To split a collection in Clojure we can use the split-with and split-at functions. The split-with function takes a predicate as first argument and a colletion as second argument. The function will return a vector with two items. The first item is the result of the function take-while with the given predicate. The second item in the result vector is the resul of the drop-while function with the same predicate.

We use the split-at function with a number as first argument followed by a collection to split based on a given number of items. Instead of using a predicate we can define the number of items that we want as the first item in the result vector. The first item in the result vector is the result of invoking the take function. The resulting number of items of the collection will be the second item in the result vector and is achieved by invoking the drop function.

In Clojure we can use the shuffle function with a collection argument to get a new collection where the items of the input collection are re-ordered randomly. The function delegates to the Java java.util.Collections#shuffle method.

In the following example code we use the shuffle method:

In Clojure we can format a string using Common Lisp format syntax or the Java format string syntax. In the post we will look at the how we can use the Java format string syntax. We must use the format function in the clojure.core namespace. The method delegates to the standard JDK String#format method. The first argument is a format string followed by one or more arguments that are used in the format string. We can look up the syntax of the format string in the Javadoc for the java.util.Formatter class.

In the following example code we use the format function with different format strings:

In Clojure we can format a string using Common Lisp format syntax or the Java format string syntax. In the post we will look at the how we can use the Java format string syntax. We must use the format function in the clojure.core namespace. The method delegates to the standard JDK String#format method. The first argument is a format string followed by one or more arguments that are used in the format string. We can look up the syntax of the format string in the Javadoc for the java.util.Formatter class.

In the following example code we use the format function with different format strings:

SDKMAN! is a very useful tool to manage versions of so-called software development kits. There are a lot of SDKs supported by SDKMAN!: Java, Groovy, Kotlin, Scala, Gradle, Maven, Leiningen, Micronaut, Grails, Vert.x, JBake, AsciidoctorJ and more. When we look at Java we can use a simple install java <version> command from the command-line to install a version of Java on our computer. SDKMAN! will take care of downloading the Java version and setting all the correct system variables to use that Java version. With the use command we can switch between version in the current shell we are working in. But we can even automatically switch to a specific installed Java version when we enter a directory. This is very useful when we have to work on multiple projects on our computer and each project requires a specific Java version to be used.

To support automatic switching of a Java version we must first run the env init command in the directory of our project. This creates a new file .sdkmanrc in the directory. The file contains the Java version that was active when we invoked the env init command. It is a text file so we can change the Java version in the file, or regenerate the file by running the env init command again, but with a different active Java version.

To find the maximum or minimum value for numeric values we can use the max and min function. The functions accept one or more numeric arguments and the value that is maximum or minimum is returned. If the numbers are already in a sequence we can use apply max or apply min. If the values are not numbers we can use the max-key or min-key functions. These functions take as first argument a function that returns a number. So we can get the value that has the maximum or minimum return value for the function we pass as first argument.

In the next exmaple code we use the max, min, max-key and min-key functions:

In a previous post we learned how to replace a file in an archive with Gradle. In this blog post we use Maven to achieve the same goal. With Maven we first have to extract the contents of the archive and then assemble a new archive where we use a file replacement to replace an original file from the archive. To extract the contents we use the task unzip with the maven-antrun-plugin. To create a new archive we use the maven-assembly-plugin. The destination directory of the extracted contents is the input fileset for the assembly definition together with the files we want to overwrite. The end result is a new archive with replaced files.

In the following pom.xml we configure the maven-antrun-plugin and maven-assembly-plugin:

Sometimes we might need to replace one or more files in an existing archive file. The archive file could be a zip, jar, war or other archive. Without Gradle we would unpack the archive file, copy our new file into the destination directory of the unpacked archive and archive the directory again. To achieve this with Gradle we can simply create a single task of type Zip. To get the content of the original archive we can use the project.zipTree method. We leave out the file we want to replace and define the new file as replacement. As extra safeguard we can let the tsak fail if duplicate files are in the archive, because of our replacement.

The following code shows an example of a task to replace a README file in an archive sample.zip using Groovy and Kotlin DSL. First the Groovy DSL:

First paragraph.

My post with code

In Clojure we can take or drop elements from a collection based on a predicate using the functions take-while and drop-while. With the function take-while we take elements as long as the predicate returns true. Once the predicate returns false the function stops returning elements. Using the function drop-while we skip elements in the collection if the predicate returns true. If the predicate returns false the remaining elements in the collection are returned.

In the following example we use take-while and drop-while with different collection types:

The map data structure is used a lot in Clojure. When we want to use Java objects in our Clojure code we can convert the Java object to a map with the bean function. This function will use reflection to get all the properties of the Java object and converts each property with the property value to a key with value in the resulting map. The bean function will not recursively convert nested objects to a map, only the top-level properties are turned into key value pairs.

We see several examples of using the bean function in the following code snippet:

It is very easy to work with Java classes in Clojure. If we want to create a new object based on a Java class and invoke methods to initialize the object directly we can use the doto macro. The first argument is an expression to create a new object and the rest of the arguments are functions to invoke methods on the newly created object. The object returned from the first argument is passed as first argument to the method invocations. The doto function returns the object that is created with the first argument.

In the following example code we use the doto function in several cases: