RE: Vector question

["Joe Teff" <JoeTeff@xxxxxxxxxxxxx>]

This is where you need to pay very close attention to the API
(documentation). The return type from Vector's get() method is an Object.
You need to cast it to a MyClass before you can use it as such. The object
is still a MyClass object, but the data type is Object, so those are the
only methods/data available unless you cast.

if ((MyClass)(list.paintOptions.get(x)).isBlue)

list is a data type MyClassList that has a reference variable paintOptions.
paintOptions is a data type Vector.
A MyClass object is created and added as a object in the Vector.

Understanding how reference variables are converted (automatic or implicit)
and casted (manual or explicit) is fundamental to understanding the Java
language. Take the following line:

  Object obj = new MyClass();

obj is an Object data type and it refers to an object. The object is really
a MyClass object, but the only things you can do to it right now are those
things that an Object can do. Now take the following statement:

  MyClass abc = obj;

This statement would fail because obj is an Object data type and MyClass is
going down the inheritance structure. Thus, I have to cast to make it work.
It will automatically convert going up, not coming down. The following line
will compile:

  MyClass abc = (MyClass) obj;

This will compile. At runtime, if obj does not point to an actual MyClass
object, I will get a runtime exception.

Joe Teff

-----Original Message-----
From: owner-java400-l@midrange.com
[mailto:owner-java400-l@midrange.com]On Behalf Of Stone, Brad V (TC)
Sent: Friday, March 02, 2001 4:08 PM
To: 'JAVA400-L@midrange.com'
Subject: Vector question


Ok, here's what I'm trying to do..

public class MyClass extends java.lang.Object
{
  protected boolean isBlue = false;

  public MyClass()
  {
    this.isBlue = false;
  }

  public void setAsBlue()
  {
    this.isBlue = true;
  }
}

public class MyClassList extends java.lang.Object
{
  protected Vector paintOptions = new Vector();

  public MyClassList()
  {
    this.paintOptions.removeAllElements();
  }

  public load()
  {
    MyClass list = new MyClass();
    list.setAsBlue();
    this.paintOptions.addElement(list);
  }
}

Now, I'm trying to do this:

MyClassList list = new MyClassList();
list.load();
if (list.paintOptions.get(x).isBlue)
  blah;

I'm assuming I'm doing something way wrong (if you can even tell what I'm
trying to do).  I'm trying to make a basic class, then make another class
that is a vector of that subclass.  Wait, subclass...  Should I be declaring
the MyClassList as extends MyClass instead of Extends Object?

Brad
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