Add methods for StringSet to OPFPreferences.

gradlew

@@ -107,7 +107,7 @@ fi
 
 # For Darwin, add options to specify how the application appears in the dock
 if $darwin; then
-    GRADLE_OPTS="$GRADLE_OPTS \"-Xdock:name=$APP_NAME\" \"-Xdock:icon=$APP_HOME/media/gradle.icns\""
+    GRADLE_OPTS="$GRADLE_OPTS \"-Xdock:name=$APP_NAME\" \"-Xdock:icon=$APP_HOME/media/gradle.icns\" \"-Xmx1024m\" \"-Xms256m\" \"-XX:MaxPermSize=1024m\""
 fi
 
 # For Cygwin, switch paths to Windows format before running java

opfutils/src/main/java/org/onepf/opfutils/OPFPreferences.java

@@ -22,12 +22,27 @@
 import android.support.annotation.Nullable;
 import android.text.TextUtils;
 
+import java.util.Set;
+
 /**
  * The helper class for the easier work with the {@link android.content.SharedPreferences}.
  */
 @SuppressWarnings("PMD.AvoidDuplicateLiterals")
 public class OPFPreferences {
 
+    private static void put(@NonNull final SharedPreferences preferences,
+                            @NonNull final String key,
+                            @NonNull final Set<String> value) {
+        preferences.edit().putStringSet(key, value).apply();
+    }
+
+    @NonNull
+    private static Set<String> getStringSet(@NonNull final SharedPreferences preferences,
+                                            @NonNull final String key,
+                                            @NonNull final Set<String> value) {
+        return preferences.getStringSet(key, value);
+    }
+
     private static void put(@NonNull final SharedPreferences preferences,
                             @NonNull final String key,
                             @NonNull final String value) {
@@ -148,6 +163,22 @@ public SharedPreferences getPreferences() {
         return preferences;
     }
 
+    public void put(@NonNull final String key, @NonNull final Set<String> value) {
+        put(preferences, key, value);
+    }
+
+    @NonNull
+    public Set<String> getStringSet(@NonNull final String key,
+                                    @NonNull final Set<String> defValue) {
+        return getStringSet(preferences, key, defValue);
+    }
+
+    @SuppressWarnings("ConstantConditions")
+    @Nullable
+    public Set<String> getStringSet(@NonNull final String key) {
+        return getStringSet(key, null);
+    }
+
     public void put(@NonNull final String key, @NonNull final String value) {
         put(preferences, key, value);
     }