Null safety

Probably the most notorious exception in the Java world is NullPointerException. 

The reason behind this exception is that every object in Java can be null. The code here shows us why:

String s = "Hello";
...
s = null;
System.out.println(s.length); // Causes NullPointerException

In this case, marking s as final would prevent the exception.

But what about this one:

public class Printer {    
    public static void printLength(final String s) {
       System.out.println(s.length);
    }
}

From anywhere in the code it's still possible to pass null:

Printer.printLength(null); // Again, NullPointerException

Since Java 8, there's been an optional construct:

if (optional.isPresent()) {
    System.out.println(optional.get());
}

In a more functional style:

optional.ifPresent(System.out::println);

But... it doesn't solve our problem. We can still pass null instead of the proper Optional.empty() and crash the program.

Kotlin checks it even earlier—during compile time:

val s : String = null // Won't compile

Let's go back to our printLength() function:

fun printLength(s: String) {
    println(s.length)
}

Calling this function with null won't compile any more:

printLength(null) // Null can not be a value of a non-null type String

If you specifically want your type to be able to receive nulls, you'll need to mark it as nullable using the question mark:

val notSoSafe : String? = null