How can I add method to an existing object in Java?

Question

I'm using a Processing library to build my project in Java. I use a function that returns me an object of type PShape (which I don't have access to source).

I need to make this object of type Shape (a class I designed that extends PShape).

How can I make that?

Basically I have:

PShape pShape = loadShape(filename);

Where loadShape is a function I don't have access to source code.

I want to somehow do:

class Shape extends PShape {...}

and then

Shape shape = (Shape) loadShape(filename);

But it won't work, once loadShape() will give me a PShape, not a Shape

How can I make loadShape returns a Shape?

Thank you

John3136 · Accepted Answer · 2017-07-21 00:29:46Z

7

If loadShape() returns a PShape, then it returns a PShape. You can't make it return a subclass of PShape.

Easiest approach would be Shape either copies the PShape into a new instance: e.g.

Shape myLoadShape(String filename)
{
    return new Shape(loadShape(filename));
    // Assumes you have a `Shape(PShape)` constructor.
}

or perhaps Shape isn't a subclass, but it contains a PShape data member.

class Shape
{
    // No one picked up my C++ syntax goof ;-)
    protected PShape pshape;

    // Using a constructor is just one way to do it.
    // A factory pattern may work or even just empty constructor and a
    // load() method.
    public Shape(String filename)
    {
        pshape = loadShape(filename);
        // Add any Shape specific setup
    }


}

edited Jul 21, 2017 at 0:29

answered Jul 20, 2017 at 23:34

John3136

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10 Comments

Zabuzard Over a year ago

To extend the answer: What differentiates the Shape class from the PShape class? Exactly add that into the above code for creating a Shape object using an existing PShape object.

Gustavo Lopes Over a year ago

I didn't want to use the Decorator Pattern or having the PShape object as a member of Shape. How can I have a Shape(PShape) constructor and PShape not being a member of Shape?

Gustavo Lopes Over a year ago

@Zabuza, I need to add some functionalities to PShape object and use it in other methods (which expects a PShape input and returns a PShape object)

Zabuzard Over a year ago

Well, your desired structure is not totally clear to me. Imagine PShape being a class Animal and Shape a class Dog. It directly follows that Dog inherits all properties of an Animal but a method that acts on Animal will not be able to use the exclusive properties of a Dog as not all Animal can for example bark. However you can indeed pass a Dog into a method that wants Animal. But you will not be able to cast an Animal to a Dog, a Cat is no Dog.

Gustavo Lopes Over a year ago

@Zabuza. It's the opposite I am wanting to achieve. Imagine PShape being a class Dog. I want Shape to be a class Animal. So whenever I create a Dog using the desired method, I want to actually create an Animal, because the Dog class only have bark() method (and I can't edit it) and I want to add a bite() method. (All the Animals I'll have are going to be Dogs)

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