The Google Java Style Guide §4.8.4.3 requires each switch statement to include a default statement group, even if it contains no code.

NOTE: A switch statement for an enum type may omit the default statement group, if it includes explicit cases covering all possible values of that type. See MissingCasesInEnumSwitch for more information.

Without a default, the reader does not always know whether execution might silently “fall out” of the entire block, having executed no code within it. This is undesirable for most of the same reasons silent fall-through is undesirable.

If the unhandled cases should be impossible, add a default clause that throws AssertionError:

enum State { READY, DONE, RUNNING, BLOCKED }

switch (state) {
  case READY:
    return true;
  case DONE:
    return false;
  default:
    throw new AssertionError("unexpected state: " + state);
}

If having execution fall out of the switch is intentional, add a default clause with a comment:

enum State { READY, DONE, RUNNING, BLOCKED }

switch (state) {
  case READY:
    return true;
  case DONE:
    return false;
  default: // continue below to handle RUNNING/BLOCKED/etc.
}