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README.html
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<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Kotlin Beginners Notes</title>
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<h1 id="kotlin-beginners-notes">Kotlin Beginners Notes</h1>
<p>These are all personal notes taken from the Udacity Course (ud9011) of Kotlin Bootcamp for Programmers by Google as well as other resources. You can use it to learn Kotlin if you are a <strong>beginner</strong>, I have taken most of the things mentioned in the all sections/videos of this course including some brief pieces from official documentation and official video of JetBrains as well. I also cared about the order of the topics, so it starts from the basics from top to bottom goes to the more advanced ones. This is not an official documentation. You will be probably finding more personal notes and human mistakes :)</p>
<p><img src="https://user-images.githubusercontent.com/36234545/127040534-c2448cfd-a4c4-43cd-9bb3-c03787b37e51.png" alt="2021-07-26 21_33_44-Editing Kotlin-Notes_README md at main · shtanriverdi_Kotlin-Notes — Mozilla Fir"></p>
<h1 id="table-of-contents">Table of Contents</h1>
<ul>
<li><a href="#l1">Lesson 1 & 2 Introduction | Kotlin Basics</a></li>
<li><a href="#l3">Lesson 3 | Functions</a></li>
<li><a href="#l4">Lesson 4 | Classes</a></li>
<li><a href="#l5">Lesson 5 | Kotlin Essentials: Beyond The Basics</a></li>
</ul>
<h2 id="l1">Lesson 1 & 2 Introduction | Kotlin Basics</h2>
<h3 id="package-definition-and-imports">Package Definition and Imports</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">// Package specification should be at the top of the source file.</span>
<span class="hljs-keyword">package</span> my.demo
<span class="hljs-keyword">import</span> kotlin.text.*
<span class="hljs-comment">// Main function</span>
<span class="hljs-function"><span class="hljs-keyword">fun</span> <span class="hljs-title">main</span><span class="hljs-params">(args: <span class="hljs-type">Array</span><<span class="hljs-type">String</span>>)</span></span> {
printHello ()
}
<span class="hljs-comment">// Alternative main, no need to write parameters!</span>
<span class="hljs-function"><span class="hljs-keyword">fun</span> <span class="hljs-title">main</span><span class="hljs-params">()</span></span> {
printHello ()
}
<span class="hljs-comment">// Prints "Hello Kotlin", println() puts newline, print()</span>
<span class="hljs-function"><span class="hljs-keyword">fun</span> <span class="hljs-title">printHello</span> <span class="hljs-params">()</span></span> {
println(<span class="hljs-string">"Hello Kotlin!"</span>)
}
<span class="hljs-comment">// A function that returns "OK" string</span>
<span class="hljs-function"><span class="hljs-keyword">fun</span> <span class="hljs-title">start</span><span class="hljs-params">()</span></span>: String = <span class="hljs-string">"OK"</span>
<span class="hljs-comment">// Function returns "Genesis", no need to write "return"</span>
<span class="hljs-function"><span class="hljs-keyword">fun</span> <span class="hljs-title">returnString</span> <span class="hljs-params">()</span></span> : String = <span class="hljs-string">"Genesis"</span>
<span class="hljs-comment">// Function returns "Genesis2" Alternative Way</span>
<span class="hljs-function"><span class="hljs-keyword">fun</span> <span class="hljs-title">returnString2</span> <span class="hljs-params">()</span></span> : String {
<span class="hljs-keyword">return</span> <span class="hljs-string">"Genesis2"</span>
}
</div></code></pre>
<h3 id="operators-----">Operators, -, +, /, *</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">// Returns integer</span>
println(<span class="hljs-number">1</span> + <span class="hljs-number">1</span>) <span class="hljs-comment">// Prints: 2</span>
println(<span class="hljs-number">53</span> - <span class="hljs-number">3</span>) <span class="hljs-comment">// Prints: 50</span>
println(<span class="hljs-number">50</span> / <span class="hljs-number">10</span>) <span class="hljs-comment">// Prints: 5</span>
println(<span class="hljs-number">1</span> / <span class="hljs-number">2</span>) <span class="hljs-comment">// Prints: 0</span>
println(<span class="hljs-number">6</span> * <span class="hljs-number">50</span>) <span class="hljs-comment">// Prints: 300</span>
<span class="hljs-comment">// Returns double</span>
println(<span class="hljs-number">1.0</span> / <span class="hljs-number">2.0</span>) <span class="hljs-comment">// Prints: 0.5</span>
println(<span class="hljs-number">1.0</span> / <span class="hljs-number">2</span>) <span class="hljs-comment">// Prints: 0.5</span>
println(<span class="hljs-number">2</span> / <span class="hljs-number">2.0</span>) <span class="hljs-comment">// Prints: 1.0</span>
<span class="hljs-comment">// Kotlin let's you overwrite the basic operators</span>
<span class="hljs-comment">// You can call methods on variables</span>
<span class="hljs-keyword">val</span> fish = <span class="hljs-number">2</span>
println(fish.times(<span class="hljs-number">6</span>)) <span class="hljs-comment">// Prints: 12</span>
println(fish.div(<span class="hljs-number">10.0</span>)) <span class="hljs-comment">// Prints: 0.2</span>
println(fish.plus(<span class="hljs-number">3</span>)) <span class="hljs-comment">// Prints: 5</span>
println(fish.minus(<span class="hljs-number">3</span>)) <span class="hljs-comment">// Prints: -1</span>
<span class="hljs-comment">// You can use numbers(basic types) as if they were objects</span>
<span class="hljs-comment">// Use primitive 'int' as an object</span>
<span class="hljs-number">1</span>.toLong() <span class="hljs-comment">// 1</span>
println(<span class="hljs-literal">false</span>.not()) <span class="hljs-comment">// true</span>
</div></code></pre>
<h3 id="boxing">Boxing</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">// Boxing describes the process of converting a primitive value to an object and unboxing therefore the inverse</span>
<span class="hljs-comment">// All numerical types in Kotlin have a supertype called Number</span>
<span class="hljs-comment">// Store value one in a variable of type Number</span>
<span class="hljs-comment">// It'll need to be placed in an object wrapper</span>
<span class="hljs-comment">// This is called boxing</span>
<span class="hljs-keyword">val</span> boxed: Number = <span class="hljs-number">1</span>
^ ^ ^
name type value
<span class="hljs-keyword">val</span> num: <span class="hljs-built_in">Int</span> = <span class="hljs-number">2</span>
<span class="hljs-keyword">val</span> dob: <span class="hljs-built_in">Double</span> = <span class="hljs-number">2.0</span>
<span class="hljs-comment">// Both lines do the exact same thing internally</span>
Integer x = <span class="hljs-number">42</span>;
Integer y = Integer.valueOf(<span class="hljs-number">42</span>);
<span class="hljs-comment">// Eventhough this is very handy, it unfortunately leads to a decrease in performance</span>
<span class="hljs-comment">// We can avoid creating these objets wrappers by not storing numbers in objects</span>
<span class="hljs-comment">// There are two types of variables in Kotlin</span>
<span class="hljs-comment">// Changeable & Unchangeable</span>
<span class="hljs-comment">// var val</span>
<span class="hljs-comment">// With "val" you can assign value only once</span>
<span class="hljs-keyword">val</span> aquarium = <span class="hljs-number">1</span>
aquarium = <span class="hljs-number">2</span> <span class="hljs-comment">// -> ERROR! cannot be reassigned</span>
<span class="hljs-comment">// You can assign vals;</span>
<span class="hljs-keyword">val</span> str = <span class="hljs-string">"string"</span>
<span class="hljs-keyword">val</span> numInt = <span class="hljs-number">1</span>
<span class="hljs-keyword">val</span> numDouble = <span class="hljs-number">1.0</span>
<span class="hljs-keyword">val</span> bool = <span class="hljs-literal">false</span>
<span class="hljs-comment">// With "var" you can assign a value, and then you can change it</span>
<span class="hljs-keyword">var</span> fish = <span class="hljs-number">2</span>
fish = <span class="hljs-number">50</span>
<span class="hljs-comment">// Type is inferred meaning that compiler can figure out the type from the context</span>
<span class="hljs-comment">// Even so the type is inferred, it becomes fixed at compile time,</span>
<span class="hljs-comment">// So you cannot change a type of a varible in kotlin once it's type has been determined.</span>
fish = <span class="hljs-string">"Bubbles"</span> <span class="hljs-comment">// ERROR</span>
<span class="hljs-comment">// We can use variables in operations and there is no punctuation at the end</span>
<span class="hljs-keyword">var</span> str = <span class="hljs-number">8</span>
<span class="hljs-keyword">var</span> a = <span class="hljs-number">5</span>
a + str
print(a + str)
<span class="hljs-comment">// Number types won't implicitly convert to other types, so you can't assign</span>
<span class="hljs-comment">// A short value to a long variable or a byte to an int</span>
<span class="hljs-keyword">val</span> b: <span class="hljs-built_in">Byte</span> = <span class="hljs-number">1</span>
<span class="hljs-keyword">val</span> i: <span class="hljs-built_in">Int</span> = b <span class="hljs-comment">// ERROR Type Mismatch</span>
<span class="hljs-comment">// But you can always assign them by casting like this;</span>
<span class="hljs-keyword">val</span> i: <span class="hljs-built_in">Int</span> = b.toInt()
<span class="hljs-comment">// Kotlin supports underscores in numbers</span>
<span class="hljs-keyword">val</span> oneMillion = <span class="hljs-number">1_000_000</span>
<span class="hljs-keyword">val</span> socialSecurityNumber = <span class="hljs-number">999_99_9999L</span>
<span class="hljs-keyword">val</span> hexBytes = <span class="hljs-number">0xFF_EC_DE_5E</span>
<span class="hljs-keyword">val</span> bytes = <span class="hljs-number">0b11010010_01101001_10010100_100100010</span>
<span class="hljs-comment">// You can speficy long constants in a format that makes sense to you</span>
<span class="hljs-comment">// The type is inferred by Kotlin</span>
</div></code></pre>
<h3 id="nullability">Nullability</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">// Kotlin helps avoid null pointer exceptions</span>
<span class="hljs-comment">// When you declare a variables type expicitly, by default its value cannot be null</span>
<span class="hljs-keyword">var</span> rocks: <span class="hljs-built_in">Int</span> = <span class="hljs-literal">null</span>
<span class="hljs-comment">// Use the question mark operator to indicate that a variable can be null</span>
<span class="hljs-keyword">var</span> rocks: <span class="hljs-built_in">Int</span>? = <span class="hljs-literal">null</span>
<span class="hljs-comment">// Whe you have complex data types such as a list, </span>
<span class="hljs-keyword">var</span> lotsOfFish: List<String?> = listOf(<span class="hljs-literal">null</span>, <span class="hljs-literal">null</span>)
<span class="hljs-comment">// You can allow for the list to be null, but if it is not null its elements cannot be null</span>
<span class="hljs-keyword">var</span> evenMoreFish: List<String>? = <span class="hljs-literal">null</span>
<span class="hljs-keyword">var</span> definitelyFish: List<String?>? = <span class="hljs-literal">null</span>
<span class="hljs-comment">// Or you can allow both the list or the elements to be null</span>
definitelyFish = listOf(<span class="hljs-literal">null</span>, <span class="hljs-literal">null</span>)
<span class="hljs-comment">// Samples</span>
<span class="hljs-comment">// Creating List</span>
<span class="hljs-keyword">var</span> names5: List<String> = listOf(<span class="hljs-string">"asd"</span>, <span class="hljs-string">"adsad3"</span>)
<span class="hljs-comment">// Allow list to be null</span>
<span class="hljs-keyword">var</span> names: List<String>? = <span class="hljs-literal">null</span>
<span class="hljs-comment">// Allow list items to be null</span>
<span class="hljs-comment">// But list cannot be null</span>
<span class="hljs-keyword">var</span> names2: List<String?> = listOf()
<span class="hljs-comment">// ERROR, List cannot be null</span>
<span class="hljs-keyword">var</span> names3: List<String?> = <span class="hljs-literal">null</span>
<span class="hljs-comment">// Allow both list items and list itself to be null</span>
<span class="hljs-comment">// But list cannot be null</span>
<span class="hljs-keyword">var</span> names4: List<String?>? = <span class="hljs-literal">null</span>
<span class="hljs-keyword">var</span> a: String = <span class="hljs-string">"abc"</span> <span class="hljs-comment">// Regular initialization means non-null by default</span>
a = <span class="hljs-literal">null</span> <span class="hljs-comment">// compilation error</span>
<span class="hljs-comment">// it's guaranteed not to cause an NPE, so you can safely say:</span>
<span class="hljs-keyword">val</span> l = a.length
<span class="hljs-comment">// To allow nulls, you can declare a variable as nullable string, written String?:</span>
<span class="hljs-keyword">var</span> b: String? = <span class="hljs-string">"abc"</span> <span class="hljs-comment">// can be set null</span>
b = <span class="hljs-literal">null</span> <span class="hljs-comment">// ok</span>
<span class="hljs-keyword">val</span> l = b.length <span class="hljs-comment">// error: variable 'b' can be null</span>
print(b)
<span class="hljs-comment">// List with some null items</span>
<span class="hljs-keyword">var</span> listWithNulls: List<String?> = listOf(<span class="hljs-string">"Kot"</span>, <span class="hljs-literal">null</span>, <span class="hljs-string">"melo"</span>, <span class="hljs-literal">null</span>)
<span class="hljs-comment">// Checking for null in conditions</span>
<span class="hljs-comment">// Option 1: First, you can explicitly check if b is null, and handle the two options separately:</span>
<span class="hljs-keyword">val</span> l = <span class="hljs-keyword">if</span> (b != <span class="hljs-literal">null</span>) b.length <span class="hljs-keyword">else</span> -<span class="hljs-number">1</span>
<span class="hljs-comment">// Option 2: Safe calls, Your second option is the safe call operator, written "?."</span>
<span class="hljs-keyword">val</span> a = <span class="hljs-string">"Kotlin"</span>
<span class="hljs-keyword">val</span> b: String? = <span class="hljs-literal">null</span>
println(b?.length)
println(a?.length) <span class="hljs-comment">// Unnecessary safe call</span>
<span class="hljs-comment">// This returns "b.length" if "b is not null", and "null" otherwise. The type of this expression is Int?</span>
<span class="hljs-comment">// "b?.length" is equal to "if (b != null) b.length else -1"</span>
<span class="hljs-keyword">var</span> b: String? = <span class="hljs-string">"abc"</span>
<span class="hljs-keyword">val</span> l = <span class="hljs-keyword">if</span> (b != <span class="hljs-literal">null</span>) b.length <span class="hljs-keyword">else</span> -<span class="hljs-number">1</span>
<span class="hljs-keyword">val</span> l2 = b?.length
<span class="hljs-comment">// Such a chain returns null if any of the properties in it is null.</span>
bob?.department?.head?.name
<span class="hljs-comment">// If you want to do an operation on the non-null items</span>
<span class="hljs-comment">// To perform a certain operation only for non-null values, you can use the safe call operator together with let: </span>
<span class="hljs-keyword">val</span> listWithNulls: List<String?> = listOf(<span class="hljs-string">"Kotlin"</span>, <span class="hljs-literal">null</span>)
<span class="hljs-keyword">for</span> (item <span class="hljs-keyword">in</span> listWithNulls) {
item?.let { println(it) } <span class="hljs-comment">// prints Kotlin and ignores null</span>
}
<span class="hljs-comment">// Print also nulls</span>
<span class="hljs-keyword">var</span> listWithNulls: List<String?> = listOf(<span class="hljs-string">"Genesis"</span>, <span class="hljs-literal">null</span>, <span class="hljs-string">"melo"</span>, <span class="hljs-literal">null</span>)
print(listWithNulls)
</div></code></pre>
<h3 id="elvis-operator------">Elvis operator " ?: "</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">/*If the expression to the left of ?: is not null, the elvis operator returns it, otherwise it returns the expression to the right. Note that the right-hand side expression is evaluated only if the left-hand side is null.*/</span>
<span class="hljs-keyword">val</span> l: <span class="hljs-built_in">Int</span> = <span class="hljs-keyword">if</span> (b != <span class="hljs-literal">null</span>) b.length <span class="hljs-keyword">else</span> -<span class="hljs-number">1</span>
<span class="hljs-keyword">val</span> l = b?.length ?: -<span class="hljs-number">1</span>
<span class="hljs-comment">/*If the expression to the left of ?: is not null, the elvis operator returns it, otherwise it returns the expression to the right. Note that the right-hand side expression is evaluated only if the left-hand side is null.*/</span>
<span class="hljs-keyword">var</span> b: String? = <span class="hljs-literal">null</span>
<span class="hljs-keyword">var</span> l = b?.length ?: -<span class="hljs-number">1</span>
print(l)
<span class="hljs-comment">/*Since throw and return are expressions in Kotlin, they can also be used on the right hand side of the elvis operator. This can be very handy, for example, for checking function arguments:
*/</span>
<span class="hljs-function"><span class="hljs-keyword">fun</span> <span class="hljs-title">foo</span><span class="hljs-params">(node: <span class="hljs-type">Node</span>)</span></span>: String? {
<span class="hljs-keyword">val</span> parent = node.getParent() ?: <span class="hljs-keyword">return</span> <span class="hljs-literal">null</span>
<span class="hljs-keyword">val</span> name = node.getName() ?: <span class="hljs-keyword">throw</span> IllegalArgumentException(<span class="hljs-string">"name expected"</span>)
<span class="hljs-comment">// ...</span>
}
</div></code></pre>
<h3 id="the----operator----not-null-assertion-operator---">The " ! " Operator & Not-null Assertion Operator " !! "</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">/*This is unsafe nullable type (T?) conversion to a non-nullable type (T), !// will throw NullPointerException if the value is null.*/</span>
<span class="hljs-comment">/*The third option is for NPE-lovers: the not-null assertion operator (!!) converts any value to a non-null type and throws an exception if the value is null. You can write b!!, and this will return a non-null value of b (for example, a String in our example) or throw an NPE if b is null:*/</span>
<span class="hljs-keyword">val</span> l = b!!.length
<span class="hljs-comment">/*Thus, if you want an NPE, you can have it, but you have to ask for it explicitly, and it does not appear out of the blue.*/</span>
</div></code></pre>
<h3 id="safe-casts">Safe casts</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">/*Regular casts may result into a ClassCastException if the object is not of the target type. Another option is to use safe casts that return null if the attempt was not successful:*/</span>
<span class="hljs-comment">// Safe Casts</span>
<span class="hljs-keyword">var</span> a = <span class="hljs-string">"1"</span>
<span class="hljs-keyword">var</span> b = <span class="hljs-number">5</span>
<span class="hljs-keyword">var</span> aInt: <span class="hljs-built_in">Int</span>? = a <span class="hljs-keyword">as</span>? <span class="hljs-built_in">Int</span>
<span class="hljs-keyword">var</span> bInt: <span class="hljs-built_in">Int</span>? = b <span class="hljs-keyword">as</span>? <span class="hljs-built_in">Int</span>
print(aInt) <span class="hljs-comment">// null</span>
print(bInt) <span class="hljs-comment">// 5</span>
</div></code></pre>
<h3 id="collections-of-a-nullable-type">Collections of a nullable type</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">/*If you have a collection of elements of a nullable type and want to filter non-null elements, you can do so by using filterNotNull:*/</span>
<span class="hljs-keyword">val</span> nullableList: List<<span class="hljs-built_in">Int</span>?> = listOf(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-literal">null</span>, <span class="hljs-number">4</span>)
<span class="hljs-keyword">val</span> intList: List<<span class="hljs-built_in">Int</span>> = nullableList.filterNotNull()
<span class="hljs-keyword">var</span> nullableList: List<<span class="hljs-built_in">Int</span>?> = listOf(<span class="hljs-number">1</span>, <span class="hljs-literal">null</span>, <span class="hljs-number">2</span>, <span class="hljs-literal">null</span>, <span class="hljs-literal">null</span>, <span class="hljs-number">5</span>)
<span class="hljs-keyword">var</span> nonNullList = nullableList.filterNotNull()
print(nonNullList) <span class="hljs-comment">// Prints: [1, 2, 5]</span>
<span class="hljs-comment">// You can do some cool null testing with the question mark operator saving you the pain of many if else statements</span>
<span class="hljs-comment">// You can check if an object or variable is non null before accessing one of its methods</span>
<span class="hljs-keyword">val</span> fishFoodTreats: <span class="hljs-built_in">Int</span>? = <span class="hljs-literal">null</span>
<span class="hljs-keyword">return</span> fishFoodTreats?.dec() ?: <span class="hljs-number">0</span>
<span class="hljs-keyword">val</span> fishFoodTreats = <span class="hljs-number">5</span>
<span class="hljs-keyword">return</span> fishFoodTreats?.dec() ?: <span class="hljs-number">0</span>
<span class="hljs-comment">// You can also chain null tests in an expression</span>
<span class="hljs-comment">/*If "fishFoodTreats" is not null use a treat and return a new value and otherwise return the value after the colon which is zero*/</span>
</div></code></pre>
<h3 id="practice-time-basic-operations">Practice Time: Basic Operations</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">/*
Solve the following using the operator methods in one line of code.
If you start with 2 fish, and they breed twice, producing 71 offspring the first time, and 233 offspring the second time, and then 13 fish are swallowed by a hungry moray eel, how many fish do you have left? How many aquariums do you need if you can put 30 fish per aquarium?
Hint: You can chain method calls.
Hint: You can call the methods on numbers, and Kotlin will convert them to objects for you.
Bonus question: What is special about all the numbers of fish?*/</span>
<span class="hljs-comment">// Solution Code</span>
<span class="hljs-number">2</span>.plus(<span class="hljs-number">71</span>).plus(<span class="hljs-number">233</span>).minus(<span class="hljs-number">13</span>).div(<span class="hljs-number">30</span>).plus(<span class="hljs-number">1</span>)
<span class="hljs-comment">// Bonus question: If you've noticed, all fish numbers above are prime.</span>
<span class="hljs-comment">// My One Line Solution</span>
println(((<span class="hljs-number">2</span>.times(<span class="hljs-number">71</span>).plus(<span class="hljs-number">2</span>.times(<span class="hljs-number">233</span>))).minus(<span class="hljs-number">13</span>)).div(<span class="hljs-number">30</span>).plus((<span class="hljs-keyword">if</span> (<span class="hljs-number">595</span>.mod(<span class="hljs-number">30</span>) > <span class="hljs-number">0</span>) <span class="hljs-number">1</span> <span class="hljs-keyword">else</span> <span class="hljs-number">0</span>)))
<span class="hljs-comment">// To find how many fishes left</span>
println((<span class="hljs-number">2</span>.times(<span class="hljs-number">71</span>).plus(<span class="hljs-number">2</span>.times(<span class="hljs-number">233</span>))).minus(<span class="hljs-number">13</span>))
<span class="hljs-comment">// To find Aquariums Needed</span>
println(<span class="hljs-number">595</span>.div(<span class="hljs-number">30</span>) + <span class="hljs-keyword">if</span> (<span class="hljs-number">595</span>.mod(<span class="hljs-number">30</span>) > <span class="hljs-number">0</span>) <span class="hljs-number">1</span> <span class="hljs-keyword">else</span> <span class="hljs-number">0</span>)
</div></code></pre>
<h3 id="practice-time-variables">Practice Time: Variables</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">/*Create a String variable rainbowColor, set its color value, then change it.
Create a variable blackColor whose value cannot be changed once assigned. Try changing it anyway.*/</span>
<span class="hljs-keyword">var</span> rainbowColor: String = <span class="hljs-string">"green"</span>
rainbowColor = <span class="hljs-string">"blue"</span>
<span class="hljs-keyword">val</span> blackColor: String = <span class="hljs-string">"you cannot change me// I am pure Black!"</span>
blackColor = <span class="hljs-string">"White!"</span>
<span class="hljs-comment">//Alternative</span>
<span class="hljs-keyword">var</span> rainbowColor = <span class="hljs-string">"green"</span>
rainbowColor = <span class="hljs-string">"blue"</span>
<span class="hljs-keyword">val</span> blackColor = <span class="hljs-string">"black"</span>
blackColor = <span class="hljs-string">"white"</span> <span class="hljs-comment">// Error</span>
</div></code></pre>
<h3 id="practice-time-nullability">Practice Time: Nullability</h3>
<pre><code class="language-kotlin"><div><span class="hljs-comment">// Try to set rainbowColor to null. Declare two variables, greenColor and blueColor. Use two different ways of setting them to null.</span>
<span class="hljs-keyword">var</span> rainbowColor = <span class="hljs-string">"red"</span>
rainbowColor = <span class="hljs-literal">null</span> <span class="hljs-comment">// Error</span>
<span class="hljs-keyword">var</span> greenColor = <span class="hljs-literal">null</span>
<span class="hljs-keyword">var</span> blueColor: <span class="hljs-built_in">Int</span>? = <span class="hljs-literal">null</span>
</div></code></pre>
<h3 id="practice-time-nullabilitylists">Practice Time: Nullability/Lists</h3>