if (monitor.enterWhen(notEmpty, timeout, unit)) {
      try {
        return extract();
      } finally {
        monitor.leave();
      }
    } else {
      return null;
    }
  }

  @CanIgnoreReturnValue
  @Override
  public E take() throws InterruptedException {
    final Monitor monitor = this.monitor;
    monitor.enterWhen(notEmpty);
    try {
      return extract();

  private static final long serialVersionUID = 5595510919245408276L;

  final PriorityQueue<E> q;
  final Monitor monitor = new Monitor(true);
  private final Monitor.Guard notEmpty =
      new Monitor.Guard(monitor) {
        @Override
        public boolean isSatisfied() {
          return !q.isEmpty();
        }
      };

  /**

constraints.ModCheck.message                = The check digit for ${value} is invalid, ${modType} checksum failed.
constraints.NotBlank.message                = {item} may not be empty.
constraints.NotEmpty.message                = {item} may not be empty.
constraints.ParametersScriptAssert.message  = script expression "{script}" didn't evaluate to true.
constraints.Range.message                   = {item} must be between {min} and {max}.

constraints.ModCheck.message                = Le chiffre de contrôle pour ${value} n'est pas valide, la somme de contrôle ${modType} a échoué.
constraints.NotBlank.message                = {item} ne peut pas être vide.
constraints.NotEmpty.message                = {item} ne peut pas être vide.
constraints.ParametersScriptAssert.message  = l'expression de script "{script}" n'a pas été évaluée comme vraie.

constraints.Mod11Check.message = {value}의 Modulo 11 checksum이 올바르지 않습니다.
constraints.ModCheck.message = {value}의 {modType} checksum이 올바르지 않습니다.
constraints.NotBlank.message = {item}은 미입력입니다.
constraints.NotEmpty.message = {item}은 미입력입니다.
constraints.ParametersScriptAssert.message = 스크립트 식「{script}」이 true가 없습니다.
constraints.Range.message = {item}는 {min}에서 {max}의 범위에 있어야합니다.
constraints.SafeHtml.message = {item}은 위험한 HTML 콘텐츠가 포함되어 있습니다.

constraints.ModCheck.message                = Die Prüfziffer für ${value} ist ungültig, die ${modType}-Prüfsumme ist fehlgeschlagen.
constraints.NotBlank.message                = {item} darf nicht leer sein.
constraints.NotEmpty.message                = {item} darf nicht leer sein.
constraints.ParametersScriptAssert.message  = Der Skript-Ausdruck "{script}" hat kein wahres Ergebnis geliefert.

constraints.ModCheck.message                = The check digit for ${value} is invalid, ${modType} checksum failed.
constraints.NotBlank.message                = {item} may not be empty.
constraints.NotEmpty.message                = {item} may not be empty.
constraints.ParametersScriptAssert.message  = script expression "{script}" didn't evaluate to true.
constraints.Range.message                   = {item} must be between {min} and {max}.

constraints.ModCheck.message                = The check digit for ${value} is invalid, ${modType} checksum failed.
constraints.NotBlank.message                = {item} may not be empty.
constraints.NotEmpty.message                = {item} may not be empty.
constraints.ParametersScriptAssert.message  = script expression "{script}" didn't evaluate to true.
constraints.Range.message                   = {item} must be between {min} and {max}.

constraints.Mod11Check.message = {value} の Modulo 11 チェックサムが正しくありません。
constraints.ModCheck.message = {value} の {modType} チェックサムが正しくありません。
constraints.NotBlank.message = {item} は未入力です。
constraints.NotEmpty.message = {item} は未入力です。
constraints.ParametersScriptAssert.message = スクリプト式「{script}」がtrueではありません。
constraints.Range.message = {item} は {min} から {max} の範囲にある必要があります。
constraints.SafeHtml.message = {item} は危険な HTML コンテンツを含んでいます。

    /** The key of the message: {item} may not be empty. */
    public static final String CONSTRAINTS_NotEmpty_MESSAGE = "{constraints.NotEmpty.message}";

    /** The key of the message: script expression "{script}" didn't evaluate to true. */
    public static final String CONSTRAINTS_ParametersScriptAssert_MESSAGE = "{constraints.ParametersScriptAssert.message}";