<code Java>
import javax.tools.*; // Compilation
</code>

<code Java>
                File helloWorldJava = new File ("HelloWorld.java"); // source file

                JavaCompiler compiler = ToolProvider.getSystemJavaCompiler ();
 
                DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject> ();
                StandardJavaFileManager fileManager = compiler.getStandardFileManager (diagnostics, null, null);
 
                ArrayList<String> optionList = new ArrayList<String> ();
                // optionList.add("-classpath");
                // optionList.add("directory");
 
                Iterable<? extends JavaFileObject> compilationUnit =
                        fileManager.getJavaFileObjectsFromFiles (Arrays.asList (helloWorldJava));
 
                JavaCompiler.CompilationTask task = compiler.getTask (
                        null,
                        fileManager,
                        diagnostics,
                        optionList,
                        null,
                        compilationUnit);

                if (task.call ())
                {
                   /* ... */
                }
</code>

<code Java>    
   URLClassLoader classLoader = 
       new URLClassLoader (new URL[] { new File ("./").toURI ().toURL () });

   Class loadedClass = classLoader.loadClass ("testcompile.HelloWorld");
 
  Object obj = loadedClass.newInstance ();

  try
  {
      Method m = loadedClass.getMethod ("doStuff");
      m.invoke (obj);
  } 
  catch (NoSuchMethodException ex) { }
  catch (SecurityException ex) { } 
  catch (IllegalArgumentException ex) { } 
  catch (InvocationTargetException ex) { }

</code>

/*https://stackoverflow.com/questions/21544446/how-do-you-dynamically-compile-and-load-external-java-classes */

<code Java>    
    private void compile ()
    {
        /*
        StringBuilder sb = new StringBuilder (64);
        sb.append ("package testcompile;\n");
        sb.append ("public class HelloWorld {\n");
        sb.append ("    public void doStuff() {\n");
        sb.append ("        System.out.println(\"Hello world\");\n");
        sb.append ("    }\n");
        sb.append ("}\n");
        */

        String source = editor.getText ();
        File helloWorldJava = new File ("testcompile/HelloWorld.java");
        if (helloWorldJava.getParentFile ().exists () || helloWorldJava.getParentFile ().mkdirs ())
        {

            try
            {
                Writer writer = null;
                try
                {
                    writer = new FileWriter (helloWorldJava);
                    // writer.write (sb.toString ());
                    writer.write (source);
                    writer.flush ();
                } 
                finally
                {
                    try
                    {
                       writer.close ();
                    } 
                    catch (Exception e) 
                    {
                    }
                }

                DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject> ();
                JavaCompiler compiler = ToolProvider.getSystemJavaCompiler ();
                StandardJavaFileManager fileManager = compiler.getStandardFileManager (diagnostics, null, null);

                // This sets up the class path that the compiler will use.
                // I've added the .jar file that contains the DoStuff interface within in it...
                ArrayList<String> optionList = new ArrayList<String> ();
                // optionList.add("-classpath");
                // optionList.add(System.getProperty("java.class.path") + File.pathSeparator + "dist/InlineCompiler.jar");

                Iterable<? extends JavaFileObject> compilationUnit =
                        fileManager.getJavaFileObjectsFromFiles (Arrays.asList (helloWorldJava));

                JavaCompiler.CompilationTask task = compiler.getTask (
                        null,
                        fileManager,
                        diagnostics,
                        optionList,
                        null,
                        compilationUnit);

                if (task.call ())
                {
                    // Create a new custom class loader, pointing to the directory that contains the compiled
                    // classes, this should point to the top of the package structure!
                    URLClassLoader classLoader = new URLClassLoader (new URL[]
                    {
                        new File ("./").toURI ().toURL ()
                    });
                    // Load the class from the classloader by name....
                    Class loadedClass = classLoader.loadClass ("testcompile.HelloWorld");

                    Object obj = loadedClass.newInstance ();
                    try
                    {
                        Method m = loadedClass.getMethod ("doStuff");
                        m.invoke (obj);
                    } 
                    catch (NoSuchMethodException ex) { }
                    catch (SecurityException ex) { } 
                    catch (IllegalArgumentException ex) { } 
                    catch (InvocationTargetException ex) { }
                }
                else
                {
                    for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics.getDiagnostics ())
                    {
                        System.out.format ("Error on line %d in %s%n",
                                diagnostic.getLineNumber (),
                                diagnostic.getSource ().toUri ());
                    }
                }
                fileManager.close ();
            } catch (IOException | ClassNotFoundException | InstantiationException | IllegalAccessException exp)
            {
                exp.printStackTrace ();
            }
        }
    }
</code>