diff --git a/src/com/fsck/k9/K9.java b/src/com/fsck/k9/K9.java
index d45e8f72e..9eead455a 100644
--- a/src/com/fsck/k9/K9.java
+++ b/src/com/fsck/k9/K9.java
@@ -88,7 +88,7 @@ public class K9 extends Application {
      *
      * @see ApplicationAware
      */
-    private static List<ApplicationAware> observers = new ArrayList<ApplicationAware>();
+    private static final List<ApplicationAware> observers = new ArrayList<ApplicationAware>();
 
     /**
      * This will be {@code true} once the initialization is complete and {@link #notifyObservers()}
diff --git a/src/com/fsck/k9/cache/EmailProviderCache.java b/src/com/fsck/k9/cache/EmailProviderCache.java
index ba3741f49..972d3abdb 100644
--- a/src/com/fsck/k9/cache/EmailProviderCache.java
+++ b/src/com/fsck/k9/cache/EmailProviderCache.java
@@ -42,9 +42,9 @@ public class EmailProviderCache {
 
 
     private String mAccountUuid;
-    private Map<Long, Map<String, String>> mMessageCache = new HashMap<Long, Map<String, String>>();
-    private Map<Long, Map<String, String>> mThreadCache = new HashMap<Long, Map<String, String>>();
-    private Map<Long, Long> mHiddenMessageCache = new HashMap<Long, Long>();
+    private final Map<Long, Map<String, String>> mMessageCache = new HashMap<Long, Map<String, String>>();
+    private final Map<Long, Map<String, String>> mThreadCache = new HashMap<Long, Map<String, String>>();
+    private final Map<Long, Long> mHiddenMessageCache = new HashMap<Long, Long>();
 
 
     private EmailProviderCache(String accountUuid) {
diff --git a/src/com/fsck/k9/controller/MessagingController.java b/src/com/fsck/k9/controller/MessagingController.java
index 7b979371b..d71c06997 100644
--- a/src/com/fsck/k9/controller/MessagingController.java
+++ b/src/com/fsck/k9/controller/MessagingController.java
@@ -319,7 +319,7 @@ public class MessagingController implements Runnable {
     };
 
     // Key is accountNumber
-    private ConcurrentHashMap<Integer, NotificationData> notificationData = new ConcurrentHashMap<Integer, NotificationData>();
+    private final ConcurrentMap<Integer, NotificationData> notificationData = new ConcurrentHashMap<Integer, NotificationData>();
 
     private static final Flag[] SYNC_FLAGS = new Flag[] { Flag.SEEN, Flag.FLAGGED, Flag.ANSWERED, Flag.FORWARDED };
 
diff --git a/src/com/fsck/k9/mail/store/ImapStore.java b/src/com/fsck/k9/mail/store/ImapStore.java
index c830ce1e8..1874d91b6 100644
--- a/src/com/fsck/k9/mail/store/ImapStore.java
+++ b/src/com/fsck/k9/mail/store/ImapStore.java
@@ -31,6 +31,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.Date;
+import java.util.Deque;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
@@ -439,7 +440,7 @@ public class ImapStore extends Store {
 
     private static final SimpleDateFormat RFC3501_DATE = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
 
-    private LinkedList<ImapConnection> mConnections =
+    private final Deque<ImapConnection> mConnections =
         new LinkedList<ImapConnection>();
 
     /**
@@ -453,7 +454,7 @@ public class ImapStore extends Store {
      * requests. This cache lets us make sure we always reuse, if possible, for a given
      * folder name.
      */
-    private HashMap<String, ImapFolder> mFolderCache = new HashMap<String, ImapFolder>();
+    private final Map<String, ImapFolder> mFolderCache = new HashMap<String, ImapFolder>();
 
     public ImapStore(Account account) throws MessagingException {
         super(account);
@@ -3463,7 +3464,7 @@ public class ImapStore extends Store {
         final PushReceiver mReceiver;
         private long lastRefresh = -1;
 
-        HashMap<String, ImapFolderPusher> folderPushers = new HashMap<String, ImapFolderPusher>();
+        final Map<String, ImapFolderPusher> folderPushers = new HashMap<String, ImapFolderPusher>();
 
         public ImapPusher(ImapStore store, PushReceiver receiver) {
             mStore = store;