code: public synchronized void setTransactionIsolation(
              int level) throws SQLException {
  
          checkClosed();
  
          switch (level) {
  
              case TRANSACTION_READ_UNCOMMITTED :
              case TRANSACTION_READ_COMMITTED :
              case TRANSACTION_REPEATABLE_READ :
              case TRANSACTION_SERIALIZABLE :
                  break;
              default :
                  throw Util.invalidArgument();
          }
  
          try {
              sessionProxy.setIsolationDefault(level);
          } catch (HsqlException e) {
              throw Util.sqlException(e);
          }
      }
docs: <!-- start generic documentation -->
  
  <code>Connection</code> object to the one given.
  The constants defined in the interface <code>Connection</code>
  are the possible transaction isolation levels.
  <P>
  <B>Note:</B> If this method is called during a transaction, the result
  is implementation-defined.
  
  <!-- end generic documentation -->
  <!-- start release-specific documentation -->
  <div class="ReleaseSpecificDocumentation">
  <h3>HSQLDB-Specific Information:</h3> <p>
  
  HSQLDB 2.0 supports all isolation levels. If database transaction
  control is MVCC, <code>Connection.TRANSACTION_READ_UNCOMMITED</code>
  is promoted to <code>Connection.TRANSACTION_READ_COMMITED</code>.
  Calling this method during a transaction always fails.<p>
  
  </div> <!-- end release-specific documentation -->
  
  @param level one of the following <code>Connection</code> constants:
  <code>Connection.TRANSACTION_READ_UNCOMMITTED</code>,
  <code>Connection.TRANSACTION_READ_COMMITTED</code>,
  <code>Connection.TRANSACTION_REPEATABLE_READ</code>, or
  <code>Connection.TRANSACTION_SERIALIZABLE</code>.
  (Note that <code>Connection.TRANSACTION_NONE</code> cannot be used
  because it specifies that transactions are not supported.)
  @exception SQLException if a database access error occurs, this
  (JDBC4 Clarification:)
  method is called on a closed connection
  (:JDBC4 End Clarification)
  or the given parameter is not one of the <code>Connection</code>
  constants
  @see JDBCDatabaseMetaData#supportsTransactionIsolationLevel
  @see #getTransactionIsolation