We need to create validators for each status. The validator will check if transition from current state is allowed or not. 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 {

    private static final Map<WorkflowExecutionStatus, StatusValidator> statusValidatorMap = new HashMap<>();

    static {
        statusValidatorMap.put(WorkflowExecutionStatus.WAITING_FOR_INPUT, new WaitingForInputValidator());
        statusValidatorMap.put(WorkflowExecutionStatus.COMPLETED, new CompletedValidator());
        statusValidatorMap.put(WorkflowExecutionStatus.FAILED, new FailedValidator());
    }

    public static void validate(WorkflowExecution workflowExecution, WorkflowExecutionStatus targetStatus) {
        StatusValidator statusValidator = statusValidatorMap.get(workflowExecution.getStatus());
        if (statusValidator == null) {
            throw new IllegalStateException("No validator for status " + workflowExecution.getStatus());
        }
        statusValidator.validate(workflowExecution, targetStatus);
    }
}