diff --git a/java/src/main/java/org/msgpack/CustomConverter.java b/java/src/main/java/org/msgpack/CustomConverter.java
index a82e3e1f..b9fb0b3f 100644
--- a/java/src/main/java/org/msgpack/CustomConverter.java
+++ b/java/src/main/java/org/msgpack/CustomConverter.java
@@ -30,7 +30,7 @@ public class CustomConverter {
 	public static void register(Class<?> target, MessageConverter converter) {
 		LOG.debug("register a MessageConverter object for the type: "
 				+ target.getName());
-		map.putIfAbsent(target, converter);
+		map.put(target, converter);
 	}
 
 	public static MessageConverter get(Class<?> target) {
diff --git a/java/src/main/java/org/msgpack/CustomPacker.java b/java/src/main/java/org/msgpack/CustomPacker.java
index 6a65fc3b..0c128b80 100644
--- a/java/src/main/java/org/msgpack/CustomPacker.java
+++ b/java/src/main/java/org/msgpack/CustomPacker.java
@@ -30,7 +30,7 @@ public class CustomPacker {
 	public static void register(Class<?> target, MessagePacker packer) {
 		LOG.debug("register a MessagePacker object for the type: "
 				+ target.getName());
-		map.putIfAbsent(target, packer);
+		map.put(target, packer);
 	}
 
 	public static MessagePacker get(Class<?> target) {
diff --git a/java/src/main/java/org/msgpack/CustomUnpacker.java b/java/src/main/java/org/msgpack/CustomUnpacker.java
index 8e801150..fbf64b78 100644
--- a/java/src/main/java/org/msgpack/CustomUnpacker.java
+++ b/java/src/main/java/org/msgpack/CustomUnpacker.java
@@ -30,7 +30,7 @@ public class CustomUnpacker {
 	public static void register(Class<?> target, MessageUnpacker converter) {
 		LOG.debug("register a MessageUnpacker object for the type: "
 				+ target.getName());
-		map.putIfAbsent(target, converter);
+		map.put(target, converter);
 	}
 
 	public static MessageUnpacker get(Class<?> target) {
diff --git a/java/src/main/java/org/msgpack/util/codegen/DynamicCodeGen.java b/java/src/main/java/org/msgpack/util/codegen/DynamicCodeGen.java
index 0a01f62e..43f5b3d9 100644
--- a/java/src/main/java/org/msgpack/util/codegen/DynamicCodeGen.java
+++ b/java/src/main/java/org/msgpack/util/codegen/DynamicCodeGen.java
@@ -71,7 +71,7 @@ class DynamicCodeGen extends DynamicCodeGenBase implements Constants {
 	}
 
 	public void setTemplates(Class<?> type, Template[] tmpls) {
-		tmplCache.putIfAbsent(type.getName(), tmpls);
+		tmplCache.put(type.getName(), tmpls);
 	}
 
 	public Template[] getTemplates(Class<?> type) {
@@ -79,7 +79,7 @@ class DynamicCodeGen extends DynamicCodeGenBase implements Constants {
 	}
 
 	public void setMessagePackers(Class<?> type, MessagePacker[] pks) {
-		pkCache.putIfAbsent(type.getName(), pks);
+		pkCache.put(type.getName(), pks);
 	}
 
 	public MessagePacker[] getMessagePackers(Class<?> type) {