Java interface code error

Question

I can't understand why I'm getting an error on the line indicated below. if there is a specific topic which covers it please provide a line (or an explanation).

public interface Button {
 void press();
}
public class Bomb implements Button {
 public void press() {
 System.out.println("Doesn't work");
 }
 void boom() {
 System.out.println("Exploded");
 }
}
public class Test {
 public static void main(String[] args) {
 Button redButton = new Bomb();
 redButton.press();
 redButton.boom(); // <-- Error is on this line.
 }
}

• awksp: Well, what's the error?
• user3550204: I'm not sure it's just saying line 16 can't run
• awksp: If you're using any decent IDE, the compiler error has got to give more information than that...
• Thilo: even the command line compiler will give a lot more information...

Answer 1

Button redButton = new Bomb();

The interface Button does not define a method boom().

The runtime instance may be a Bomb (which has boom()), but the compiler does not know that (it only sees the compile time type, which is Button).

You need to use the interface defined by class Bomb:

Bomb redButton = new Bomb();

• Thilo: Because there is a method press() in the interface Button.
• Sal Prima: Interface have different concept compared to Inheritance (Super class & Sub class). Interface is used when you need several different implementations of the same method name. In the real life opening coke bottle vs opening the door have different ways, so you need same method name which is open() but the way of opening its different in the other hand, when you need same ways with exactly same method name you may use Inheritance. CMIIW.