The chunked extension method is added to the Iterable Java class and makes it possible to split an interable into fixed sized lists. We define the size of the lists as argument to the chunked method. The return result is a list of lists. Each of the lists will have the number of elements we have specified as argument. The last list can have less elements if the total number of elements cannot be divided exactly by the size we specified as argument. We can specify a lambda transformation function as second argument. The lambda function has the new sublist as argument and we can write code to transform that sublist.

The method partition is available in Kotlin for arrays and iterable objects to split it into two lists. We pass a predicate lambda function to the partition method. The predicate should return either true or false based on a condition for each element from the array or iterable. The return result is a Pair instance where the first element is a List object with all elements that returned true from the predicate. The second element in the Pair object contains all elements for which the predicate returned false. As a String can be seen as an iterable of characters we can also use partition on a String instance.

Kotlin adds a lot of extension methods to the String class. For example we can use the take method to get a certain number of characters from the start of a string value. With the drop method where we remove a given number of characters from the start of the string to get a new string. We can also take and drop a certain number of characters from the end of a string using the methods takeLast and dropLast.

Instead of using the number of characters we want to take or drop we can also use a condition defined with a predicate lambda function. We take or drop characters as long as the lambda returns true. The names of the methods for taking characters are takeWhile and takeLastWhile and for dropping characters dropWhile and dropLastWhile.

If we want to find the longest shared prefix or suffix for two string values we can use the String extension methods commonPrefixWith and commonSuffixWith. The result is the prefix or suffix value that is common for both values. We can pass a second argument to the method to indicate if we want to ignore the casing of the letters. The default value for this argument is false, so if we don’t set it explicitly the casing of the letters should also match.

Kotlin gives us the associate method for collection objects, like lists, iterables and arrays. With this method we can convert the items in the collection to a new Map instance. The associate method accepts a lambda function as argument and we must return a Pair from the lambda. The first item of the pair will be the key and the second element is the value of the key/value pair in the resulting map.

If we want to use the elements in our collection as key, but want to transform the value we must use associateWith. The lambda for this method must return the value part of our key/value pair. Alternatively if we only want to transform the key value we can use associateBy with one lambda function. The lambda function must return the result for the key in the key/value pair of the map. The method associateBy is overloaded where we can pass two lambda functions. The first lambda function is for transforming the key and the second lambda function is for transforming the value.

Kotlin extends the String class with a couple of padding methods. These methods allows us to define a fixed width a string value must occupy. If the string itself is less than the fixed width then the space is padded with spaces or any other character we define. We can pad to the left or the right of the string using the padStart and padEnd methods. When we don’t define an argument a space character is used for padding, but we can also add our own custom character as argument that will be used as padding character.

Multiline strings are very useful. But sometimes we want use the multiline string without the leading spaces that are added because of code formatting. To remove leading spaces we can use the trimIndent method. This method will find the least amount of leading spaces and removes that amount of spaces from each line. Also a first and last empty line are removed.

If we want a bit more control we can also add a character to the start of each line to show where the line starts. And then we use the method trimMargin and all spaces before that character are removed. The default character is the pipe symbol, |, but we can also define our own and pass it as argument to the trimMargin method.

If we want to transform items in a collection we can use the map method. If we also want to use the index of the element in the collection in the transformation we must use the mapIndexed method. We must provide a lambda function with 2 arguments, where the first argument is the index of the element in the collection and the second argument is the element in the collection.

Kotlin adds a lot of useful extensions to the collection classes. One of them is the indices property. The indices property returns the indices of the elements in the collection as an IntRange.

Context with receivers is a new experimental Kotlin feature. So let’s explore this feature a bit and see what it is all about.

One of the great features of Kotlin is its interoperability with Java code. This allows you to easily call 'traditional' Java code from your Kotlin code, but it also helps you the other way around: calling Kotlin code from Java.

Sometimes, a little extra work is needed to make some shiny Kotlin feature work with Java code. For example, Kotlin supports default parameter values, which are not supported in Java. In this case, the @JvmOverloads annotation can be used to generate overloads for functions that contain parameters with default values. This annotation does not only work on functions, but can also be applied on constructors. In this post I will explain how to use this feature on the primary constructor, as it might be confusing where to place the annotation.

After completing the Kotlin for Java Developers Course on Coursera I was looking for an excuse to put my freshly gained Kotlin knowledge into action. I decided to address my frustration about the large amount of falsely detected movements by one of my security camera’s.

One of the core components of that solution is a REST API that receives an image and returns a list of detected objects. I decided to develop that using Kotlin, KotlinDL and KTor.

This blog posts describes the core components of the solution. The source code of the example is available at GitHub.