# V6055. Expression inside assert statement can change object's state.

The analyzer has detected a problem where an 'assert' statement contains a method modifying an object's state. How exactly such methods are called depends on the settings of the Java Virtual Machine, which means the program's behavior may differ from what is expected.

Consider the following example:

    void someFunction(List<String> listTokens)
    {
      ....
      assert "<:>".equals(listTokens.remove(0));
      ....
    }

What we are interested in here is the call 'listTokens.remove(0)'. This method removes the first element in 'listTokens' and thus modifies the collection. The removed string is then returned to be compared with some expected string. The problem is that if assertions are disabled for the project, the expression will not be evaluated and, therefore, the first element of the collection will not be removed, which may affect the program's subsequent behavior.

To avoid such situations, make sure your assert statements do not contain calls to functions that change objects' contents.

This is what the fixed version looks like:

    void someFunction(List<String> listTokens)
    {
      ....
      boolean isFirstStr = "<:>".equals(listTokens.remove(0));
      assert isFirstStr;
      ....
    }

This diagnostic is classified as:

CERT-EXP06-J