diff --git a/src/main/java/com/cloudhopper/smpp/pdu/Pdu.java b/src/main/java/com/cloudhopper/smpp/pdu/Pdu.java
index d7808070..ddc5e503 100644
--- a/src/main/java/com/cloudhopper/smpp/pdu/Pdu.java
+++ b/src/main/java/com/cloudhopper/smpp/pdu/Pdu.java
@@ -27,7 +27,10 @@
 import com.cloudhopper.smpp.tlv.Tlv;
 import com.cloudhopper.smpp.transcoder.PduTranscoderContext;
 import com.cloudhopper.smpp.util.ChannelBufferUtil;
+
 import java.util.ArrayList;
+import java.util.List;
+
 import org.jboss.netty.buffer.ChannelBuffer;
 
 public abstract class Pdu {
@@ -147,7 +150,7 @@ public int getOptionalParameterCount() {
      * added, this will return null.
      * @return Null if no parameters added yet, or the list of optional parameters.
      */
-    public ArrayList<Tlv> getOptionalParameters() {
+    public List<Tlv> getOptionalParameters() {
         return this.optionalParameters;
     }
 
diff --git a/src/main/java/com/cloudhopper/smpp/type/NotEnoughDataInBufferException.java b/src/main/java/com/cloudhopper/smpp/type/NotEnoughDataInBufferException.java
index 20f8762d..950cb916 100644
--- a/src/main/java/com/cloudhopper/smpp/type/NotEnoughDataInBufferException.java
+++ b/src/main/java/com/cloudhopper/smpp/type/NotEnoughDataInBufferException.java
@@ -31,8 +31,8 @@
 public class NotEnoughDataInBufferException extends RecoverablePduException {
     static final long serialVersionUID = 1L;
     
-    private int available;
-    private int expected;
+    private final int available;
+    private final int expected;
 
     /**
      * Constructs an instance of <code>AtNotEnoughDataInBufferException</code>
diff --git a/src/main/java/com/cloudhopper/smpp/type/SmppProcessingException.java b/src/main/java/com/cloudhopper/smpp/type/SmppProcessingException.java
index 95b47008..a6657a68 100644
--- a/src/main/java/com/cloudhopper/smpp/type/SmppProcessingException.java
+++ b/src/main/java/com/cloudhopper/smpp/type/SmppProcessingException.java
@@ -30,7 +30,7 @@
 public class SmppProcessingException extends Exception {
     static final long serialVersionUID = 1L;
 
-    private int errorCode;
+    private final int errorCode;
 
     public SmppProcessingException(int errorCode) {
         this(errorCode, null);
diff --git a/src/main/java/com/cloudhopper/smpp/util/DeliveryReceipt.java b/src/main/java/com/cloudhopper/smpp/util/DeliveryReceipt.java
index a0e2f10c..2c34af39 100644
--- a/src/main/java/com/cloudhopper/smpp/util/DeliveryReceipt.java
+++ b/src/main/java/com/cloudhopper/smpp/util/DeliveryReceipt.java
@@ -336,7 +336,7 @@ static private DateTime parseDateTimeHelper(String value, DateTimeZone zone) {
 	}
 
 	static public void findFieldAndAddToTreeMap(String normalizedText,
-			String field, TreeMap<Integer, String> fieldsByStartPos) {
+			String field, Map<Integer, String> fieldsByStartPos) {
 		int startPos = normalizedText.indexOf(field);
 		// logger.debug("Found field " + field + " at startPos " + startPos);
 		if (startPos >= 0) {