What's the difference between !! and ? in Kotlin?

Question

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

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 } 

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

Answer 1

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

a!!.length 

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 

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a?.length 

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

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

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To sum up:

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

Might be useful: What is a NullPointerException?