Calculating Fields

Just like lookup fields, calculated fields are virtual fields which you can add to a DataTable object (in this case the Tasks table). A calculated field differs from a lookup field in that rather than looking up its value from another table based on a key, it will calculate its value using code which we provide. Calculated fields in Data Abstract work exactly the same way as they do in any other dataset.

To demonstrate this we are going to improve how the data from the Done column is presented. We have a couple of possible options:

• Show a checkbox which is clear when the task is open, and checked when the task is done
• Show the text Open when the task is open, and Done when the task is done.

For ease of demonstrating calculated fields, we will use the later and leave the former as an exercise for the reader.

Setting up the Calculated Field

In DataModule.java we add our new CalculatedDataColumn after the LookupDataColumn we added on the previous page.

The CalculatedDataColumn's constructor takes three arguments:

• the name of the column, here we set it to Status
• the column's data type, here we set it to a String
• an Evaluator class, here we create an anonymous class using the new Lambda functionality to remove the boiler plate code. We retrieve the value from the Done column of the dataRow and return the strings Done or Open based on its value.

private void setupTable() {
    //snipped code

    CalculatedDataColumn calculatedDoneColumn = new CalculatedDataColumn("Status", String.class, dataRow -> {
        long doneAsLong =  ((Long)dataRow.getField("Done")).longValue();
        return doneAsLong == 1 ? "Done" : "Open";
    });
    tasksTable.getColumns().add(calculatedDoneColumn);    
  }

Creating the CalculatedDataColumn without the new Lambda functionality in Java looks like this

    CalculatedDataColumn calculatedDoneColumn = new CalculatedDataColumn("Status", String.class, new CalculatedDataColumn.Evaluator() {
      @Override
        public Object evaluate(DataRow dataRow) {
          long doneAsLong =  ((Long)dataRow.getField("Done")).longValue();
          return doneAsLong == 1 ? "Done" : "Open";
      }
    });

Display the Calculated Field

Swap over to TodoApp.java and change the tweakTable method to use the new CalculatedDataColumn we added to the table model. Simply change the name of the column passed to getColumn from Done to Status.

private void tweakTable() {
    // snipped code

    //  TableColumn statusColumn = this.table.getColumn("Done");
    TableColumn statusColumn = this.table.getColumn("Status");

    // snipped code
}

Now when you run the app now you will see that the Status column displays the appropriate text rather than 0 or 1.

Now that we are happy with how the data is presented to the user, its time that we add the ability to add, edit and delete tasks. Over the next few pages thats exactly what we shall do.