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.

下面有两个片段: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
}

<小时>

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

推荐答案

正如 中所说Kotlin 参考，!! 是 NPE 爱好者的选择:)

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

a!!.length

将返回一个 a.length 的非空值，或者如果 a 是 null 则抛出一个 NullPointerException:

will return a non-null value of a.length or throw a NullPointerException if a is null:

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

<小时>

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

<小时>

总结:

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

可能有用:什么是空指针异常?

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