guava/src/com/google/common/util/concurrent/AbstractService.java
@GuardedBy("monitor")
  private void checkCurrentState(State expected) {
    State actual = state();
    if (actual != expected) {
      if (actual == FAILED) {
        // Handle this specially so that we can include the failureCause, if there is one.
        throw new IllegalStateException(
            "Expected the service " + this + " to be " + expected + ", but the service has FAILED",
            failureCause());
      }
      throw new IllegalStateException(
          "Expected the service " + this + " to be " + expected + ", but was " + actual);
    }
  }
@GuardedBy("monitor")
  private void checkCurrentState(State expected) {
    State actual = state();
    if (actual != expected) {
      if (actual == FAILED) {
        // Handle this specially so that we can include the failureCause, if there is one.
        throw new IllegalStateException(
            "Expected the service " + this + " to be " + expected + ", but the service has FAILED",
            failureCause());
      }
      throw new IllegalStateException(
          "Expected the service " + this + " to be " + expected + ", but was " + actual);
    }
  }
Checks that the current state is equal to the expected state.