code: public Group withKeys(String... keys) {
          if (this.keys == null) {
              setKeys(new java.util.ArrayList<String>(keys.length));
          }
          for (String ele : keys) {
              this.keys.add(ele);
          }
          return this;
      }
docs: <p>
  The keys that are included in this group.
  </p>
  <p>
  <b>NOTE:</b> This method appends the values to the existing list (if any). Use
  {@link #setKeys(java.util.Collection)} or {@link #withKeys(java.util.Collection)} if you want to override the
  existing values.
  </p>
  
  @param keys
  The keys that are included in this group.
  @return Returns a reference to this object so that method calls can be chained together.