[IMPALA-7711] Hash code improvements for CatalogdMetaProvider - ASF JIRA

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Consider the classes nested within CatalogdMetaProvider. A number of hashing-related items could use attention.

First, the code uses com.google.common.base.Objects, however, the Guava docs state:

Note for Java 7 and later: This method should be treated as deprecated; use Objects.hash(java.lang.Object...) instead.

Second, hash codes are combined incorrectly. Consider:

    public int hashCode() {
      return Objects.hashCode(super.hashCode(), version_);
    }

Note that this takes the hash code from the super can hashes it again in the subclass. The Java library combines hash codes as follows:

    public int hashCode() {
      return super.hashCode() * 31 + Objects.hashCode(version_);
    }

Could find no reference to a standard JDK or Guava function that combines hash codes, perhaps we should create one.

Finally, some methods include the class when computing the hash, but no subclasses exist:

  private static class DbCacheKey {
    ...
    public int hashCode() {
      return Objects.hash(getClass(), dbName_, type_);
    }

No need to include the class if either a) no subclasses exist, or b) subclasses include attributes not in the base class.