diff --git a/smack-core/src/main/java/org/jivesoftware/smack/filter/FlexiblePacketTypeFilter.java b/smack-core/src/main/java/org/jivesoftware/smack/filter/FlexiblePacketTypeFilter.java
index 80d764a12..5ad7d2409 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/filter/FlexiblePacketTypeFilter.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/filter/FlexiblePacketTypeFilter.java
@@ -28,7 +28,7 @@ import org.jivesoftware.smack.packet.Packet;
  */
 public abstract class FlexiblePacketTypeFilter<P extends Packet> implements PacketFilter {
 
-    final Class<P> packetType;
+    protected final Class<P> packetType;
 
     public FlexiblePacketTypeFilter(Class<P> packetType) {
         this.packetType = packetType;
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/filter/FromMatchesFilter.java b/smack-core/src/main/java/org/jivesoftware/smack/filter/FromMatchesFilter.java
index 83eb2648c..f793c032f 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/filter/FromMatchesFilter.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/filter/FromMatchesFilter.java
@@ -32,11 +32,12 @@ import org.jxmpp.util.XmppStringUtils;
  */
 public class FromMatchesFilter implements PacketFilter {
 
-    private String address;
+    private final String address;
+
     /**
      * Flag that indicates if the checking will be done against bare JID addresses or full JIDs.
      */
-    private boolean matchBareJID = false;
+    private final boolean matchBareJID;
 
     /**
      * Creates a filter matching on the "from" field. The from address must be the same as the
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/filter/NotFilter.java b/smack-core/src/main/java/org/jivesoftware/smack/filter/NotFilter.java
index 42f252e69..5ab86104c 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/filter/NotFilter.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/filter/NotFilter.java
@@ -27,7 +27,7 @@ import org.jivesoftware.smack.packet.Packet;
  */
 public class NotFilter implements PacketFilter {
 
-    private PacketFilter filter;
+    private final PacketFilter filter;
 
     /**
      * Creates a NOT filter using the specified filter.
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/filter/PacketIDFilter.java b/smack-core/src/main/java/org/jivesoftware/smack/filter/PacketIDFilter.java
index 56498373a..4c31bcdfe 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/filter/PacketIDFilter.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/filter/PacketIDFilter.java
@@ -18,6 +18,7 @@
 package org.jivesoftware.smack.filter;
 
 import org.jivesoftware.smack.packet.Packet;
+import org.jivesoftware.smack.util.StringUtils;
 
 /**
  * Filters for packets with a particular packet ID.
@@ -26,7 +27,7 @@ import org.jivesoftware.smack.packet.Packet;
  */
 public class PacketIDFilter implements PacketFilter {
 
-    private String packetID;
+    private final String packetID;
 
     /**
      * Creates a new packet ID filter using the specified packet's ID.
@@ -43,8 +44,8 @@ public class PacketIDFilter implements PacketFilter {
      * @param packetID the packet ID to filter for.
      */
     public PacketIDFilter(String packetID) {
-        if (packetID == null) {
-            throw new IllegalArgumentException("Packet ID must not be null.");
+        if (StringUtils.isNullOrEmpty(packetID)) {
+            throw new IllegalArgumentException("Packet ID must not be null or empty.");
         }
         this.packetID = packetID;
     }
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/filter/PacketTypeFilter.java b/smack-core/src/main/java/org/jivesoftware/smack/filter/PacketTypeFilter.java
index 8742ff0ef..acd6b3fd7 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/filter/PacketTypeFilter.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/filter/PacketTypeFilter.java
@@ -37,7 +37,7 @@ public class PacketTypeFilter implements PacketFilter {
     public static final PacketTypeFilter PRESENCE = new PacketTypeFilter(Presence.class);
     public static final PacketTypeFilter MESSAGE = new PacketTypeFilter(Message.class);
 
-    Class<? extends Packet> packetType;
+    private final Class<? extends Packet> packetType;
 
     /**
      * Creates a new packet type filter that will filter for packets that are the
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/filter/ThreadFilter.java b/smack-core/src/main/java/org/jivesoftware/smack/filter/ThreadFilter.java
index aa77ee7c0..ca45733ea 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/filter/ThreadFilter.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/filter/ThreadFilter.java
@@ -19,6 +19,7 @@ package org.jivesoftware.smack.filter;
 
 import org.jivesoftware.smack.packet.Packet;
 import org.jivesoftware.smack.packet.Message;
+import org.jivesoftware.smack.util.StringUtils;
 
 /**
  * Filters for message packets with a particular thread value.
@@ -27,7 +28,7 @@ import org.jivesoftware.smack.packet.Message;
  */
 public class ThreadFilter implements PacketFilter {
 
-    private String thread;
+    private final String thread;
 
     /**
      * Creates a new thread filter using the specified thread value.
@@ -35,8 +36,8 @@ public class ThreadFilter implements PacketFilter {
      * @param thread the thread value to filter for.
      */
     public ThreadFilter(String thread) {
-        if (thread == null) {
-            throw new IllegalArgumentException("Thread must not be null.");
+        if (StringUtils.isNullOrEmpty(thread)) {
+            throw new IllegalArgumentException("Thread must not be null or empty.");
         }
         this.thread = thread;
     }