Distribution.java correct equals method

api/src/main/java/org/apache/gravitino/rel/expressions/distributions/Distribution.java

@@ -46,16 +46,20 @@ default Expression[] children() {
   /**
    * Indicates whether some other object is "equal to" this one.
    *
-   * @param distribution The reference distribution object with which to compare.
+   * @param obj The reference object with which to compare.
    * @return returns true if this object is the same as the obj argument; false otherwise.
    */
-  default boolean equals(Distribution distribution) {
-    if (distribution == null) {
+  default boolean equals(Distribution obj) {
+    if (this == obj) {
+      return true;
+    }
+    if (obj == null || getClass() != obj.getClass()) {
       return false;
     }
-
-    return strategy().equals(distribution.strategy())
-        && number() == distribution.number()
-        && Arrays.equals(expressions(), distribution.expressions());
+
+    Distribution that = (Distribution) obj;
+    return number() == that.number()
+    && strategy().equals(that.strategy())
+    && Arrays.equals(expressions(), that.expressions());
   }
 }