code: public String toTableId(String tableName) {
      Preconditions.checkNotNull(tableName, "Table name cannot be null");
      String tablesPrefix = instanceName + TABLE_SEPARATOR;
      Preconditions.checkState(tableName.startsWith(tablesPrefix),
          "'%s' does not start with '%s'", tableName, tablesPrefix);
      String tableId = tableName.substring(tablesPrefix.length()).trim();
      Preconditions.checkState(!tableId.isEmpty(), "Table id is blank");
      return tableId;
    }
docs: Transforms a tableName within this instance of the form
  projects/(projectId)/instances/(instanceId)/tables/(tableId) to (tableId).
  
  @param tableName a {@link java.lang.String} object.
  @return a {@link java.lang.String} object.