Understanding How to Call a Method in a Running Thread Object in Java

Learn how to properly call methods in a running Java Thread Object and fix common errors like NullPointerException with this step-by-step guide.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Calling a Method in a running Thread Object

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
How to Call a Method in a Running Thread Object in Java

As a beginner in Java, encountering errors while working with threads can be quite frustrating. One common issue arises when you try to invoke a method on a running Thread object, but instead, you end up with a NullPointerException. This guide addresses that problem and provides a detailed solution to ensure your thread methods are called correctly.

Understanding the Problem

When you attempt to call methods like next() or last() on your Graphic_handler thread, you may run into the following exception:

[[See Video to Reveal this Text or Code Snippet]]

This error indicates that the grap variable is null during the action event handling. Let's explore why this happens.

Why the NullPointerException Occurs

The issue arises from variable scope. In Java, when you declare a variable inside a method, it can hide an instance variable of the same name. In your code, the main method also defines a local variable named grap, which hides the class-level variable grap. As a result, when you try to use the instance variable later, it remains uninitialized, leading to a NullPointerException.

Here's the flawed part of your code:

[[See Video to Reveal this Text or Code Snippet]]

The Solution

To resolve this issue, you need to avoid hiding the instance variable by removing the local declaration inside the main method. This will ensure that you are using the correct instance variable that is initialized.

Here’s how to modify your code:

[[See Video to Reveal this Text or Code Snippet]]

Key Changes Made:

Removed Local Declaration: By only assigning to grap without declaring it again, you ensure that you are referencing the instance variable.

No Scope Confusion: This prevents scope issues where the local variable overshadows an instance variable, allowing your methods to be called without encountering a null reference.

Conclusion

Debugging Java applications, especially those involving multithreading, can be challenging. Understanding variable scope plays a crucial role in avoiding common pitfalls like NullPointerException. By following the solution outlined in this post, you can successfully call methods on running thread objects in Java.

Use this newfound clarity to advance your Java programming skills and build more robust applications. Happy coding!