Re: Generics question

From:

Larry Coon <lmcoon_nospam@cox.net>

Newsgroups:

comp.lang.java.help

Date:

Fri, 13 Oct 2006 20:43:28 -0700

Message-ID:

<45305CE0.12B3@cox.net>

Lew wrote:

The problem is that the compiler cannot know which subclass of History is in
the type variable, only that it is some subclass of History. Specifically, it
cannot know that the 'history'.get() result is of type 'TitleHistory'.

I'm not entirely sure of the right answer, but I think it's either to declare
history of type ContinuousHistory<TitleHistory> (or '...<? extends
TitleHistory>'), or to use a run-time cast

TitleHistory th = (TitleHistory)history.get(row);

and hope that the get() returns the type that you expect.

I am still learning generics myself, so I welcome more expert comment.

Thanks for the reply.

I at least got the following to compile, but it doesn't seem like the
right approach.

I moved the declaration of the ContinuousHistory<> out of the
HistoryTableModel base class, and into each of the subclasses,
without the generic aspect.

So I have:

public abstract class HistoryTableModel extends AbstractTableModel {
  protected static int[] COLUMN_WIDTHS;

  // This depends on the history, so it's now abstract here.
  public abstract int getRowCount();

  // Doesn't depend on history -- can be implemented here.
  public final int getColumnCount() {
    return COLUMN_WIDTHS.length;
  }

  // etc..
}

Now my subclasses contain stuff that I was trying to define in the
base class:

public class UnitsTableModel extends HistoryTableModel {
  private ContinuousHistory<UnitsHistory> history;

  public UnitsTableModel(ContinuousHistory<UnitsHistory> history) {
    COLUMN_WIDTHS = new int[] {20, 20, 50, 50};

    this.history = history;
  }

  public final int getRowCount() {
    return history.getHistoryData().size();
  }

  // etc.
}

public class TitleTableModel extends HistoryTableModel {
  private ContinuousHistory<TitleHistory> history;

  public TitleTableModel(ContinuousHistory<TitleHistory> history) {
    COLUMN_WIDTHS = new int[] {20, 20, 150};

    this.history = history;
  }

  public final int getRowCount() {
    return history.getHistoryData().size();
  }

  // etc.
}

So now every subclass declares history, which is identical in every
implementation, except for the <> history type, which is what I was
trying to make generic. Each subclass also has to provide an
identical implementation of getRowCount() and any other method which
refers to history.

To me, these are big red flags that I'm doing it the wrong way. I
just can't find the right way.