One major advantage in Kotlin is its Type System. It eliminates the the danger of having null references.

For example, if we declare and initialize a variable, it cannot be set to null anytime after.

var name: String = "John"

name = null 

In the above example, name = null will cause a compilation error.

Nullable References

To allow variables to accept null values, we use the "?" symbol after the type. 

var nane: String? = "John"

Now we can set the variable to null.

name = null

However, when we try to access a property of a nullable variable, it will not allow us to. We will get a compilation error. 

var numberOfCharactersInName = name.length

Handling Nullable References

To safely access a property of a nullable reference, we need to use the Safe Call operator (?).

var numberOfCharactersInName = name?.length

In the example, the Safe Call operator is placed before the Dot Operator to access the length property from the name variable.

Another way we can safely access a property of a nullable reference is to check if the reference is not null using a If Statement.

if(name != null) {

or

var numberOfCharactersInName = if(name != null) name.length else 0

Elvis Operator

The Elvis Operator is similar to an If Statement.

Syntax

[expression] ?: [value_to_return_if_null] 

Instead of using the full If statement - if(name != null) ... else ... - we can shorten the code with the Elvis Operator.

var numberOfCharactersInName = name?.length?: 20;

If the expression on the left of the (?:) is not null, it will return that value, otherwise, it will return the value on the right.

Not-Null Assertion Operator (!!)

The Not-Null Assertion Operator (!!) forces the compiler to ignore its checks on the reference to check whether the value is null or not.

var numberOfCharactersInName = name!!.length;

If name = null, it will throw an error.