diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPAddress.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPAddress.java
index ed8a11b..54465bf 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPAddress.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPAddress.java
@@ -28,6 +28,9 @@
     public abstract int hashCode();
 
     public static IPAddress<?> of(String ip) {
+        if (ip == null) {
+            throw new NullPointerException("String ip must not be null");
+        }
         if (ip.indexOf('.') != -1)
             return IPv4Address.of(ip);
         else if (ip.indexOf(':') != -1)
diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPAddressWithMask.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPAddressWithMask.java
index d0632c5..654e72c 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPAddressWithMask.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPAddressWithMask.java
@@ -10,6 +10,9 @@
     public abstract IPVersion getIpVersion();
 
     public static IPAddressWithMask<?> of(String ip) {
+        if (ip == null) {
+            throw new NullPointerException("String ip must not be null");
+        }
         if (ip.indexOf('.') != -1)
             return IPv4AddressWithMask.of(ip);
         else if (ip.indexOf(':') != -1)
@@ -18,6 +21,7 @@
             throw new IllegalArgumentException("IP Address not well formed: " + ip);
     }
 
+    @Override
     public String toString() {
         StringBuilder res = new StringBuilder();
         res.append(value.toString());
diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4Address.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4Address.java
index 970a65c..87167fb 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4Address.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4Address.java
@@ -49,6 +49,9 @@
     }
 
     public static IPv4Address of(final byte[] address) {
+        if (address == null) {
+            throw new NullPointerException("Address must not be null");
+        }
         if (address.length != LENGTH) {
             throw new IllegalArgumentException(
                     "Invalid byte array length for IPv4Address address: " + address.length);
@@ -67,6 +70,9 @@
     }
 
     public static IPv4Address of(final String string) {
+        if (string == null) {
+            throw new NullPointerException("String must not be null");
+        }
         int start = 0;
         int shift = 24;
 
diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4AddressWithMask.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4AddressWithMask.java
index 1f23f2d..9b60c6a 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4AddressWithMask.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4AddressWithMask.java
@@ -22,10 +22,19 @@
     }
 
     public static IPv4AddressWithMask of(IPv4Address value, IPv4Address mask) {
+        if (value == null) {
+            throw new NullPointerException("Value must not be null");
+        }
+        if (mask == null) {
+            throw new NullPointerException("Mask must not be null");
+        }
         return new IPv4AddressWithMask(value, mask);
     }
 
     public static IPv4AddressWithMask of(final String string) {
+        if (string == null) {
+            throw new NullPointerException("String must not be null");
+        }
         int slashPos;
         String ip = string;
         int maskBits = 32;
diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6Address.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6Address.java
index 531dadf..63ae700 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6Address.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6Address.java
@@ -53,6 +53,9 @@
     }
 
     public static IPv6Address of(final byte[] address) {
+        if (address == null) {
+            throw new NullPointerException("Address must not be null");
+        }
         if (address.length != LENGTH) {
             throw new IllegalArgumentException(
                     "Invalid byte array length for IPv6 address: " + address.length);
@@ -98,6 +101,9 @@
     private final static Pattern colonPattern = Pattern.compile(":");
 
     public static IPv6Address of(final String string) {
+        if (string == null) {
+            throw new NullPointerException("String must not be null");
+        }
         IPv6Builder builder = new IPv6Builder();
         String[] parts = colonPattern.split(string, -1);
 
diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6AddressWithMask.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6AddressWithMask.java
index 8376e5e..7259c7f 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6AddressWithMask.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6AddressWithMask.java
@@ -16,11 +16,20 @@
     }
 
     public static IPv6AddressWithMask of(IPv6Address value, IPv6Address mask) {
+        if (value == null) {
+            throw new NullPointerException("Value must not be null");
+        }
+        if (mask == null) {
+            throw new NullPointerException("Mask must not be null");
+        }
         return new IPv6AddressWithMask(value, mask);
     }
 
 
     public static IPv6AddressWithMask of(final String string) {
+        if (string == null) {
+            throw new NullPointerException("String must not be null");
+        }
         int slashPos;
         String ip = string;
         int maskBits = 128;