Notes on Session O3: Interfaces and Polymorphism

Experiment O3.2, Step 3. Because PointPrinter's print method was designed to work with Points and not NewPoints, Java won't let you call the print method on a NewPoint.

Experiment O3.2, Step 4. Since PointPrinter needs to support both Points and NewPoints, it makes sense to maintain the existing method and add an additional method that differs only in that we use NewPoint instead of Point. The method might look like

  /**
   * Print a point using a particular output object.
   */
  public void print(SimpleOutput out, NewPoint pt) {
    out.println("(" + pt.getX() + "," + pt.getY() + ")");
    out.println("  distance from origin: " + 
                pt.distanceFromOrigin());
  } // print(SimpleOutput, NewPoint)

Experiment O3.2, Step 5. If you have done everything appropriately, compilation should be successful in this step. Why? Before there was no print method that could accept a NewPoint as a parameter. Now there is.

Experiment O3.3, Step 3. In Experiment O3.2, you found that it is not possible to pass a NewPoint as a parameter to the print method of PointPrinter. In this experiment, you found that it is possible to pass an ExtendedPoint as a parameter to the same function without modifying print! This illustrates the principle that you can use an element of a subclass wherever an element of a superclass is called for.

Experiment O3.4, Step 3. You will need to create the appropriate method using code similar to that which appears in the discussion. In addition, you will need to import the new Printable class.