Using Enum's ordinal value in switch-case statement

user_699908 0
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For my project I am using enums, and I need to implement the switch-case statement, where ordinal numbers of values of the specific Enum are checked, like this way:

        switch ( variable )
        {
        case MyEnum.A.ordinal():
            return true;
        case MyEnum.B.ordinal():
            return true;
        default:
            return false;
        }

Note: return values are only an example.

Unfortunately, Eclipse (I'm using 1.6 JDK) gives my compilation error "case expressions must be constant expressions". What I should do? Is there any other method than static lookup table, described here: Convert from enum ordinal to enum type ?

switch-statement enums java
7 Answers

It is because the compiler sees differences. For example this enum code, we can see:

public enum TrafficLight {RED, YELLOW, GREEN}

TrafficLight trafficLights = ...
switch (trafficLights) {
  case RED: {/* do stuff */}
  case YELLOW: {/* do stuff */}
  case GREEN: {/* do stuff */}
}

BUT the compiler see:

switch (trafficLights.ordinal()) {
      case 0: {/* do stuff */}
      case 1: {/* do stuff */}
      case 2: {/* do stuff */}
    }

That's why it throws NPE when trafficLights are NULL and you also don't know why it throws NPE in the ordinal() function even though we didn't call that method.

SOLUTION is checking NULL the ENUM before it reaches to the switch-case.

if (trafficLights != null) {
  switch (trafficLights) {
  case RED: {/* do stuff */}
  case YELLOW: {/* do stuff */}
  case GREEN: {/* do stuff */}
  }
}
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