Kotlin supports a more strict type system when compared to Java, and divides all types into two groups, as follows:

• Nullable
• No-nullable

One of the most popular causes of an app crashing is the NullPointerException. This happens as a result of accessing a member of a null reference. Kotlin provides a mechanism that helps us to avoid this error by using a type system.

The following diagram shows what the class hierarchy looks like in Kotlin:

In Kotlin, nullable types have the same names as no-nullable types, except with the ? character at the end.

If we use a no-nullable variable, we can't assign null to it, and the following code can't be compiled:

var name = "Igor"
name = null

To be able to compile this code, we have to explicitly declare the name variable as nullable:

var name: String? = "Igor"
name = null

After doing this, we cannot compile the following code:

name.length

To access members of nullable types, we have to use the ?. operator, like in the following example:

name?.length

One expression can contain the ?. operator as many times as needed:

name?.length?.compareTo(4)

If a member in this chain is null, the next member can't be invoked.

To provide an alternative program flow, if null is encountered, we can use the Elvis operator (?:). This can be used in the following way:

name?.length?.compareTo(4) ?: { println("name is null") }()

The preceding snippet demonstrates that the Elvis operator can be used if we want invoke a block of code if an expression returns as null.