將帶有空字段的對象轉換爲具有非空字段的對象或爲空

我有一個由API返回的對象，該對象具有不同的可空字段。我想用非空字段創建另一個對象，如果不可能則返回null。最適合這種情況的語言習語是什麼？目前我使用?.let {}，它看起來很醜陋：將帶有空字段的對象轉換爲具有非空字段的對象或爲空

fun convertAnswer(userAnswer: AnswerGson, answerResponse: AnswerResponseGson, 
        correctAnswerText: String): AnswerResponseUi? { 
    return userAnswer.id?.let { userAnswerId -> 
     userAnswer.text?.let { userAnswerText -> 
      answerResponse.answer?.id?.let { correctAnswerId -> 
       answerResponse.points?.let { points -> 
        answerResponse.discount?.let { discount -> 
         answerResponse.booster?.let { booster -> 
          return AnswerResponseUi(userAnswerId, correctAnswerId, userAnswerText, 
            correctAnswerText, points, discount, booster.name ?: "") 
         } 
        } 
       } 
      } 
     } 
    } 
}

來源

2016-10-19 TpoM6oH

如果你的類AnswerGson和AnswerResponseGson聲明爲val它們的屬性，您可以將您的代碼轉換爲這樣的事情：

fun convertAnswer(userAnswer: AnswerGson, answerResponse: AnswerResponseGson, 
        correctAnswerText: String): AnswerResponseUi? { 
    if (userAnswer.id == null || 
     userAnswer.text == null || 
     answerResponse.discount == null || 
     answerResponse.points == null || 
     answerResponse.booster == null || 
     answerResponse.answer == null || 
     answerResponse.answer.id == null 
    ) return null 

    return AnswerResponseUi(userAnswer.id, answerResponse.answer.id, userAnswer.text, 
          correctAnswerText, answerResponse.points, 
          answerResponse.discount, answerResponse.booster.name ?: "") 
}

本示例使用smart casts：編譯器分析控制流並證明如果最後一條語句是reac hed然後沒有上面檢查的值是null。

這些檢查不適用於安全解除鏈，我必須先檢查answerResponse.answer，然後再檢查answerResponse.answer.id。

如果你有var性能，智能轉換不能應用，因爲它被選中後的值可能會改變。

fun convertAnswer(userAnswer: AnswerGson, answerResponse: AnswerResponseGson, 
        correctAnswerText: String): AnswerResponseUi? { 
    val answerId = userAnswer.id ?: return null 
    val correctAnswerId = answerResponse.answer?.id ?: return null 
    val userAnswerText = userAnswer.text ?: return null 
    val points = answerResponse.points ?: return null 
    val discount = answerResponse.discount ?: return null 
    val booster = answerResponse.booster ?: return null 

    return AnswerResponseUi(answerId, correctAnswerId, userAnswerText, 
          correctAnswerText, points, 
          discount, booster.name ?: "") 
}

而且，作爲@mfulton26注意到，你甚至可以內嵌這些：

在這種情況下，你仍然可以通過提取值到本地變量，並在同一時間檢查它們減少代碼嵌套變量（適用於val s）：

fun convertAnswer(userAnswer: AnswerGson, answerResponse: AnswerResponseGson, 
        correctAnswerText: String): AnswerResponseUi? { 
    return AnswerResponseUi(userAnswer.id ?: return null, 
          answerResponse.answer?.id ?: return null, 
          userAnswer.text ?: return null, 
          correctAnswerText, 
          answerResponse.points ?: return null, 
          answerResponse.discount ?: return null, 
          (answerResponse.booster ?: return null).name ?: "") 
}

來源

2016-10-19 16:52:09 hotkey

哇......我剛纔意識到，如果你不想要它們，甚至可以內聯所有這些局部變量。例如'... return AnswerResponseUi（userAnswer.id？：return null，answerResponse.answer？.id？：return null，...' – mfulton26

可讀性是關鍵，所以不要嘗試內聯所有內容;）但這確實令人印象深刻！ – voddan

將帶有空字段的對象轉換爲具有非空字段的對象或爲空

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