Quick Answer

Fix invalid object state operations.

Understanding the Issue

This exception occurs when a method is called at an inappropriate time or when the object is not in the correct state.

The Problem

This code demonstrates the issue:

Java Error 
Iterator<String> it = list.iterator();
it.remove(); // Throws without next()

The Solution

Here's the corrected code:

Java Fixed 
// Correct usage:
Iterator<String> it = list.iterator();
if (it.hasNext()) {
    it.next();
    it.remove(); // Now valid
}

// Alternative with Collections:
list.removeIf(item -> item.equals("target"));

// State validation:
public void process() {
    if (!isInitialized) {
        throw new IllegalStateException("Not initialized");
    }
    // Proceed
}

Key Takeaways

Always verify object state before operations.