In a method that accepts an object as an argument, writing code that accidentally modifies the object. When a reference variable is passed as an argument to a method, the method has access to the object that the variable references. When writing a method that receives a reference variable as an argument, you must take care not to accidentally modify the contents of the object that is referenced by the variable. Why is this an error? Why shouldn't a method external to an object be able to modify attributes of an object?