Same design strategy of Solution #1 Take #1 but with the brand new Java ...

java/build.gradle

@@ -1,6 +1,6 @@
 apply plugin: 'java'
 
-sourceCompatibility = 1.7
+sourceCompatibility = 1.8
 group = 'it.tug'
 version = '0.1'
 

java/src/main/java/it/tug/Main/AskResult.java

@@ -0,0 +1,11 @@
+package it.tug.Main;
+
+public abstract class AskResult {
+  public AskResult onOk(Runnable action) {
+    return this;
+  }
+
+  public AskResult onFail(Runnable action) {
+    return this;
+  }
+}

java/src/main/java/it/tug/Main/FailAskResult.java

@@ -0,0 +1,9 @@
+package it.tug.Main;
+
+public class FailAskResult extends AskResult {
+  @Override
+  public AskResult onFail(Runnable action){
+    action.run();
+    return this;
+  }
+}

java/src/main/java/it/tug/Main/Formatter.java

@@ -1,22 +1,22 @@
 package it.tug.Main;
 
-public class Formatter {
+import java.util.concurrent.atomic.AtomicReference;
 
-    private Service service;
+public class Formatter {
+    private final Service service;    
 
     public Formatter(Service service) {
         this.service = service;
     }
 
     public String doTheJob(String theInput) {
-        String response = service.askForPermission();
-        switch (response) {
-            case "FAIL":
-                return "error";
-            case "OK":
-                return String.format("%s%s", theInput, theInput);
-            default:
-                return null;
-        }
+        AtomicReference<String> doTheJobResult = new AtomicReference<>();
+
+        service
+          .askForPermission()
+          .onFail(() -> doTheJobResult.set("error"))
+          .onOk(() -> doTheJobResult.set(String.format("%s%s", theInput, theInput)));
+
+        return doTheJobResult.get();
     }
 }

java/src/main/java/it/tug/Main/OkAskResult.java

@@ -0,0 +1,9 @@
+package it.tug.Main;
+
+public class OkAskResult extends AskResult {
+  @Override
+  public AskResult onOk(Runnable action) {
+    action.run();
+    return this;
+  }
+}

java/src/main/java/it/tug/Main/Service.java

@@ -1,5 +1,5 @@
 package it.tug.Main;
 
 public interface Service {
-    String askForPermission();
+  AskResult askForPermission();
 }

java/src/test/java/it/tug/FormatterTest.java

@@ -3,8 +3,7 @@
 import static org.junit.Assert.assertEquals;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
-import it.tug.Main.Formatter;
-import it.tug.Main.Service;
+import it.tug.Main.*;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -19,31 +18,34 @@ public void setup() {
         service = mock(Service.class);
         sut = new Formatter(service);
     }
-    
+
     @Test
     public void shouldReturnAnErrorMessageIfServiceReturnsNull() {
-        when(this.service.askForPermission()).thenReturn("FAIL");
-
+        when(this.service.askForPermission())
+          .thenReturn(new FailAskResult());
+
         String actual = sut.doTheJob("foo");
-        
+
         assertEquals("error", actual);
     }
 
     @Test
     public void shouldReturnAnTheStringDoubledIfServiceReturnsOK() {
-        when(this.service.askForPermission()).thenReturn("OK");
-
+        when(this.service.askForPermission())
+          .thenReturn(new OkAskResult());
+
         String actual = sut.doTheJob("foobar");
-        
+
         assertEquals("foobarfoobar", actual);
     }
-    
+
     @Test
     public void shouldReturnNullIfServiceRepliesDifferentlyThanOKOrFAIL() {
-        when(this.service.askForPermission()).thenReturn("luchino");
-
+        when(this.service.askForPermission())
+          .thenReturn(new AskResult(){});
+
         String actual = sut.doTheJob("foobar");
-        
+
         assertEquals(null, actual);
     }
 }