Question

I am trying to understand Java interfaces like the millions of others around the world. How do I test that I’m really using my interface? If I remove the “implements” in my TestBubbles class I still get the same results. I can change either method definition and get a compilation failure but how do I test the data I’m passing?

public interface Bubbles {
   public void addAir(String bubbleType, float bubbleOne, float bubbleTwo );
}

---

public class TestBubbles implements Bubbles {

    public static void main(String [] args){
      String type = "wiggly";
      float sizeOne = 42.01f;
      float sizeTwo = 80.10f;

      TestBubbles tb = new TestBubbles();
      tb.addAir(type, sizeOne, sizeTwo);

}


   public void addAir(String rType, float fOne, float fTwo ){
       System.out.println(rType + " " + fOne + " " + fTwo);

   }

}

Answer 1

You test that, and you should code that by defining your variables with the type of the interface instead the type of your class.
Look at your line:

TestBubbles tb = new TestBubbles();
tb.addAir(type, sizeOne, sizeTwo);

When programming to interfaces that should be coded as follows:

Bubbles tb = new TestBubbles();
tb.addAir(type, sizeOne, sizeTwo);

That way you later could exchange TestBubbles with SuperTrooperTestBubbles which implements the same Interface by only changing one line of code, and the rest will work:

Bubbles tb = new SuperTrooperTestBubbles();
tb.addAir(type, sizeOne, sizeTwo);

Where

public class SuperTrooperTestBubbles implements Bubbles { ....