diff --git a/protocol/src/main/java/com/zfoo/protocol/collection/ArrayUtils.java b/protocol/src/main/java/com/zfoo/protocol/collection/ArrayUtils.java
index 09385728..1e2f9bac 100644
--- a/protocol/src/main/java/com/zfoo/protocol/collection/ArrayUtils.java
+++ b/protocol/src/main/java/com/zfoo/protocol/collection/ArrayUtils.java
@@ -34,8 +34,6 @@ public abstract class ArrayUtils {
     public static final double[] EMPTY_DOUBLE_ARRAY = new double[0];
     public static final char[] EMPTY_CHAR_ARRAY = new char[0];
 
-    public static final Object EMPTY_OBJECT = new Object();
-
     /**
      * unbox
      */
diff --git a/protocol/src/main/java/com/zfoo/protocol/collection/HashSetByte.java b/protocol/src/main/java/com/zfoo/protocol/collection/HashSetByte.java
index ea48ab40..7fb27715 100644
--- a/protocol/src/main/java/com/zfoo/protocol/collection/HashSetByte.java
+++ b/protocol/src/main/java/com/zfoo/protocol/collection/HashSetByte.java
@@ -24,7 +24,7 @@ import java.util.Objects;
  */
 public class HashSetByte extends AbstractSet<Byte> {
 
-    private final ByteObjectHashMap<Object> map;
+    private final ByteObjectHashMap<Boolean> map;
 
     public HashSetByte() {
         map = new ByteObjectHashMap<>();
@@ -56,16 +56,16 @@ public class HashSetByte extends AbstractSet<Byte> {
 
     @Override
     public boolean add(Byte e) {
-        return map.put(e, ArrayUtils.EMPTY_OBJECT) == null;
+        return map.put(e, Boolean.TRUE) == null;
     }
 
     public boolean add(byte e) {
-        return map.put(e, ArrayUtils.EMPTY_OBJECT) == null;
+        return map.put(e, Boolean.TRUE) == null;
     }
 
     @Override
     public boolean remove(Object o) {
-        return map.remove(o) == ArrayUtils.EMPTY_OBJECT;
+        return map.remove(o) != null;
     }
 
     @Override
diff --git a/protocol/src/main/java/com/zfoo/protocol/collection/HashSetInt.java b/protocol/src/main/java/com/zfoo/protocol/collection/HashSetInt.java
index 27ea8e03..d9523a10 100644
--- a/protocol/src/main/java/com/zfoo/protocol/collection/HashSetInt.java
+++ b/protocol/src/main/java/com/zfoo/protocol/collection/HashSetInt.java
@@ -24,7 +24,7 @@ import java.util.Objects;
  */
 public class HashSetInt extends AbstractSet<Integer> {
 
-    private final IntObjectHashMap<Object> map;
+    private final IntObjectHashMap<Boolean> map;
 
     public HashSetInt() {
         map = new IntObjectHashMap<>();
@@ -56,16 +56,16 @@ public class HashSetInt extends AbstractSet<Integer> {
 
     @Override
     public boolean add(Integer e) {
-        return map.put(e, ArrayUtils.EMPTY_OBJECT) == null;
+        return map.put(e, Boolean.TRUE) == null;
     }
 
     public boolean add(int e) {
-        return map.put(e, ArrayUtils.EMPTY_OBJECT) == null;
+        return map.put(e, Boolean.TRUE) == null;
     }
 
     @Override
     public boolean remove(Object o) {
-        return map.remove(o) == ArrayUtils.EMPTY_OBJECT;
+        return map.remove(o) != null;
     }
 
     @Override
diff --git a/protocol/src/main/java/com/zfoo/protocol/collection/HashSetLong.java b/protocol/src/main/java/com/zfoo/protocol/collection/HashSetLong.java
index cf80db24..b3a32697 100644
--- a/protocol/src/main/java/com/zfoo/protocol/collection/HashSetLong.java
+++ b/protocol/src/main/java/com/zfoo/protocol/collection/HashSetLong.java
@@ -56,16 +56,16 @@ public class HashSetLong extends AbstractSet<Long> {
 
     @Override
     public boolean add(Long e) {
-        return map.put(e, ArrayUtils.EMPTY_OBJECT) == null;
+        return map.put(e, Boolean.TRUE) == null;
     }
 
     public boolean add(long e) {
-        return map.put(e, ArrayUtils.EMPTY_OBJECT) == null;
+        return map.put(e, Boolean.TRUE) == null;
     }
 
     @Override
     public boolean remove(Object o) {
-        return map.remove(o) == ArrayUtils.EMPTY_OBJECT;
+        return map.remove(o) != null;
     }
 
     @Override
diff --git a/protocol/src/main/java/com/zfoo/protocol/collection/HashSetShort.java b/protocol/src/main/java/com/zfoo/protocol/collection/HashSetShort.java
index bfb0af73..ea8ea25d 100644
--- a/protocol/src/main/java/com/zfoo/protocol/collection/HashSetShort.java
+++ b/protocol/src/main/java/com/zfoo/protocol/collection/HashSetShort.java
@@ -24,7 +24,7 @@ import java.util.Objects;
  */
 public class HashSetShort extends AbstractSet<Short> {
 
-    private final ShortObjectHashMap<Object> map;
+    private final ShortObjectHashMap<Boolean> map;
 
     public HashSetShort() {
         map = new ShortObjectHashMap<>();
@@ -56,16 +56,16 @@ public class HashSetShort extends AbstractSet<Short> {
 
     @Override
     public boolean add(Short e) {
-        return map.put(e, ArrayUtils.EMPTY_OBJECT) == null;
+        return map.put(e, Boolean.TRUE) == null;
     }
 
     public boolean add(short e) {
-        return map.put(e, ArrayUtils.EMPTY_OBJECT) == null;
+        return map.put(e, Boolean.TRUE) == null;
     }
 
     @Override
     public boolean remove(Object o) {
-        return map.remove(o) == ArrayUtils.EMPTY_OBJECT;
+        return map.remove(o) != null;
     }
 
     @Override