You never touched Groovy, nor did you jump on the Scala train. Clojure never attracted you; and you heard about Ceylon long after the language had already died. You are one of those old-fashioned Java folks! But now, after all those years, you want to join the cool Kotlin kids. So, where to start? Let’s discover the language together by decompiling it to Java code. Today: Pattern matching!

You never touched Groovy, nor did you jump on the Scala train. Clojure never attracted you; and you heard about Ceylon long after the language had already died. You are one of those old-fashioned Java folks! But now, after all those years, you want to join the cool Kotlin kids. So, where to start? Let’s discover the language together by decompiling it to Java code. Today: Covariance, contravariance and invariance!

You never touched Groovy, nor did you jump on the Scala train. Clojure never attracted you; and you heard about Ceylon long after the language had already died. You are one of those old-fashioned Java folks! But now, after all those years, you want to join the cool Kotlin kids. So, where to start? Let’s discover the language together by decompiling it to Java code. Today: the Companion Object and top level declarations!

You never touched Groovy, nor did you jump on the Scala train. Clojure never attracted you; and you heard about Ceylon long after the language had already died. You are one of those old-fashioned Java folks! But now, after all those years, you want to join the cool Kotlin kids. So, where to start? Let’s discover the language together by decompiling it to Java code. Today: Null Safety and the Safe Call Operator!

You never touched Groovy, nor did you jump on the Scala train. Clojure never attracted you; and you heard about Ceylon long after the language had already died. You are one of those old-fashioned Java folks! But now, after all those years, you want to join the cool Kotlin kids. So, where to start? Let’s discover the language together by decompiling it to Java code. Today: the Data class!

You never touched Groovy, nor did you jump on the Scala train. Clojure never attracted you; and you heard about Ceylon long after the language had already died. You are one of those old-fashioned Java folks! But now, after all those years, you want to join the cool Kotlin kids. So, where to start? Let’s discover the language together by decompiling it to Java code. Today: the Kotlin Class!

You never touched Groovy, nor did you jump on the Scala train. Clojure never attracted you; and you heard about Ceylon long after the language had already died. You are one of those old-fashioned Java folks! But now, after all those years, you want to join the cool Kotlin kids. So, where to start? Let’s discover the language together by decompiling it to Java code. Today: Functions!

A lot of applications seem to either only use runtime exceptions or only use error monads like the Optional for error handling. In this blog I will try to dive a bit deeper into when to use one over the other (tldr: you probably need both)

Since Java 16 we can use the method mapMulti(BiConsumer) of the Stream API. This method allows us to map each element of the stream to multiple elements. We can also do that with the flatMap(Function) method, but if we want to map a limited set of elements, mapMulti is more convenient. Internally a shared stream is used and we don’t have the cost of creating a new stream for each element. Another use case is if the logic to map an element to multiple elements is complex and is hard to implement by returning a stream. Then mapMulti allows us to write that logic in a BiConsumer instead of a Function.

For a lot of types AssertJ has special assertion methods. Also for the type Optional. If we want to assert an Optional value we can use several methods that AssertJ provides. For example to check if an Optional is present we can use isPresent() or the alias isNotEmpty(). To check if the Optional is empty we can use isEmpty() or the alias isNotPresent(). Checking the value of an Optional (if it is indeed set) can be done with hasValue() or contains(). For more fine grained assertions on the value we can use hasValueSatisfying(Condition) or hasValueSatisfying(Consumer). With the map(Function) and flatMap(Function) methods we can map the Optional, if not empty, to another value and assert that value.