Constructor question

[eric moon]

I have a utility that reads and writes objects to disk. I have an object Params that gets stored to disk via the utility. Params is a singleton that gets created when the user first starts the program, and initializes the users data. So in the private constructor to Params, I first check whether the file exists on disk, and if it does, I want to use it to create my Params object. I though maybe I could just use "this", but the compiler is complaining. How best to do this?

private Params() {
        if (new XFile(Global.ROOT, "Params.obj").exists())  {
            XFile paramFile = new XFile(Global.ROOT, "Params.obj");
            try {
                //compiler complains about this line:
                this = (Params) paramFile.readObject();
            }
            catch(Exception e) {
                System.out.println("Problem reading note file off disk");
                e.printStackTrace();
            }
        }
        else {         //it's the first time the user has run the program
            noteSet = new NoteSet();
            treeRoot = new NoteNode(new Search("Searches",noteSet));
            treeModel = new SearchTreeModel(treeRoot);
        }
    }
    public Params getInstance()  {
        if (paramsExists) return this;
        else return new Params();
    }

[This message has been edited by eric moon (edited January 12, 2001).]

[Peter den Haan]

Originally posted by eric moon:
I have a utility that reads and writes objects to disk. I have an object Params that gets stored to disk via the utility. Params is a singleton that gets created when the user first starts the program, and initializes the users data. So in the private constructor to Params, I first check whether the file exists on disk, and if it does, I want to use it to create my Params object. I though maybe I could just use "this", but the compiler is complaining. How best to do this?

As you found out you can't assign a new object to "this". Since you are already using a factory method for the singleton pattern, why not put the creation detail, i.e. the choice between deserializing from disk, or creation from scratch, in that factory?

public class Params {
  private Params params = null;
  public static getParams() {
    if (params == null) {
      XFile paramFile = new XFile(Global.ROOT, "Params.obj");
      if (paramFile.exists()) {
        try {
          params = (Params) paramFile.readObject();
        ... etc ...
      } else {
        params = new Params();
      }
    }
  }
  private Params() {
    // it's the first time the user has run the program
    ... bla ...
  }
}

HTH
- Peter

[eric moon]

Oh, that's how I do it?? My class actually contains a single static instance of itself?? Trippy freak-out weird! I'll try it.
Thanks!
e