalStateException might occur:

1. Using a Closed Resource:

import java.io.*;

public class ResourceExample {
    public static void main(String[] args) {
        try (FileInputStream fis = new FileInputStream("example.txt")) {
            // Perform some operations
        } catch (IOException e) {
            e.printStackTrace();
        }
        // Attempting to use the closed resource
        // This will throw IllegalStateException
        int data = fis.read();
    }
}

2. Modifying a Collection during Iteration:

import java.util.ArrayList;
import java.util.List;

public class CollectionExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("One");
        list.add("Two");
        list.add("Three");

        for (String str : list) {
            if (str.equals("Two")) {
                // Modifying the list during iteration
                // This will throw IllegalStateException
                list.remove(str);
            }
        }
    }
}

3. Using a Closed Scanner:

import java.util.Scanner;

public class ScannerExample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        // Closing the scanner
        scanner.close();

        // Attempting to read input after closing
        // This will throw IllegalStateException
        String input = scanner.nextLine();
    }
}

Handling IllegalStateException:

To handle IllegalStateException, you can either catch it using try-catch blocks or prevent it by ensuring that the conditions leading to it are avoided.

try {
    // Code that may throw IllegalStateException
} catch (IllegalStateException e) {
    // Handle the exception
}

Conclusion:

Understanding when and why IllegalStateException occurs is crucial for writing robust Java applications. By being mindful of the state of objects and following method contracts, you can minimize the occurrence of this exception in your code.