Julia - Call Java Functions

Julia is a high-level, high-performance programming language for technical computing, with syntax that is similar to that of MATLAB or Python. Julia was designed to be used in a wide range of applications, including numerical computing, data science, machine learning, and parallel computing. Some of its key features include:

• Speed: Julia is designed to be fast, and its Just-In-Time (JIT) compiler can generate machine code that is comparable in performance to C or Fortran.
• Interactive: Julia has a built-in package manager and a REPL (Read-Eval-Print Loop) that allows you to quickly test and experiment with code.
• Dynamic: Julia is a dynamic language, meaning that the type of a variable is determined at runtime. This makes it more flexible and easier to use for some types of problems.
• General-purpose: Julia is a general-purpose programming language, meaning that it can be used for a wide range of tasks, such as web development, data visualization, and scientific computing.
• Multi-paradigm: Julia supports multiple programming paradigms such as functional, object-oriented, and imperative programming.
• Built-in parallelism: Julia has built-in support for parallel computing, which makes it well-suited for multi-core and distributed systems.
• Interoperability: Julia can call code written in other languages, such as C and Python, and can be called from other languages such as Python and R. 

Julia provides a way to call Java functions using the JavaCall package. The JavaCall package is a Julia package that provides a way to interact with Java classes and objects from Julia. It provides a set of macros and functions that allow you to import Java classes, create instances of those classes, and call their methods.

Usage

One can use the @java_import macro to import a Java class and create an instance of it. Once there is an instance, one can call its methods just as one would in Java. 

• The JavaCall package also provides a way to pass and return any types that have a corresponding Java type.
• This enables developers to use the strengths of both languages in a single project, leveraging Java's vast libraries and tools in Julia, and also taking advantage of Julia's high-performance computing capabilities.

Installation

It should be noted that the JavaCall package is not included in Julia by default, it needs to be installed by running the command 

using Pkg; 

Pkg.add("JavaCall")

To use this package, you need to have a Java runtime environment installed on your machine.

Implementation

1. Start the Julia REPL and load the package by running the command:

using JavaCall

2. To call a Java function, you need to first import the class that contains the function using the "@java_import" macro. For example, to call a function in the "java.util.ArrayList" class, you would use the following code:

@java_import java.util.ArrayList

3. Once the class is imported, you can create an instance of the class and call the function on it. For example, to create an ArrayList object and add an element to it, you would use the following code:

list = ArrayList()

list.add("Hello, Julia!")

4. To call a static function, you can directly call it in the class. For example, to call the "getProperty" function in the "System" class, you would use the following code:

@java_import java.lang.System

property = System.getProperty("os.name")

Julia allows you to pass and return any types that have a corresponding Java type. You may need to cast the Julia type to the corresponding java type to be able to call the function, Any exceptions thrown by the Java function will be propagated as a JavaException in Julia. You can catch and handle these exceptions using a try-catch block, Once you are done with the Java objects, you can release the memory by calling the gc() function.

Here are the examples Code examples:

Run the following commands in Julia REPL in order to initialize the required environments:

julia> using JavaCall

julia> JavaCall.init(["-Xmx128M"])

Example 1:

julia> jlm = @jimport java.lang.Math

JavaObject{:java.lang.Math} (constructor with 2 methods))

julia> jcall(jlm, "sin", jdouble, (jdouble,), pi/2)

Output: 

Example 2:

julia> jlm = @jimport java.lang.Math

JavaObject{:java.lang.Math} (constructor with 2 methods))

julia> jcall(jlm, "cos", jdouble, (jdouble,), pi/4)

Output: