Doubt on Dynamic polymorphism

The following program illustrates dynamic polymorphism by passing required object at run time by "command line arguments". The program is very simple finding out area of rectangle and square using Hierarchial inheritance.(Shape is super class)

MY Problem is that in the last second line of the code " x = args[0] " I am trying to pass the object (square) using command line arguments. But the error is getting displayed as "incompatible type one is string and other is object"
Kindly someone help me .
Thanks
// I did not post full code //
class Shapedemo
{
public static void main(String args[])
{
Shape x;
Rectangle rectangle = new Rectangle();

Square square = new Square();

rectangle.setvalue();
square.input();

x= args[0];
x.area();


}
}
[ December 11, 2008: Message edited by: Martin Arun Paul Perumala ]

Welcome to JavaRanch. Please use code tags when you post code.

args is String that is declared in the main.(i dont know what do you mean by saying that you are sending object through Command line)
X is a shape class reference.
Now string class does not extend shape.
So String class and Shape class has no relation.
So the error pops.
[ December 11, 2008: Message edited by: James Tharakan ]

I mean I am trying to execute the program like

" Java Shapedemo square" where I am passing square object as commandline argument.

Or in another way can a object be passed as a commanline argument ?

without commandline argument the code would look like

x=rectangle;
x.area();

x=square;
x.area();

I want to calculate Square area only(not both) at run time..how can I do it ?
Thanks .
[ December 11, 2008: Message edited by: Martin Arun Paul Perumala ]

i totally understood what is your doubt and what is that you are trying to do.
But i really dont know how to explain that to you.
And as far as i know It is impossible

args[] is declared as String in the main.
So what ever argument you send to main would be taken as String.
No matter what you send (shape or rectangle object as you are thinking) it would consider it as a string.
[ December 11, 2008: Message edited by: James Tharakan ]

Thanks anyways.. Atleast now I know that it is impossible. loll

There are ways of passing Rectangle as a command-line argument and getting a Rectangle object; you can use a Map<String, Shape> or a factory method which takes the name of the class as a parameter.

Dear Campbell,
Can you explain it how with a simple example?
Because even if we pass map , it would be taken as only strings in main method right?
Regards.

You would have to populate the Map before starting the application.

Not impossible, but hard... And you need to know a thing or two about how Java works behind the scene.

Take a look at this code.

First, the abstract Shape-class.
package lime; public abstract class Shape { public Shape(){ System.out.println("Constructing a shape"); } public abstract int area(); }

Then the implementing Rectangle class.

package lime; public class Rectangle extends Shape{ public Rectangle(String aHeight, String aWidth){ System.out.println("Creating a Shape called Rectangle"); System.out.println("Height: " + aHeight); System.out.println("Width: " + aWidth); } public int area() { System.out.println("Calculating the area of a rectangel (Area 42)"); return 42; } }

And now the magic... Comments in code...

package lime; import java.lang.reflect.Constructor; import java.lang.reflect.InvocationTargetException; class ShapeDemo { public static void main(String args[]) { if (args.length != 3) { System.out.println("Usage: java ShapeDemo <java class name> <height> <width>"); System.exit(-1); } // Read the arguments. String aClassName = args[0]; String aHeight = args[1]; String aWidth = args[2]; // Convert the width and height to try { // Use the classloader to load the named class. Sort of the same as when you type java lime.ShapeDemo. Class theShapeClass = ClassLoader.getSystemClassLoader().loadClass(aClassName); // Get the constructor that takes two strings. Constructor aConstructor = theShapeClass.getConstructor(String.class, String.class); // Create a new instance. Shape x = (Shape) aConstructor.newInstance(aHeight, aWidth); // Calculate the area. x.area(); } catch (ClassNotFoundException e) { e.printStackTrace(); } catch (NoSuchMethodException e) { e.printStackTrace(); } catch (InvocationTargetException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } catch (InstantiationException e) { e.printStackTrace(); } } }

Thanks Lindstrom I appreciate that..

Originally posted by Martin Arun Paul Perumala:
Thanks Lindstrom I appreciate that..

YW... But still... it is easier to take a string that represents a shape and then look up the shape in ha Map of some sort.

while calling the main method with the Rectangle class, campbell said to populate the map;
This is what I've done:
package lime; import java.util.*; public class Map4Rectangle extends HashMap { public static void main(String[] args) { Rectangle r=new Rectangle("20","34"); HashMap h=new HashMap(); h.put("rectangle",r); } }
output:
C:\sai>java lime.ShapeDemo lime.Map4Rectangle In main method Usage: java ShapeDemo <java class name> <height> <width>

Regards.

Let me tell you this once more, and VERY SLOWLY.

You can NOT send in an OBJECT from the command prompt!!!

The only thing you can do is send in Strings and then react depending on the what string that was sent in.

My very, almost, pseudo-code take a name of a java class, a height and a width AS STRINGS!

What you should do is create a map that has a key-value pair, where the key is the string that you want the user to sent into the program from the command prompt and the value is the class/object that you want to use.

So if you populate the map like this:
Map<String, Class<? extends Shape> myShapes = new HashMap<String, Class<? extends Shape>();
myShapes.put("rectangle", Rectangle.class);
myShapes.put("circle", Circle.class);

then you can get the class from the map and then create a new instance of that class.


EDIT:

Also...

Originally posted by ramya narayanan:
package lime; import java.util.*; public class Map4Rectangle extends HashMap { public static void main(String[] args) { Rectangle r=new Rectangle("20","34"); HashMap h=new HashMap(); h.put("rectangle",r); } }
output:
C:\sai>java lime.ShapeDemo lime.Map4Rectangle In main method Usage: java ShapeDemo <java class name> <height> <width>

Regards.

This it totaly rubbish. When starting a Java application you send in ONE and only ONE java class as argument and that is the one with the main-method. From there you should create any other objects.

Also, if you extend a HashMap, you are a HashMap and there is no need to instanciate another HahsMap.

My suggestion is that you take two steps back and start over again with the basics of Java.
[ December 11, 2008: Message edited by: Ove Lindstr�m ]

If not, then how can you populate a map & send it at run time as an commandline argument.
Regards.

Originally posted by ramya narayanan:
If not, then how can you populate a map & send it at run time as an commandline argument.
Regards.

Do you understand how a map works? It has a key that is unique and value that is some sort of object. The key can be a String. The value a class. Then you can send in a STRING from the commandline and lookup the class in the map, using the STRING that you got from the commandline as the key.

And I'm not yelling. My tone of voice just guarantee that I'm not being misunderstood or ignored...

Thanks & now I have understood.
Regards.