Generics Problem

[robbie.26]

I'm trying to set/get values to/from a Property<T> class.
I can set a Property's value like this: myNode.getProperty( "My_Prop" ).setValue( 0.25 ).
However, I cannot retrieve a Property's value without having to cast.

I want to be able to do this: Double dbl = myNode.getProperty( "My_Prop" ).getValue().
For some reason I cannot figure out, getValue() is returning an Object instead of a Double.

Any guidance is appreciated.

public class Property<T>
{
	public Property( String id, T initialValue )
	{
		this.id = id;
		value = initialValue;
	}

	public T getValue()
	{
		return value;
	}

	public void setValue( T newValue )
	{
		value = newValue;
	}

	public String getID()
	{
		return id;
	}

	private T value;
	private String id;
}

import java.util.*;

public abstract class Node<T>
{
	public Node()
	{
		properties = new ArrayList<Property>();
		inputs = new ArrayList<Node>();
	}

	public abstract T process();

	public Property getProperty( String id )
	{
		for ( Property prop : properties )
		{
			if ( prop.getID().equals( id ) )
				return prop;
		}
		return null;
	}

	public ArrayList<Property> properties;
	public ArrayList<Node> inputs;
}

import java.awt.image.*;
import javax.imageio.*;
import java.io.File;

public class ImageRead extends Node<BufferedImage>
{
	public ImageRead()
	{
		super();
		Property<String> imageFile = new Property<String>( "Image File", "bitmap.jpg" );
		properties.add( imageFile );
	}

	public BufferedImage process()
	{
		return null;
	}
}

public class Tester
{
	public static void main( String[] args )
	{
		ImageRead read = new ImageRead();
		
        read.getProperty( "Image File" ).setValue( "testImage.jpg" );          //works
        
        String str = read.getProperty( "Image File" ).getValue();              //doesn't work
        
        String str2 = (String) read.getProperty( "Image File" ).getValue();    //works with cast		
	}
}

[doWhile]

Where do you define the generic type for the Property objects contained in the Node class?

[robbie.26]

Originally I defined all the Property objects in Node as Property<?> , but that bombarded me with exceptions like this:

C:\Users\Robbie\Desktop\Reaction\Reaction.java:6: error: method setValue in class Property<T> cannot be applied to given types;
		read.getProperty( "Image File" ).setValue( "testImage.jpg" );
		                                ^
  required: CAP#1
  found: String
  reason: actual argument String cannot be converted to CAP#1 by method invocation conversion
  where T is a type-variable:
    T extends Object declared in class Property
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Object from capture of ?

[doWhile]

A wildcard (?) should extend something.

One way to get the result you are after would be to define Node to have 2 generic types, the second of which is assigned to Property.

public abstract class Node<T,V>{
    ...
    properties = new ArrayList<Property<V>>();
    ...

    public Property<V> getProperty()
    ....
}

[robbie.26]

A wildcard (?) should extend something.

One way to get the result you are after would be to define Node to have 2 generic types, the second of which is assigned to Property.

public abstract class Node<T,V>{
    ...
    properties = new ArrayList<Property<V>>();
    ...

    public Property<V> getProperty()
    ....
}

What if I need a Node to hold a collection of different types of Properties?

[doWhile]

What if I need a Node to hold a collection of different types of Properties?

What if? I gave you a solution - add 2 generic types....did you try to implement that solution into your code?

[robbie.26]

What if? I gave you a solution - add 2 generic types....did you try to implement that solution into your code?

When I define a sub-class of Node like ImageRead, I would need to pass in a second type:

public class ImageRead extends Node<BufferedImage, String> { ... }

Wouldn't this limit the properties ArrayList of an ImageRead object to contain only Properties of type String?

[doWhile]

Wouldn't this limit the properties ArrayList of an ImageRead object to contain only Properties of type String?

Yes, which is exactly what you are doing in the constructor of ImageRead.

[robbie.26]

Ok. Well I need a Node to have an ArrayList of any type of Property. How would I do that?

[doWhile]

Ok. Well I need a Node to have an ArrayList of any type of Property. How would I do that?

I just told you - specify Node as having 2 generic types, one for the object contained in Node and one for the Parameter type. I recommend you try, and post the code if you have problems.