Solution #1 - Using a DefaultMap as lookup table and a Strategy Pattern.

java/build.gradle

@@ -22,6 +22,8 @@ repositories {
 }
 
 dependencies {
+    compile 'org.apache.commons:commons-collections4:4.0'
+
     testCompile 'junit:junit:4.11'
     testCompile 'org.mockito:mockito-all:1.9.5'
 }

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

@@ -0,0 +1,10 @@
+package it.tug.Main;
+
+public class DoubleStringStrategy implements Strategy {
+
+    @Override
+    public String workOn(String theInput) {
+        return String.format("%s%s", theInput, theInput);
+    }
+
+}

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

@@ -0,0 +1,10 @@
+package it.tug.Main;
+
+public class ErrorStrategy implements Strategy {
+
+    @Override
+    public String workOn(String theInput) {
+        return "error";
+    }
+
+}

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

@@ -1,22 +1,25 @@
 package it.tug.Main;
 
+import org.apache.commons.collections4.map.DefaultedMap;
+
+
 public class Formatter {
 
     private Service service;
+    private DefaultedMap<String, Strategy> strategies;
 
     public Formatter(Service service) {
         this.service = service;
+
+        strategies = new DefaultedMap<String, Strategy>(new NullStrategy());
+        strategies.put("FAIL", new ErrorStrategy());
+        strategies.put("OK", new DoubleStringStrategy());
     }
 
     public String doTheJob(String theInput) {
         String response = service.askForPermission();
-        if (response == "FAIL") {
-            return "error";
-        } else if (response == "OK") {
-            return String.format("%s%s", theInput, theInput);
-        } else {
-            return null;
-        }
+        Strategy strategy = strategies.get(response);
+        return strategy.workOn(theInput);
     }
 
 }

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

@@ -0,0 +1,9 @@
+package it.tug.Main;
+
+public class NullStrategy implements Strategy {
+
+    @Override
+    public String workOn(String theInput) {
+        return null;
+    }
+}

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

@@ -0,0 +1,7 @@
+package it.tug.Main;
+
+public interface Strategy {
+
+    public String workOn(String theInput);
+
+}