[3197] in java-interest
Object Creation using Class.newInstance()
daemon@ATHENA.MIT.EDU (Suresh Srinivasan)
Thu Nov 2 21:28:01 1995
Date: Thu, 2 Nov 1995 17:14:33 -0500
From: suresh@thomtech.com (Suresh Srinivasan)
To: java-interest@java.sun.com
Hello all:
I'm not a OO programmer (yet!) so these may be easy to answer.
Suppose I have an abstract class say Shape which is subclassed
into Rect, Arc, Ellipse, etc. somewhat analogous to the GraphicsTest
demo.
abstract class Shape {
abstract void draw();
abstract void fill();
...
}
class Rect extends Shape {
void draw() {...}
void fill() {...}
}
And I want to stuff an array with Shapes so I can call its abstract
methods without worrying about the type of object it represents.
The type of shape I want to put in is specified in another array
containing the string representation of the subclass name as in:
String shapeNames[] = {"Rect", "Arc", "Rect", "Ellipse"};
Shape shapeArray[] = new Shape[shapeNames.length];
for (int i=0; i<shapeNames.length; i++) {
Class c = Class.forName(shapeNames[i]);
shapeArray[i] = new (Shape)c; // option A
shapeArray[i] = (Shape) c.newInstance(); // option B
}
Option A generates a compile time error of the form:
'new(...)' not supported.
Option B compiles OK, except my shape constructors take arguments
and the newInstance method clearly does not.
Anybody see a way to do this more elegantly?
Another related question, is there a notion of abstract instance
variable? Can I access an instance variable of Rect, say, if I
have a Shape reference? (I'm assuming not). For example:
for (int i=0; i<shapeNames.length; i++) {
shapeArray[i].size += 1;
}
Must 'size' be an instance variable in Shape, an abstract class?
Thanks for reading this and any help you can provide,
--Suresh
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