This lesson is all about nothing!
It turns out that nothing can cause us some trouble!
But, because we’re using Kotlin, we have a language designed around dealing with
nothing safely.
Let’s try to make sense of those statements, and also continue to practice writing String
algorithms.
Wait: why are we suddenly talking about Java?
To understand how Kotlin handles null
, we need to take a brief digression and discuss a different programming language: Java.
Why?
Because Kotlin is designed to improve on but also to interoperate with Java code.
So in some ways null
is a problem that Kotlin inherits from Java, even if Kotlin provides us with much better solutions for working with this special value safely.
Java has a special value that is used to indicate that an object is uninitialized: null
.
Here’s an example using String
s, one of the objects we have been working with:
Any Java object can be initialized to null
.
Because Kotlin interoperates with Java, this is where the problem creeps in…
null
seems harmless and maybe even kind of cute!
But it has left a trail of damage and tears in its wake.
In fact, famous computer scientist Tony Hoare,
who is credited for inventing null
as part of the programming language ALGOL, refers to it as his “billion dollar mistake”.
Let’s look at why.
Again, this example examines Java code: we’ll get back to Kotlin in just a minute.
null
has caused so many problems over the years, that Kotlin makes working with null
safely a core design goal.
One of the ways that it does it does this is that, unlike Java, Kotlin carefully tracks what variables can and can’t hold null
in our programs.
Consider the following example:
If you try to run the code above you’ll see that it fails.
The reason is that, while value
can store String
s, and we can change its value to another String
, it cannot store null
.
So, for each variable in our program, Kotlin knows whether or not it can hold null
and will fail if we try to assign null
to a variable that can’t contain it.
Sometimes null
is a useful value in our programs to indicate that a variable is unset or uninitialized.
So: what if we want a String
that can contain null
?
Here’s how we do that:
Note how we used the type String?
on the variable value
.
By appending an ?
to any type name in Kotlin we indicate that the variable can also store null
.
You can think of the ?
as hinting that maybe the variable stores the type, but maybe it stores null
(nothing)!
This also works for the other kinds of variables we have been using:
Types that end with ?
and can contain null
are referred to as nullable in Kotlin.
Because null
can be used to initialize any value, Kotlin’s type inference fails if we try and initialize a value with null
:
A value that is initialized with null
has the inferred type Nothing?
, which is fairly useless since a Nothing?
variable can’t store anything but null
!
Instead, when we need a nullable variable we will need to specify the type explicitly, even if we initialize it with a value:
null
null
From this point forward we’re going to try and keep null
in the back of our minds.
Always.
Whenever we have a variable that could be null
, we need to make sure that it isn’t null
before we do anything with it!
Happily Kotlin provides us with a lot of help here!
We’ll get to know Kotlin’s null
safely capabilities bit-by-bit as we go.
But here’s a few tips to get us started.
When declaring a method we have two options.
First, we can prevent the method from accepting null
by using a non-nullable
type:
Alternatively, if the method must accept a nullable type, we can simply check for it at the beginning. Let’s see how to do that:
One of the biggest problems with null
in Java is errors caused by trying to use dot-notation on a variable that contains null
:
(Again, this is Java code.)
In Kotlin, whenever a variable could be null
, plain dot notation will not work:
Instead, we have to use something called the safe call operator. Let’s see how that works:
Declare and implement a function called arrayMax
.
It should accept a nullable DoubleArray
as its single argument, and return the maximum value stored in the array.
If the array is empty or null
, you should return 0.0.
String
sString
sNow let’s continue developing our algorithmic and String
manipulation capabilities.
Let’s apply our skills to determining whether two String
s are anagrams.
An anagram is created by rearrange the letters from one word to form another:
For our implementation we will not ignore whitespace and capitalization. Some anagrams do: for example, “New York Times” and “monkeys write” are anagrams, but the first string has two spaces while the second has only one. To us those would not be anagrams. When we are done, we can discuss how to make our approach more flexible.
Note that there are better ways to implement this algorithm. Perhaps we’ll return to it later and experiment with one or even two alternate approaches. That’s part of what makes computer science so exciting! There is always more than one way to solve any problem…
Write a method named arrayCountGreaterThan
.
It should accept a non-nullable Int
array as its first argument and an Int
as its second,
and return a count of how many values in the array are strictly greater than the second parameter.
Write a method named arrayAllMultiples
that, given an array of Int
values greater than 0, returns whether the
values are all integer multiples of one of the values in the array, which we'll call the base.
For example, given the array {4, 2, 8}
you would return true
, since 4 and 8 are integer multiples of the base 2.
Given the array {2, 2, 5}
you would return false
, since there is no value that all others are integer multiples
of.
(If the array contains 1, you should always return true
, but there's no need to handle this case specifically.)
You should approach this problem in two steps. First, identify the base—the value in the array that you are going to check whether others are multiples of. This may sound complicated, but it's straightforward. You do not need to and should not check whether every value is the base! Your solution should not contain a nested loop.
Next, check all the values in the array and look for a counterexample—a value that is not a multiple of the base.
The passed array may be null
or empty.
In those cases you should return false
.
Valid arrays will contain only integers strictly greater than 0.
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