之间有什么区别！和 ?在科特林? [英] What's the difference between !! and ? in Kotlin?

问题描述

我是Kotlin的新手.我想知道下面的代码中这两个!!和?之间的区别.

I am new to Kotlin. I want to know the difference between this two !! and ? in below code.

下面有两个代码段:第一个使用!!表示mCurrentDataset，另一个使用?表示相同的变量.

Below, there are two snippets: the first uses !! for mCurrentDataset and another having ? for same variable.

if(!mCurrentDataset!!.load(mDataSetString.get(mCurrentDataSelectionIndex), STORAGE_TYPE.STORAGE_APPRESOURCE))
{
    Log.d("MyActivity","Failed to load data.")
    return false
}

---

if(!mCurrentDataset?.load(mDataSetString.get(mCurrentDataSelectionIndex), STORAGE_TYPE.STORAGE_APPRESOURCE)!!)
{
    Log.d("MyActivity","Failed to load data.")
    return false
}

推荐答案

Kotlin参考书，!!是NPE爱好者的选择:)

As it said in Kotlin reference, !! is an option for NPE-lovers :)

a!!.length

如果a为null，

将返回非null值a.length或引发 NullPointerException :

val a: String? = null
print(a!!.length) // >>> NPE: trying to get length of null

---

a?.length

如果a不是null，则返回a.length，否则返回null:

returns a.length if a is not null, and null otherwise:

val a: String? = null
print(a?.length) // >>> null is printed in the console

---

总结:

---

To sum up:

+------------+--------------------+---------------------+----------------------+
| a: String? |           a.length |           a?.length |           a!!.length |
+------------+--------------------+---------------------+----------------------+
|      "cat" | Compile time error |                   3 |                    3 |
|       null | Compile time error |                null | NullPointerException |
+------------+--------------------+---------------------+----------------------+

可能有用:什么是NullPointerException?

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