Why does this code print "Woof" twice when executed even though I only called super.speak() once?

public class Dog
{
    public void speak()
    {
        System.out.println("woof!");
    }

    public static void main(String[] args)
    {
        Dog d = new Dog();
        d.speak();
        Dog b = new Beagle();
        b.speak();
    }
}

class Beagle extends Dog
{
    public void speak()
    {
        super.speak();
        System.out.println("arf arf");
    }
}

This is super confusing, because I am not sure why the speak method of the super class is being run twice even if I called it just once. The final result is "Woof \n Woof \n arf arf".

> I am not sure why the speak method of the super class is being run twice even if I called it just once.*.

You call it twice.

1. You call it directly when you invoke d.speak();
2. Then you call it indirectly when you invoke b.speak() since b.speak() first invokes super.speak().

Edit:

> because we are calling b.speak, d.speak -

b.speak() does NOT display "woof" directly. There is no println(...) statement to display "woof".

It invokes super.speak(). The super.speak() simply means execute the code in the speak() method of the parent class which happens to be the Dog class. So "woof" is displayed.

Then it displays "arf arf".

So:

1. d.speak() causes "woof" to be displayed.
2. b.speak() causes "woof" and "arf arf"to be displayed.

Change your code to be:

System.out.println("Dog is about to speak:");
d.speak();
Dog b = new Beagle();
System.out.println("Beagle is about to speak:");
b.speak();

to see the output as described above.

See annotations in comments.

public static void main(String[] args)
{
    Dog d = new Dog();
    d.speak();  // calls Dog.speak() -> prints "woof!"
    Dog b = new Beagle();
    b.speak(); // calls Beagle.speak() -> 
               // prints "woof!" via call to super.speak() 
               // and then "arf arf"
} 

Basically you are making the call twice when you invoque super.speak() in Beagle class. If you want to eliminate the duplicate call, you might want to delete the super.speak() call. When you call super, you are invoking the super class method.

public class Main {

    public static void main(String[] args) {
        Dog d = new Dog();
        d.speak();
        Dog b = new Beagle();
        b.speak();

    }
}

class Dog
{
    public void speak()
    {
        System.out.println("woof!");
    }

}

class Beagle extends Dog
{
    @Override
    public void speak()
    {
//        super.speak();
        System.out.println("arf arf");
    }
}

Also, you might want to annotate the Beagle.speak method with @Override, so that is understandable that you are overriding the method from the super class.

Side Note:
I have also separated the main method from the Super class... If you are getting started with Java programming, I would take a look at SOLID principles, first one been "S" for Single Responsibility. Basically this means that every class and method should have only one responsibility.