Kotlin Null Safety

• ?: allow null
• ?.: safe access
• !!: not null assertion
• as?: safe casts

NPE

The only possible causes of an NPE in Kotlin are:

• An explicit call to throw NullPointerException().
• Usage of the not-null assertion operator !!.
• Data inconsistency during initialization, such as when:
  • An uninitialized this available in a constructor is used somewhere else (a “leaking this ”).
  • A superclass constructor calling an open member whose implementation in the derived class uses an uninitialized state.
• Java interoperation:
  • Attempts to access a member of a null reference of a platform type.
  • Nullability issues with generic types. For example, a piece of Java code adding null into a Kotlin MutableList<String>, which would require MutableList<String?> to handle it properly.
  • Other issues caused by external Java code.
• Null safety | Kotlin Documentation