"?" Added after the variable name, the system will not report in any case it is a null pointer exception.
"!!" followed by the variable name, if the object is null, then the system will report an exception!
var variable is a variable which is a variable can be changed to another value by redistribution. This way declare variables and methods in java declare variables like. val is a read-only variables, this embodiment corresponds to declare variables in the final java variables. When a val created must be initialized, because the future can not be changed.
In Kotlin, the address comparison indicates === == shows a comparison of two values Size
