Remove duplicate code in ConstantPool class

Motivation:

Currently, valueOf() and newInstance() use almost same code to create new constant.
For maintainability, it's better to share duplicate code among them.

Motification:

Added new private functions.
- checkNotNullAndNotEmpty() is for checking whether the name of a constant is null and empty.
- newConstant0() is for creating a new constant.

Result:

- Compact source code
- Improvement of maintainability

common/src/main/java/io/netty/util/ConstantPool.java

@@ -55,31 +55,24 @@ public T valueOf(Class<?> firstNameComponent, String secondNameComponent) {
      * @param name the name of the {@link Constant}
      */
     public T valueOf(String name) {
-        if (name == null) {
-            throw new NullPointerException("name");
-        }
-
-        if (name.isEmpty()) {
-            throw new IllegalArgumentException("empty name");
-        }
+        T c;
 
         synchronized (constants) {
-            T c = constants.get(name);
-            if (c == null) {
-                c = newConstant(nextId, name);
-                constants.put(name, c);
-                nextId ++;
+            if (exists(name)) {
+                c = constants.get(name);
+            } else {
+                c = newInstance0(name);
             }
-
-            return c;
         }
+
+        return c;
     }
 
     /**
      * Returns {@code true} if a {@link AttributeKey} exists for the given {@code name}.
      */
     public boolean exists(String name) {
-        ObjectUtil.checkNotNull(name, "name");
+        checkNotNullAndNotEmpty(name);
         synchronized (constants) {
             return constants.containsKey(name);
         }
@@ -91,27 +84,35 @@ public boolean exists(String name) {
      */
     @SuppressWarnings("unchecked")
     public T newInstance(String name) {
-        if (name == null) {
-            throw new NullPointerException("name");
+        if (exists(name)) {
+            throw new IllegalArgumentException(String.format("'%s' is already in use", name));
         }
 
-        if (name.isEmpty()) {
-            throw new IllegalArgumentException("empty name");
-        }
+        T c = newInstance0(name);
 
+        return c;
+    }
+
+    // Be careful that this dose not check whether the argument is null or empty.
+    private T newInstance0(String name) {
         synchronized (constants) {
-            T c = constants.get(name);
-            if (c == null) {
-                c = newConstant(nextId, name);
-                constants.put(name, c);
-                nextId ++;
-            } else {
-                throw new IllegalArgumentException(String.format("'%s' is already in use", name));
-            }
+            T c = newConstant(nextId, name);
+            constants.put(name, c);
+            nextId++;
             return c;
         }
     }
 
+    private String checkNotNullAndNotEmpty(String name) {
+        ObjectUtil.checkNotNull(name, "name");
+
+        if (name.isEmpty()) {
+            throw new IllegalArgumentException("empty name");
+        }
+
+        return name;
+    }
+
     protected abstract T newConstant(int id, String name);
 
     @Deprecated