code: public List<MockActiveContext> getIdleActiveContexts() {
      final List<MockActiveContext> idleActiveContexts = new ArrayList<>();
      final Set<String> activeContextsWithRunningTasks = new HashSet<>();
      for (final MockRunningTask task : this.runningTasks) {
        activeContextsWithRunningTasks.add(task.getActiveContext().getEvaluatorId());
      }
      for (final MockActiveContext context : this.activeContexts) {
        if (!activeContextsWithRunningTasks.contains(context.getEvaluatorId())) {
          idleActiveContexts.add(context);
        }
      }
      return idleActiveContexts;
    }
docs: Pass these active contexts to the {@link org.apache.reef.driver.parameters.DriverRestartContextActiveHandlers}.
  These active contexts have no tasks running.
  @return