WorkflowExecutionValidator will pick up an appropriate validator from a list and check if transition is allowed. Validators are implemented for each workflow execution status Validators are applied in production code

public class WorkflowExecutionValidator  {
  public static boolean isTransitionAllowed(WorkflowExecution wf, WorkflowExecution.State nextState) {
    if (wf.getState() == nextState) {
      // no transition if previous state and next state are the same
      return false;
    }
    return getValidator(wf.getState()).isTransitionAllowed(nextState);
  }

  private static StateValidator getValidator(WorkflowExecution.State state) {
    switch (state) {
    case STARTED:
      return new StartedValidator();
    case RUNNING:
      return new RunningValidator();
    case SUCCEEDED:
      return new SucceededValidator();
    case FAILED:
      return new FailedValidator();
    case CANCELLED:
      return new CancelledValidator();
    case TIMED_OUT:
      return new TimedOutValidator();
    default:
      throw new IllegalArgumentException("State not expected: " + state);
    }