How to solve: java.lang.IllegalStateException: Cannot perform this action on a sealed instance

The error message "java.lang.IllegalStateException: Cannot perform this action on a sealed instance" in Java typically occurs when you attempt to modify an object that has been declared as a "sealed" instance. In Java, sealed classes and interfaces were introduced in JDK 15 as part of the JEP 360 feature.

A sealed class or interface restricts its subclassing to a limited set of classes or interfaces. The sealed modifier is applied using the sealed keyword in the class or interface declaration.

Here's an example of how a sealed class is defined:

javaCopy code
public sealed class SealedClass permits Subclass1, Subclass2 {
    // Class definition
}

In this example, the SealedClass is declared as sealed, and it permits only Subclass1 and Subclass2 to be its direct subclasses.

The error message you encountered indicates that you are trying to create a new subclass or modify the sealed instance in a way that is not allowed based on the class's sealed declaration.

To resolve this issue, you have a few options:

1. Follow Sealed Class Restrictions: Make sure that you are adhering to the restrictions defined by the sealed class or interface. If the sealed class specifies limited subclasses, ensure that you are not trying to subclass it with an unauthorized class.

2. Use Permitted Subclasses: If you want to create a subclass of the sealed class, make sure you use one of the permitted subclasses specified in the permits clause of the sealed class declaration.

3. Review the Code Logic: Check the code that leads to the error carefully. Ensure that you are not inadvertently trying to modify a sealed instance in a way that is not allowed.

4. Check JDK Version: Sealed classes were introduced in JDK 15. If you are using an earlier version of Java, you won't be able to use sealed classes. Update to JDK 15 or a later version to use sealed classes.

5. Remove the Sealed Modifier: If you don't intend to use the sealed feature, remove the sealed keyword from the class or interface declaration to make it open for normal subclassing.

Remember that sealed classes and interfaces are designed to provide controlled access to inheritance, and they can help enforce better encapsulation and API design. Make sure to use them appropriately based on your specific requirements.

If you are still encountering the issue after reviewing and adjusting the code, please provide more context or code snippets so that we can better understand the problem and offer more specific assistance.