The Kotlin double-bang !! operator is used to assert that a nullable value is not null and convert it to a non-nullable type[1][2][3]. It is also known as the "not-null assertion operator"[1][2].
When you use the double-bang operator on a nullable value, you are telling the compiler that you know for sure the value is not null and it is safe to access its properties or methods[1]. However, if the value is actually null, a NullPointerException will be thrown at runtime[1][2][3].
The double-bang operator should be used with caution, as it can lead to runtime exceptions if the value is unexpectedly null[3][4]. It is generally better to use null-safe calls with the safe call operator ?. or the Elvis operator ?:[3][4].
