Can you give an example of why exactly a Java abstract method cannot exist in a non-abstract class?

I'd like a real-world type example that illustrates why exactly a Java abstract method cannot exist in a non-abstract class.

I appreciate the reasons why that can't happen - abstract classes forcing implementation of any abstract methods contained in them - but an understandable concrete example would really help me reason it out in my head, many thanks.

One of the key differences between an abstract class and a non-abstract class is the fact that you cannot create an instance of an abstract class. This prevents a situation where a method with no definition gets called.

So if we had the following abstract class:

abstract class Elephant {    // abstract class

    public String getName() {
        return "Ollie";
    }

    public abstract void walk();   // abstract method

}

Then the following would be illegal:

Elephant e = new Elephant();   // error

Because Elephant is abstract and it cannot be instantiated.

But say for argument sake, that we make Elephant non-abstract but it is allowed to still have the abstract method walk(). i.e. we change the definition of Elephant to:

class Elephant {    // non-abstract class

    public String getName() {
        return "Ollie";
    }

    public abstract void walk();    // abstract method

}

Now what should happen if I do the following:

Elephant e = new Elephant();   // legal since Elephant is non-abstract
e.walk();   // oops, we didn't define this anywhere

Java compiler won't allow this. You could argue that there are other ways to handle a situation like this, but this simply how the Java language has decided to implement the feature.