为什么Enum被认为比常量更安全？

Question

In our example, we can choose to define an Enumerated Type that will restrict the possible assigned values (ie improved type-safety):

public class OfficePrinter {

public enum PrinterState { Ready, OutOfToner, Offline };
public static final PrinterState STATE = PrinterState.Ready;
}

static final char MY_A_CONST = 'a';

Answer 1

Imagine these two method signatures:

void rawF(char someFlag);

void enumF(MyFlags someFlag);

The latter is more restrictive as only the valid values of MyFlags are allowed. In the former case, any character could be passed - even if only the values defined in "constants" where used.

Happy coding.

Answer 2

You could pass MY_A_CONST to any method that takes a char. You could also pass any other char to a method that takes a char.

You could pass Ready, OutOfToner, Offline, and null to a method that takes a PrinterState.

You get safety by being able to limit the total set of values that can be passed to a method (or assigned to a variable).

Answer 3

Using enum over constants helps with type safety because if a function takes an enum and you pass it anything but an enum, the compiler will complain. With constants, you're accepting a pretty large range of data, most of which are invalid.