Samples for returning user objects types and exception types from JavaFX service calls.

BadFooServiceApp.java

import java.util.Random;
import javafx.application.Application;
import javafx.concurrent.*;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.VBox;
import javafx.scene.text.TextAlignment;
import javafx.stage.Stage;
 
public class BadFooServiceApp extends Application {
  public static void main(String[] args) { Application.launch(args); }
  @Override public void start(final Stage stage) throws Exception {
    final BadFooGenerator badFooGenerator = new BadFooGenerator();
    final Label badFooDisplay = new Label();
    badFooDisplay.setMinWidth(250);
    badFooDisplay.setAlignment(Pos.CENTER);
    final Button badFooMaker = new Button("Fail Foo");
    badFooMaker.setOnAction(new EventHandler<ActionEvent>() {
      @Override public void handle(ActionEvent t) {
        badFooDisplay.setText("...");
        if (badFooGenerator.getState() == Worker.State.READY) {
          badFooGenerator.start();
        } else {  
          badFooGenerator.restart();
        }  
      }
    });
    badFooMaker.disableProperty().bind(badFooGenerator.runningProperty());
    
    badFooGenerator.setOnFailed(new EventHandler<WorkerStateEvent>() {
      @Override public void handle(WorkerStateEvent t) {
        Throwable ouch = badFooGenerator.getException();
        badFooDisplay.setText(ouch.getClass().getName() + " -> " + ouch.getMessage());
      }
    });
    
    final VBox layout = new VBox(10);
    layout.setAlignment(Pos.CENTER);
    layout.getChildren().setAll(badFooMaker, badFooDisplay);
    layout.setStyle("-fx-padding: 10px; -fx-background-color: cornsilk;");

    Scene scene = new Scene(layout);
    stage.setScene(scene);
    stage.show();
  }

  class BadFooGenerator extends Service<Foo> {
    @Override protected Task createTask() {
      return new Task<Foo>() {
        @Override protected Foo call() throws Exception {
          Thread.currentThread().sleep(1000);
          throw new BadFooException();
        }
      };
    }
  }
  
  class BadFooException extends Exception {
    BadFooException() {
      super(random.nextInt(100) + "");
    }
  }

  class Foo {
    private int answer = random.nextInt(100);
    public int getAnswer() {
      return answer;
    }
  }
   
  private static final Random random = new Random();
}

FooModifierServiceApp.java

import javafx.application.Application;
import javafx.concurrent.Service;
import javafx.concurrent.Task;
import javafx.concurrent.Worker;
import javafx.concurrent.WorkerStateEvent;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

import java.util.Random;

public class FooModifierServiceApp extends Application {
  public static void main(String[] args) { Application.launch(args); }
  @Override public void start(final Stage stage) throws Exception {
    final FooModifier fooModifier = new FooModifier();

    final Label fooBeforeDisplay = new Label();
    fooBeforeDisplay.setMinWidth(300);
    fooBeforeDisplay.setAlignment(Pos.CENTER);
    final Label fooAfterDisplay  = new Label();
    fooAfterDisplay.setMinWidth(300);
    fooAfterDisplay.setAlignment(Pos.CENTER);

    final Button fooMaker = new Button("Make Foo");
    fooMaker.setOnAction(new EventHandler<ActionEvent>() {
      @Override public void handle(ActionEvent t) {
        fooModifier.reset();
        Foo foo = new Foo();
        fooModifier.setFoo(foo);
        fooBeforeDisplay.setText("Before Modification: " + foo.getAnswer());
        fooAfterDisplay.setText("...");
        if (fooModifier.getState() == Worker.State.READY) {
          fooModifier.start();
        } else {
          fooModifier.restart();
        }
      }
    });
    fooMaker.disableProperty().bind(fooModifier.runningProperty());

    fooModifier.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
      @Override public void handle(WorkerStateEvent t) {
        Foo myModifiedFoo = fooModifier.getValue();
        fooAfterDisplay.setText("After Modification: " + myModifiedFoo.getAnswer());
      }
    });

    final VBox layout = new VBox(10);
    layout.setAlignment(Pos.CENTER);
    layout.getChildren().setAll(fooMaker, fooBeforeDisplay, fooAfterDisplay);
    layout.setStyle("-fx-padding: 10px; -fx-background-color: cornsilk;");

    Scene scene = new Scene(layout);
    stage.setScene(scene);
    stage.show();
  }

    class FooModifier extends Service<Foo> {
      private Foo foo;

      void setFoo(Foo foo) {
        this.foo = foo;
      }

      @Override protected Task createTask() {
        return new FooModifierTask(foo);
      }

      private class FooModifierTask extends Task<Foo> {
        final private Foo fooInput;

        FooModifierTask(Foo fooInput) {
          this.fooInput = fooInput;
        }

        @Override protected Foo call() throws Exception {
          Thread.currentThread().sleep(1000);
          return new Foo(fooInput);
        }
      }
    }


    class Foo {
      private final int answer;

      Foo() {
        answer = random.nextInt(100);
      }

      Foo(Foo input) {
        answer = input.getAnswer() + 42;
      }

      public int getAnswer() {
        return answer;
      }
    }

  private static final Random random = new Random();
}

FooServiceApp.java

import java.util.Random;
import javafx.application.Application;
import javafx.concurrent.*;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
 
public class FooServiceApp extends Application {
  public static void main(String[] args) { Application.launch(args); }
  @Override public void start(final Stage stage) throws Exception {
    final FooGenerator    fooGenerator = new FooGenerator();

    final Label fooDisplay = new Label();
    final Button fooMaker = new Button("Make Foo");
    fooMaker.setOnAction(new EventHandler<ActionEvent>() {
      @Override public void handle(ActionEvent t) {
        fooDisplay.setText("...");
        if (fooGenerator.getState() == Worker.State.READY) {
          fooGenerator.start();
        } else {  
          fooGenerator.restart();
        }  
      }
    });
    fooMaker.disableProperty().bind(fooGenerator.runningProperty());
    
    fooGenerator.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
      @Override public void handle(WorkerStateEvent t) {
        Foo myLovelyNewFoo = fooGenerator.getValue();
        fooDisplay.setText(myLovelyNewFoo.getAnswer() + "");
      }
    });

    final VBox layout = new VBox(10);
    layout.setAlignment(Pos.CENTER);
    layout.getChildren().setAll(fooMaker, fooDisplay);
    layout.setStyle("-fx-padding: 10px; -fx-background-color: cornsilk;");

    Scene scene = new Scene(layout);
    stage.setScene(scene);
    stage.show();
  }

  class FooGenerator extends Service<Foo> {
    @Override protected Task createTask() {
      return new Task<Foo>() {
        @Override protected Foo call() throws Exception {
          Thread.currentThread().sleep(1000);
          return new Foo();
        }
      };
    }
  }

  class Foo {
    private int answer = random.nextInt(100);
    public int getAnswer() {
      return answer;
    }
  }
   
  private static final Random random = new Random();
}

Solution to the StackOverflow question:

How to communicate user defined objects and exceptions between Service and UI in JavaFX2?