Using an interface method for an object of one data type in an array of a different data type? Java

Question

So my problem is that I have a class called GeometricFigure2 which holds fields such as width and height. I have an interface called SidedObject which holds a method to display how many sides a figure has

public interface SidedObject
{
    public void displaySides();
}

I have two subclasses called Square2 and Triangle2 which extend GeometricFigure2 and implement SidedObject. Both classes contain the displaySides() method which looks like:

public void displaySides()
{
   System.out.println("The square has 4 sides.");
}

Finally I have a class called UseGeometricFigure2 which uses both subclasses. I create an array with a type of GeometricFigure2 which is used to hold two Square2 objects and two Triangle2 objects:

GeometricFigure2[] geoRef = new GeometricFigure2[4];
    geoRef[0] = new Square2();
    geoRef[1] = new Square2();
    geoRef[2] = new Triangle2();
    geoRef[3] = new Triangle2();

I then create a for loop to iterate through the array and call the displaySides() method for each object in the array:

for(int i=0; i<4; i++)
{
    geoRef[i].displaySides();
}

The problem is when I try to compile it gives me a "Cannot find symbol" error. It is looking for displaySides() in the GeometricFigure2 class which is the array type. How do I correctly call the displaySides() method in this setup?

Robby Cornelissen · Accepted Answer · 2014-11-13 01:28:58Z

1

You have three choices:

Have GeometricFigure2 implement your SidedObject interface.
Declare your array as type SidedObject[] instead of GeometricFigure2[].
Cast your array variables to SidedObject:
((SidedObject) geoRef[i]).displaySides();

answered Nov 13, 2014 at 1:28

Robby Cornelissen

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1 Comment

Thomas Wegiel Over a year ago

The first option works best for me as I have other methods in the GeometricFigure2 class. Thank you!

Elliott Frisch · Accepted Answer · 2014-11-13 01:28:51Z

0

For the displaySides() method from SideObject to work with instances of GeometricFigure2 you would need to modify GeometricFigure2 to implement SidedObject.

answered Nov 13, 2014 at 1:28

Elliott Frisch

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