Problems when importing a custom Java class

Question

I have a jar file so I tried to run the class from Mathematica following these steps, but I get all these:

Java::excptn: A Java exception occurred: java.lang.ClassNotFoundException: Buscar
    at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:471)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:588)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
    at java.base/java.lang.Class.forName0(Native Method)
    at java.base/java.lang.Class.forName(Class.java:398).

LoadJavaClass::fail: Java failed to load class Buscar.

This is the code in my notebook

Needs["JLink`"]
InstallJava[]
AddToClassPath["C:\Users\Amada\Documents\Buscar.java"]
LoadJavaClass["Buscar"]

And this is the Java codel I made it with Apache-Neatbeans 11.2 and I have JDK 12 and Mathematica 12 (if it's useful)

package buscar;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class Buscar {

    public static void main(String[] args) {
        buscador("issue");
    }
    
    public static void buscador(String palabra){
     Scanner entrada= null;
        String linea;
        int numdelinea= 1;
        boolean contiene= false;
        Scanner sc = new Scanner(System.in);
        
         try{
            
            File f= new File("C:/Program Files/Wolfram Research/Mathematica/12.1/Documentation/English/System/ExampleData/USConstitution.txt");
            entrada= new Scanner(f);
            System.out.println("Archivo: " + f.getName());
            System.out.println("Texto a bucar: " + palabra);
            
            while(entrada.hasNext()){
                linea= entrada.nextLine();
                if(linea.contains(palabra)){
                    System.out.println("Línea " + numdelinea + ": " + linea);
                    contiene = true;
                }
                numdelinea++;
            }
            if( !contiene){
                System.out.println(palabra + "no se ha encontrado");
            }
        } catch (FileNotFoundException e){
            System.out.println(e.toString());
        }catch (NullPointerException e){
            System.out.println(e.toString()+ "No se ha seleccionado ningún archivo");
        }catch (Exception e){
            System.out.println(e.toString());
        }finally{
            if(entrada !=null){
                entrada.close();
                }
            }
        }
    }

Thank you everyone.

-Luisa

Answer 1

There are a number of issues that must be addressed.

1. The AddToClassPath call should reference the directory or JAR file containing the compiled Java classes, not the source file. Since you are using NetBeans, the class files are normally compiled into the subfolder build\classes within the project folder (or possibly just build depending on the NetBeans vintage). Also, if it is a normal Java application project, then a JAR file will also have been generated into the subfolder dist.

2. When loading the Java class, the package name must be included in the class name. In this case, LoadJavaClass["buscar.Buscar"].

3. The class uses System.out.println to display its results. To see this output, you must use ShowJavaConsole[].

The following code puts this all together. You will need to adjust it to match your actual project directory structure (here, C:\NetBeansProjects\myapp):

Needs["JLink`"]
InstallJava[];

AddToClassPath["C:\NetBeansProjects\myapp\build\classes"]
(* or possibly: AddToClassPath["C:\NetBeansProjects\myapp\dist\myapp.jar"] *)

LoadJavaClass["buscar.Buscar"]

ShowJavaConsole[]

Having done all this, we can run the main program:

Buscar`main[{}]

(* output in the Java console:

Archivo: USConstitution.txt
Texto a bucar: issue
Línea 42: Authority thereof shall issue Writs of Election to fill such Vacancies.
Línea 545: no Warrants shall issue, but upon probable cause, supported by Oath or
Línea 699: executive authority of such State shall issue writs of election to fill such
Línea 845: Congress shall decide the issue, assembling within forty eight hours for that
*)

... or we can run the static method buscador instead:

Buscar`buscador["issue"]

(* same output as above *)

Manipulating the Result

You might want to consider changing the method buscador to return its result instead of printing it to the console. This would make it easier to manipulate the result within Mathematica. Perhaps it could return a string or list of strings... or something more structured like JSON. But such changes to the Java code are beyond the scope of this response.

Alternatively, you could run your application directly with Java and Import the result (no JLink required):

Import[
  "!java -cp C:\\NetBeansProjects\\myapp\\build\\classes buscar.Buscar"
, "Text"
]

(* Same output as before, but returned as a string *)