Class is not Abstract and does not Override error in Java

Question

I am getting a compile time error with Java:

MyClass is not abstract and does not override abstract method
onClassicControllerRemovedEvent(
wiiusej.wiiusejevents.wiiuseapievents.ClassicControllerRemovedEvent)
in wiiusejevents.utils.WiimoteListener)

Here is the class:

import wiiusej.WiiUseApiManager;
import wiiusej.Wiimote;
import wiiusej.wiiusejevents.physicalevents.ExpansionEvent;
import wiiusej.wiiusejevents.physicalevents.IREvent;
import wiiusej.wiiusejevents.physicalevents.MotionSensingEvent;
import wiiusej.wiiusejevents.physicalevents.WiimoteButtonsEvent;
import wiiusej.wiiusejevents.utils.WiimoteListener;
import wiiusej.wiiusejevents.wiiuseapievents.DisconnectionEvent;
import wiiusej.wiiusejevents.wiiuseapievents.NunchukInsertedEvent;
import wiiusej.wiiusejevents.wiiuseapievents.NunchukRemovedEvent;
import wiiusej.wiiusejevents.wiiuseapievents.StatusEvent;


public class MyClass implements WiimoteListener{

    public void onButtonsEvent(WiimoteButtonsEvent arg0) {
        System.out.println(arg0);
        if (arg0.isButtonAPressed()){
            WiiUseApiManager.shutdown();
        }
    }

    public void onIrEvent(IREvent arg0) {
        System.out.println(arg0);
    }

    public void onMotionSensingEvent(MotionSensingEvent arg0) {
        System.out.println(arg0);
    }

    public void onExpansionEvent(ExpansionEvent arg0) {
        System.out.println(arg0);
    }

    public void onStatusEvent(StatusEvent arg0) {
        System.out.println(arg0);
    }

    public void onDisconnectionEvent(DisconnectionEvent arg0) {
        System.out.println(arg0);
    }

    public void onNunchukInsertedEvent(NunchukInsertedEvent arg0) {
        System.out.println(arg0);
    }

    public void onNunchukRemovedEvent(NunchukRemovedEvent arg0) {
        System.out.println(arg0);
    }

    public static void main(String[] args) {
        Wiimote[] wiimotes = WiiUseApiManager.getWiimotes(1, true);
        Wiimote wiimote = wiimotes[0];
        wiimote.activateIRTRacking();
        wiimote.activateMotionSensing();
        wiimote.addWiiMoteEventListeners(new MyClass());
    }
}

Can I get a better explanation of what this error means?

Answer 1

Your class implements an interface WiimoteListener, which has a method onClassicControllerRemovedEvent. However, the methods in interfaces are abstract, which means they are essentially just contracts with no implementations. You need to do one of the things here:

1. Implement this method and all the other methods that this interface declares, which make your class concrete, or
2. Declare your class abstract, so it cannot be used to instantiate instances, only used as a superclass.

Answer 2

How to reproduce that error as simply as possible:

Java code:

package javaapplication3;
public class JavaApplication3 {
    public static void main(String[] args) {
    }
}
class Cat implements Animal{

}

interface Animal{
    abstract boolean roar();
}

Shows this compile time error:

Cat is not abstract and does not override abstract method roar() in Animal

Why won't it compile?

Because:

1. You created a class Cat which implements an interface Animal.
2. Your interface called Animal has an abstract method called roar which must be overridden.
3. You didn't provide for method roar. There are many ways to eliminate the compile time error.

Remedy 1, have Cat override the abstract method roar()

package javaapplication3;

public class JavaApplication3 {
    public static void main(String[] args) {
        Cat c = new Cat();
        System.out.println(c.roar());
    }
}
class Cat implements Animal{ 
    public boolean roar(){
        return true;
    }
}

interface Animal{
    abstract boolean roar();
}

Remedy 2, change Cat to be an abstract like this:

package javaapplication3;

public class JavaApplication3 {
    public static void main(String[] args) {
        Cat c;
    }
}
abstract class Cat implements Animal{ 

}
interface Animal{
    abstract boolean roar();
}

Which means you can't instantiate Cat anymore.

Remedy 3, have cat stop implementing Animal

package javaapplication3;

public class JavaApplication3 {
    public static void main(String[] args) {
        Cat c = new Cat();
    }
}
class Cat{ 

}

interface Animal{
    abstract boolean roar();
}

Which makes roar() no longer a contract for things that animals must know how to do.

Remedy 3, extend a class rather than implementing an interface

package javaapplication3;

public class JavaApplication3 {
    public static void main(String[] args) {
        Cat c = new Cat();
        System.out.println(c.roar());
    }
}
class Cat extends Animal{ 

}
class Animal{
    boolean roar(){
        return true;
    }
}

The remedy to use depends on what the best model is to represent the problem being represented. The error is there to urge you stop "programming by brownian motion".