warn if not serializable, also add basic unit test

src/com/fsck/k9/helper/Utility.java

@@ -709,21 +709,33 @@ public class Utility {
     }
 
     public static <T> Serializable toSerializableList(List<T> list) {
-        return list instanceof Serializable ?
+        return isExpectedCollectionClass(Serializable.class, list) ?
                 (Serializable) list :
                 new ArrayList<T>(list);
     }
 
     public static <T> ArrayList<T> toArrayList(List<T> list) {
-        return list instanceof ArrayList ?
+        return isExpectedCollectionClass(ArrayList.class, list) ?
                 (ArrayList<T>) list :
                 new ArrayList<T>(list);
     }
 
-
-    public static <T,U> Serializable toSerializableConcurrentMap(ConcurrentMap<T,U> list) {
-        return list instanceof ConcurrentHashMap ?
-                (ConcurrentHashMap<T,U>) list :
-                new ConcurrentHashMap<T,U>(list);
+    public static <T, U> Serializable toSerializableConcurrentMap(ConcurrentMap<T, U> map) {
+        return isExpectedCollectionClass(ConcurrentHashMap.class, map) ?
+                (ConcurrentHashMap<T, U>) map :
+                new ConcurrentHashMap<T, U>(map);
     }
+
+    private static boolean isExpectedCollectionClass(Class<?> expected, Object instance) {
+//        Objects.requireNonNull(instance); // uncomment when project moves to API level 19
+        if (!expected.isInstance(instance)) {
+            Log.w(K9.LOG_TAG, "Instance class [" + instance.getClass().getName()
+                    + "] would be better as [" + expected.getName()
+                    + "] for performance, to prevent collection copying");
+            return false;
+        } else {
+            return true;
+        }
+    }
+
 }

tests-on-jvm/src/com/fsck/k9/helper/UtilityTest.java

@@ -4,23 +4,23 @@ import com.fsck.k9.helper.Utility;
 
 import junit.framework.TestCase;
 
-import java.io.Serializable;
-import java.util.LinkedList;
+import java.io.*;
+import java.util.*;
 
 public class UtilityTest extends TestCase {
 
-    public void testToSerializableList() {
+    public void testToArrayList() {
         LinkedList<String> input = new LinkedList<String>(Arrays.asList("a", "b"));
 
-        Serializable serializableList = Utility.toSerializableList(input);
+        Serializable serializableList = Utility.toArrayList(input);
 
         assertEquals(serializableList, input);
     }
 
-    public void testToSerializableListAlreadySerializable() {
+    public void testToArrayListAlreadyArrayList() {
         ArrayList<String> input = new ArrayList<String>(Arrays.asList("a", "b"));
 
-        Serializable serializableList = Utility.toSerializableList(input);
+        Serializable serializableList = Utility.toArrayList(input);
 
         assertSame(serializableList, input);
     }