public Object getConstantValue()
Returns the value of this variable if this is a {@code final}
field initialized to a compile-time constant.  Returns {@code
null} otherwise.  The value will be of a primitive type or a
{@code String}.  If the value is of a primitive type, it is
wrapped in the appropriate wrapper class (such as Integer ).

Note that not all {@code final} fields will have
constant values.  In particular, {@code enum} constants are
not considered to be compile-time constants.  To have a
constant value, a field's type must be either a primitive type
or {@code String}.