I think you are missing ListProvider. In your case.You should try:
empTable = new DataGrid<Employee>(25);
ListDataProvider<Employee> provider = new ListDataProvider<Employee>(tableRowData);
provider.addDataDisplay(empTable);
.
.
.
//empTable.setRowCount(1, true);
//empTable.setRowData(0, tableRowData);
And add data by using provider.getList().add()
or something like this.