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:

The clojure.string namespace contains a lot of useful functions to work with string values. The escape function can be used to replace characters in a string with another character. The function accepts as first argument the string value and the second argument is a map. The map has characters as key that need to be replaced followed by the value it is replaced with. For example the map {\a 1 \b 2} replaces the character a with 1 and the character b with 2.

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

When we use a function as argument for the map function that returns a collection we would get nested collections. If we want to turn the result into a single collection we can concatenate the elements from the collections by applying the concat function, but we can do this directly with the function mapcat. The function mapcat takes as first argument a function (that returns a collection) and one or more collections as next arguments.

In the following examples we see several uses of mapcat:

Since beginning of time mankind has been looking for a way to separate right from wrong. Where the primeval man judged righteousness by the contributions of the tribe, the current day programmer judges right by the wishes of the customer. For many years the average programmer wrote a bunch of logic to check if the boundaries defined by the client where uphold. As time went on and programming languages involved, metadata could be added to enrich functions, methods, classes and the like.

Of course for Java, these metadata are called annotations. Very soon they were used for a lot of things. Surpressing warnings, managing transactions, building XML/JSON structures and injecting dependencies. And, as you might have guessed by now, validating objects by a set of specific rules. One of the most commonly used frameworks would be the Jakarta Bean Validation framework. But what if I told you the provided annotations of that framework could be very easily expanded.

When we are working with sets in Clojure we can use some useful functions from the clojure.set namespace. In a previous post we learned how we can get the difference of several sets. To get the union of several input sets we use the union function of the clojure.set namespace. The function returns a new set that is the union of unique elements from the input sets. A nil value is ignored by the union function.

In the following example code we use union:

If we want to get the values from a set that are not part of one or more other sets we must use the difference function in the clojure.set namespace. The function returns a set with all values from the first set that are different from values in other sets.

In the following example we use the difference with several sets:

In Clojure functions are everywhere. In a previous post we learned that sets can be functions, but Clojure also makes keywords functions. A keyword is a symbol starting with a colon (:) and is mostly used in map entries as key symbol. The keyword as function accepts a map as single argument and returns the value for the key that equals the keyword in the map or nil if the keyword cannot be found.

In the following code we use keywords as function in several examples:

One of the nice things in Clojure is that some data structures are also functions. For me this felt rather strange when learning Clojure (coming from Java), but it can be very powerful. A set in Clojure is also a function. The set as function accept a single argument and it return nil when the argument is not part of the set, otherwise the argument value is returned. This behaviour also makes a set as function a nice predicate to be used for example in collection functions.

In the following example code we use different sets as function:

The Clojure function complement can be used to created a new function that returns the opposite truth value of the old function. The new function accepts the same number of arguments as the old function. Also when we invoke the new function created by the complement the old function is actually invoked and the result is used as argument for the not function to return the opposite truth value. So if the original function returns false or nil the result for the new function is true.

In the following example code we create a new function bad-weather that is the complement of good-weather: