For the 11th post, we get something new to an old C# person - Object Expressions!

Object Expressions are very similar to Anonymous Classes in Java where you can declare a class inline rather than entirely separately in its own file.

In the first Koan we need to implement Comparator<Int> inline:

You can see on line 21, we define the new object with the object keyword and then use : Comparator<Int> to state it implements that interface. The Comparator has a single function which needs to be implemented which we do on line 22.

The second Koan takes this further and states if there is a single method in an abstract class or interface then we can use SAM, or Single Abstract Method, to avoid needing the class at all as we just need to implement the single function. To achieve this with the Koan, we use an anonymous function that handles the compare function of the Comparator:

and lastly, if we look back to previous post we can use the extension method to simply it further:

These Koans have given me more thoughts about the language than probably any previous Koans:

1. Why do classes which implement an interface use override?
   In C#, when you implement an interface you do not need to state that you are not overriding the functions (see this example). In C# only state that your are overriding when you inherit from a function and you actually override a function. The reason is that an interface in Kotlin is closer to an abstract class than in C#, to the point it can have functionality - yup, interfaces can have functions and logic!
2. So why does Kotlin have interfaces and abstract classes?
   The key difference is an abstract class can have state while the logic in an interface needs to be stateless!
3. Why bother having SAM?
   As I was working on the Koan, I was delighted by the SAM syntax… and then I wondered why I needed this at all? Why is Collections.sort taking a class as the second parameter? Why not just pass in a function, since that is all that is actually needed? Both C# and Kotlin supports passing functions so this is possible… but something I never knew about Java is that it doesn’t support passing functions! You have to use Callable, a class, to pass functions.

This post is the first to cover multiple Koans in a single post.

The 10th in our series is very simple coming from C# because Extension Functions in Kotlin are identical as Extension Methods in C#, though they are much cleaner in their implementation.

In their example for the Koan we add a lastChar and lastChar1 function to the String class.

// ‘this’ refers to the receiver (String) and can be omitted fun String.lastChar1() = get(length - 1)

For the Koan itself, we need to an r function to Int and Pair<int, int> which returns an instance of RationalNumber, which we do as follows:

An additional learning on this, was the Pair<A, B> class which is similar to the Tuple class from C#.

When we get into the second Koan, we get exposed to some of the built-in extension’s functions in Kotlin which ship out of the box; in this case, we use sortedDescending extension method with the Java collection. It is a great example of mixing Java and Kotlin too:

The goal of this Koan is to show how smart the Kotlin compiler is; in that when you use something like the is keyword to handle type checking the compiler will then know the type later on and be able to use it intelligently.

So if we want to check types in Java we would use something like this where we would use instanceof to check the type and then cast it to the right type.

In Kotlin we, by checking the type the compiler handles the casting for us, but before we get to that we also got to learn about the when which is the Kotlin form of the Switch keyword in C# or Java and it offers similar functionality, as shown in this example:

and it supports multiple values on the same branch

Where it gets awesome, is the extra actions it supports; for example when values can be functions, not just constants:

You can also use in or !in to check values in a range/collection:

It really is very cool, so let us see how we use Smart Casts and when together and how it compares with the Java code above:

Really nice and, I think, more readable than the Java code.

The next Koan looks at how Kotlin handles nulls, and it does it wonderfully; Null is explicitly opt-in. For example, in C# you can assign null to a string variable but in Kotlin unless you say you want to support nulls, which you do by adding a trailing question mark to the class, you cannot.

Their example in this Koan is a nice example:

if (q != null) q.length      // you have to check to dereference
val i: Int? = q?.length      // null
val j: Int = q?.length ?: 0  // 0

}

Let us dig into the Koan, where we get given the following Java code:

PersonalInfo personalInfo = client.getPersonalInfo();
if (personalInfo == null) return;

String email = personalInfo.getEmail();
if (email == null) return;

mailer.sendMessage(email, message);

}

and we need are going to rewrite it using the Nullable language features of Kotlin, which looks like:

mailer.sendMessage(email, message)

}

The big changes from Java:

1. The @NotNull attribute for mailer is no longer needed
2. The @Null attribute for the other parameters becomes the question mark
3. We do not need to pre-check client before calling the parameter, as you can use the null safe operator ?. to ensure you only check before we call the method.
4. Unfortunately, the null safe operator in Kotlin doesn’t support calling methods on null objects as C# currently does with its Elvis operator

This post is one in a series of stuff formally trained programmers know – the rest of the series can be found in the series index.

In this post, we will look at two related and simple data structures, the Stack and the Queue. The stack is a structure which can be implemented with either an Array or Linked List.

An important term for understanding a stack is that it is LIFO, last-in-first-out, system; namely, the last item you add (or push or bury) is the first item you take out when you retrieve an item (or peek or unbury).

Let’s have a look at what a stack would look like:

In this, we added 1, 2, 3, 4, and then 5 and when we read from the stack we read in the reverse order.

#Implementations

• Java has a Stack
• C# has a Stack

Next is the queue, which is very similar to the stack, except where the stack was LIFO; a queue is FIFO, first-in-first-out; so if we put in 1, 2, 3, 4, & 5 into the queue, then we would read them in the same order, i.e. 1, 2, 3, 4, 5.

Implementations

C# has a queue implementation and the JavaScript array can act as a queue, when you use unshift to add to the beginning of the array (also known as the head) and pop to remove from the end of the array (also known as the tail).

WOW! Data Classes are awesome! I really like a lot of how classes are handled in Kotlin, but Data Classes are the pinnacle of that work. The effort for this Koan is to convert this Java class to Kotlin:

public static class Person {
    private final String name;
    private final int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
}

}

and what does that look like in Kotlin?

Yup, a single line. The data annotation adds a number of important functions (like ToString and copy). We then declare the class with a constructor which takes two parameters which both become properties.

Another important aspect I learnt with this one is that Kotlin has both a val and var keyword for variables. We have seen var already and val is for read-only variables.

The purpose of this lesson is to show off string templating and the Koan starts with some interesting examples

fun example3(c: Boolean, x: Int, y: Int) = “Any expression can be used: ${if (c) x else y}”

If you are used to string templates from C# and JavaScript this should be fairly simple to understand.

The fourth example is interesting

This brought up something I’ve never heard of, raw strings. Raw strings seem to be the Python way of saying a Verbatim string… which I didn’t know was the official term.

The actual Koan here is about converting this fun getPattern() = """\d{2}\.\d{2}\.\d{4}""" to a different regular expression, using a string template:

It isn’t that interesting and really, if you don’t know regular expressions this could be tougher than it needs be.

So following C#, Kotlin has Lambda support with the change if => becoming -> and the Koan starts with some nice examples at the start:

val sum = { x: Int, y: Int -> x + y }
val square: (Int) -> Int = { x -> x * x }

sum(1, square(2)) == 5

}

Basically, though, the code that needs to be done is to check a collection if all items are even, which is easily done with:

Previously we covered Named Arguments and this is a small continuation from it, we start with a simple function fun foo(name: String): String = todoTask3() and we need to have it call a single Java function and to provide it with default values which ultimately looks like this:

On to our third exercise and definitely, the difficulty curve has lowered again (or should it be steep) as we have a simple lesson - how to use named arguments.

Not only is an example for it provided for how named arguments work:

We also get an example of default values for arguments

The problem we need to solve itself is simple too,

Print out the collection contents surrounded by curly braces using the library function ‘joinToString’. Specify only ‘prefix’ and ‘postfix’ arguments.

Which has this answer:

Not much to add to this lesson unfortunately.