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IllegalStateException class signals that a method has been invoked at an illegal or inappropriate time. In other words, the Java environment or Java application is not in an appropriate state for the requested operation.
What is IllegalStateException?
Common Scenarios
IllegalStateException Class Diagram
Java IllegalStateException Example
package com.javaguides.corejava;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
public class IllegalStateExceptionExample {
public static void main(String[] args) {
List < Integer > intList = new ArrayList < > ();
for (int i = 0; i < 10; i++) {
intList.add(i);
}
Iterator < Integer > intListIterator = intList.iterator(); // Initialized with index at -1
try {
intListIterator.remove(); // IllegalStateException
} catch (IllegalStateException e) {
System.err.println("IllegalStateException caught!");
e.printStackTrace();
}
}
}
IllegalStateException caught!
java.lang.IllegalStateException
at java.util.ArrayList$Itr.remove(ArrayList.java:872)
at com.javaguides.corejava.IllegalStateExceptionExample.main(IllegalStateExceptionExample.java:21)
Handling IllegalStateException
Best Practices
- Be aware of the lifecycle of objects, especially when working with frameworks and libraries.
- Clearly document the expected state and constraints for your methods when creating APIs.
- Familiarize yourself with Java's concurrency utilities when dealing with multi-threaded applications to avoid state-related issues.
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