docs: public int[] getKeys() {
          int[] keys = this.keys;
          int n = keyCount;
          int[] result = new int[n];
          for (int i = 0; n != 0; ++i) {
              int entry = keys[i];
              if (entry != EMPTY && entry != DELETED) {
                  result[--n] = entry;
              }
          }
          return result;
      }