Kotlin Null Safety Tutorial

1. Introduction

Goal

This tutorial aims to provide an in-depth look into the null safety feature in Kotlin, a significant improvement from the Java programming language.

Learning Outcomes

By the end of this tutorial, you will understand how to declare nullable and non-nullable types, how to handle null pointer exceptions, and best practices for using null safety in Kotlin.

Prerequisites

• Basic understanding of Kotlin syntax and data types
• Familiarity with the concept of nullability in programming

2. Step-by-Step Guide

In Kotlin, every variable must be declared either as nullable or non-nullable. A nullable variable can hold a null value, whereas a non-nullable variable cannot.

Non-Nullable Types

By default, variables are non-nullable in Kotlin. Here's an example:

var name: String = "Kotlin"

If you try to assign null to this variable, it will result in a compile-time error:

name = null // Compilation error

Nullable Types

To declare a variable as nullable, append a question mark to the type:

var name: String? = "Kotlin"

Now, you can assign null to this variable:

name = null // No error

Null Safety Operators

Kotlin provides several operators for safely dealing with nullable types:

• Safe Call Operator ?. : It returns null if used on a null reference.
• Non-null Assertion Operator !! : Converts nullable type to a non-nullable type, and throws a NullPointerException if the nullable type holds a null value.
• Elvis Operator ?: : Returns the left-hand expression if it's non-null; otherwise, it returns the right-hand expression.

3. Code Examples

Example 1: Safe Call Operator

var name: String? = null

// Use safe call operator
println(name?.length) // It will print 'null' because name is null.

Example 2: Non-null Assertion Operator

var name: String? = null

// Use non-null assertion operator
println(name!!.length) // It will throw NullPointerException because name is null.

Example 3: Elvis Operator

var name: String? = null

// Use Elvis operator
println(name?.length ?: "Name is null") // It will print 'Name is null' because name is null.

4. Summary

We've learned that Kotlin provides a robust system for avoiding null pointer exceptions. By distinguishing nullable and non-nullable types, and providing helpful operators, Kotlin makes your code safer and more expressive.

5. Practice Exercises

1. Declare a nullable String variable and a non-nullable Int variable. Try assigning null to both of them and observe the results.

2. Experiment with the safe call operator, non-null assertion operator, and Elvis operator. Create different scenarios to understand their functionality.

Solutions

var nullableString: String? = "Kotlin"
nullableString = null // No error

var nonNullableInt: Int = 10
nonNullableInt = null // Compilation error

2.

var name: String? = null

// Safe call operator
println(name?.length) // prints null

// Non-null assertion operator
// println(name!!.length) // Uncommenting this will throw NullPointerException

// Elvis operator
println(name?.length ?: "Name is null") // prints 'Name is null'

Continue exploring more about Kotlin's null safety features from the official Kotlin documentation.