.. role:: raw-html-m2r(raw)
   :format: html

NullPointerException
====================

Introduction
------------

``NullPointerException`` is a Scala runtime reported error which can happen when a variable is accessed before it has been initialized.

Example
-------

The following code:

.. code-block:: scala

   class TopLevel extends Component {
     a := 42
     val a = UInt(8 bits)
   }

will throw:

.. code-block:: text

   Exception in thread "main" java.lang.NullPointerException
     ***
     Source file location of the a := 42 assignment via the stack trace
     ***

A fix could be:

.. code-block:: scala

   class TopLevel extends Component {
     val a = UInt(8 bits)
     a := 42
   }

Issue explanation
^^^^^^^^^^^^^^^^^

SpinalHDL is not a language, it is a Scala library, which means that it obeys the same rules as the Scala general purpose programming language.

When running the above SpinalHDL hardware description to generate the corresponding VHDL/Verilog RTL, the SpinalHDL hardware description will be executed as a Scala program, and ``a`` will be a null reference until the program executes ``val a = UInt(8 bits)``, so trying to assign to it before then will result in a ``NullPointerException``.