Optimize the code in TypeAliasRegistry

Author: kang-hl

src/main/java/org/apache/ibatis/type/TypeAliasRegistry.java

@@ -117,10 +117,8 @@ public <T> Class<T> resolveAlias(String string) {
       }
       // issue #748
       String key = string.toLowerCase(Locale.ENGLISH);
-      Class<T> value;
-      if (typeAliases.containsKey(key)) {
-        value = (Class<T>) typeAliases.get(key);
-      } else {
+      Class<T> value = (Class<T>) typeAliases.get(key);
+      if (value == null) {
         value = (Class<T>) Resources.classForName(string);
       }
       return value;
@@ -161,9 +159,10 @@ public void registerAlias(String alias, Class<?> value) {
     }
     // issue #748
     String key = alias.toLowerCase(Locale.ENGLISH);
-    if (typeAliases.containsKey(key) && typeAliases.get(key) != null && !typeAliases.get(key).equals(value)) {
+    Class<?> cls = typeAliases.get(key);
+    if (cls != null && !cls.equals(value)) {
       throw new TypeException(
-          "The alias '" + alias + "' is already mapped to the value '" + typeAliases.get(key).getName() + "'.");
+          "The alias '" + alias + "' is already mapped to the value '" + cls.getName() + "'.");
     }
     typeAliases.put(key, value);
   }