How To Throw Illegalstateexception Java

Java Programming

Throwing an IllegalStateException in Java can be a powerful tool when it comes to handling unexpected or illegal program states. In this article, I will guide you through the process of throwing an IllegalStateException in Java and provide you with some personal touches and commentary along the way.

Understanding the IllegalStateException

The IllegalStateException is a subclass of the RuntimeException class in Java. It is typically thrown to indicate that a method has been invoked at an illegal or inappropriate time.

Before we dive into throwing this exception, let me share a personal experience. I was working on a project where I needed to implement a multi-threaded application. One of the challenges I faced was ensuring that certain methods were called in the correct order. To handle this scenario, I used the IllegalStateException to notify the caller when they attempted to invoke a method in an inappropriate state.

Throwing the IllegalStateException

To throw an IllegalStateException, you can use the throw keyword followed by a new instance of the exception class. Let’s take a look at an example:

public void doSomething() {
if (!isValidState()) {
throw new IllegalStateException("Invalid state: unable to perform action.");
// Perform the action

In this example, we have a method called doSomething(). Before performing the action, we check if the current state is valid using the isValidState() method. If the state is not valid, we throw an IllegalStateException with a descriptive message.

Adding a personal touch, I would recommend including a detailed error message that clearly explains the reason for the exception. This can greatly assist in debugging and understanding the cause of the problem.

Handling the IllegalStateException

When you throw an IllegalStateException, it is important to handle it properly to prevent your program from crashing. One way to handle the exception is by using a try-catch block. Let’s see an example:

public void performAction() {
try {
// Continue with other actions
} catch (IllegalStateException e) {
// Handle the exception appropriately
System.out.println("An IllegalStateException occurred: " + e.getMessage());

In this example, we have a method called performAction() that calls the doSomething() method. We wrap the method call in a try-catch block to catch the IllegalStateException. Inside the catch block, we handle the exception by printing a custom error message.


Throwing an IllegalStateException in Java can be a useful technique for signaling and handling illegal program states. By following the examples and recommendations provided in this article, you can effectively utilize this exception class in your Java programs. Remember to add descriptive error messages and handle the exception appropriately to ensure the stability and reliability of your code.