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Theory: Specifying Types & java.langλ︎

Clojure has types that are created dynamically when the code is compiled, with everything being represented by Java objects as its compiled to Java byte code.

Clojure simply infers the type of a value, so types do not need to be specified in code.

The built in collections (list, map, vector & set) also support mixed types too.

Calling Java codeλ︎

The Clojure project uses Jetty, a web application server written in Java. When calling the run-jetty function an Integer type must be passed to the Java object for the port number.

When running the Clojure project, the argument supplied for the port number on the command line is treated as a String object. Therefore we need to explicitly cast the port number from a Java String type to an Java Integer type.

java.lang libraryλ︎

The java.lang. library is part of all Clojure projects, so as we are going to create a Java Integer it makes sense to simply use the Integer constructor with a String argument which returns a new Integer object.

(Integer. port-number) calls the java.lang.Integer constructor.

The . is actually a macro in Clojure that provides a simple way to work with Java, allowing you to call Java objects as if they were Clojure functions. In Java you would have to use the form Type instance-name = new Type(argument). In our example you would write this in Java as String port = new String(port-number)

From the Java 8 docs for Integer class: Integer(String s) - constructs a newly allocated Integer object that represents the int value indicated by the String parameter.

Theory: Its Java Objects underneath strings & numbersλ︎

Strings and numbers are represented by Java objects underneath, so its convenient to use Java Classes to manipulate these simple data structures on the rare occasion you need a specific type.

You can see the underlying Java types in Clojure using the type or class function. In the following example you can see the Java types for strings and numbers

Clojure Types: String to Integer examples