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

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
}

---

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

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

---

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

---

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?

this is '!!' double-bang operator is always return not-null value and this is '?' safe call operator returns value if value is not null, and null otherwise

This is unsafe nullable type (T?) conversion to a non-nullable type (T). It will throw NullPointerException if the value is null.

It is documented here along with Kotlin means of null-safety.

ref - hotkey

the precedence of operators !, ?., !! is ?. > !! > !.

the !! operator will raising KotlinNullPointerException when operates on a null reference, for example:

null!!;// raise NullPointerException

the safe call ?. operator will return null when operates on a null reference, for example:

(null as? String)?.length; // return null;

the !! operator in your second approach maybe raise NullPointerException if the left side is null, for example:

mCurrentDataset?.load(..)!!
    ^-------------^
           | 
when mCurrentDataset== null || load() == null a NullPointerException raised.

you can using the elvis operator ?: instead of the !! operator in your case, for example:

!(mCurrentDataset?.load(..)?:false)

In Addition to what Alexander said and as shown in the docs too, 

the ?. safe call operator is very useful in chaining, something like this

student?.department?.hod?.name

if there is no student, returns null otherwise look for his department. If department doesn't exist returns null otherwise look for hod (head of department) and so on.

If any one of student, department or hod is null then the result will be null.

Safe Calls operator

In Kotlin

var a = x?.length;

Equivalent code in Java

int a = valueOfInt();

int valueOfInt() {
    if (x != null) {
        return x;
    } else {
        return null;
    }
}

Side chain rule

bob?.department?.head?.name

it can be read as->

If bob is not null give me the department ,

if department is not null give me the head,

if head is not null give me the name.

If any of it is null, then it returns null

? before Data type

If ? used before data type like:

val b: String? = null

it means you can assign null value to it otherwise null value can't be assigned to it.

The !! Operator

For those who like to have Null Pointer Exception (NPE) in their program.

val l = b!!.length

this will return a non-null value of b if b is not null OR throw an NPE if b is null

SafeCall Operator(?):

var a: String = "abc"
a = null  //compile time error

val b: String? = null
val result = b?.length//returns null

Assertion Operator(!!):

 val b: String? = "dd" //any value or null 
 val l = b!!.length 
//this throws null pointer exception if b is null otherwise returns actual