exception in thread "main" in java.lang.NullPointerException

This error occurs when a program attempts to use a null reference, which means that it is trying to access an object or variable that has not been initialized or has been set to null. This can happen when a variable is declared but not assigned a value, or when an object is not properly instantiated. To fix this error, you need to identify the null reference and either initialize it or assign it a valid value.