how to implement method of an abstract class in our class if our class already extends some other class

Question

In android, MainActivity already extends Activity.

Now I want to use an abstract class say CountdownTimer.

How can I do that because MainActivity already extends one class and it cannot extend another.

Answer 1

You should use interfaces for this instead of abstract classes, since you can implement multiple interfaces or even implement multiple interfaces and extend a class.

The only caveat is that interfaces must have only abstract methods.

Answer 2

Composition over inheritance

Short answer, you don't need to extend another class.

Android needs an Activity to display an interface. Your activity can "compose" a CountDownTimer

public class MainActivity extends AppCompatActivity {
    private CountDownTimer timer;

    @Override
    protected void onCreate(Bundle b) {
        super.onCreate(b);
        setContentView(R.layout.activity_main);

        timer = new CountDownTimer(...);
    }
}

Answer 3

Multiple inheritance is not allowed in Java. So literally you can extend only one class. Consider changing your abstract class to an interface since you can implement multiple interfaces.

Answer 4

In Java multiple inheritance is not permitted. It was excluded from the language as a design decision, primarily to avoid circular dependencies

Scenario 1: As you have learned the following is not possible in Java:

public class Dog extends Animal, Canine{

}

Scenario 2: However the following is possible:

public class Canine extends Animal{

}

public class Dog extends Canine{

}

The difference in these two approaches is that in

The second approach there is a clearly defined parent or super class,

while in the first approach the super class is ambiguous.

Consider if both Animal and Canine had a method drink().

• Under the first scenario which parent method would be called if we called Dog.drink()?

• Under the second scenario, we know calling Dog.drink() would call the Canine classes drink method as long as Dog had not overridden it