Exception in thread "main" java.lang.NoClassDefFoundError: com/dingtalk/api/DefaultDingTalkClient

This error occurs when the JVM is unable to find the class definition for the specified class, in this case, "com.dingtalk.api.DefaultDingTalkClient". This can happen due to various reasons such as:

1. The class is not included in the classpath.
2. The class file is not present in the specified directory.
3. The class file is corrupt or incomplete.

To resolve this issue, you can try the following steps:

1. Check if the required library or jar file is included in the classpath.
2. Make sure the class file is present in the specified directory.
3. Ensure that the version of the library or jar file is compatible with your current JVM version.
4. If the above steps don't work, try recompiling the code or rebuilding the project.

If none of the above steps work, you may need to seek help from the library or framework developers or consult the Java documentation for further guidance.