Question

I’m having trouble understanding the concept behind abstract classes. The definition I’m reading is that they have at least one method which is declared but lack implementation, thus the class can’t be instantiated. The java calendar class is abstract and can’t be instantiated using the New operator, however there’s a method called getInstance() which returns a calendar object. How does this work?

Calendar cal = new Calendar(); //doesn't work
Calendar cal = Calendar.getInstance(); //works

Answer 1

Abstract classes don’t have to have any abstract methods. That’s just how it’s usually done.

Now Calendar.getInstance() works by actually creating an instance of a subclass. You’re allowed to call it because it’s a static method. The return value refers to an instance of the relevant subclass, but the return type is just Calendar, which is fine due to the normal rules of inheritance.

Here’s an example of the same sort of approach, but without all the complexities of Calendar – the same principles apply:

abstract class AbstractParent {
    public static AbstractParent getInstance() {
        // Normally you'd have code to work out which
        // concrete class to actually use
        return new ConcreteChild();
    }

    public abstract void sayHello();
}

class ConcreteChild extends AbstractParent {
    @Override public void sayHello() {
        System.out.println("Hello from ConcreteChild");
    }
}

public class Test {
    public static void main(String[] args) {
        AbstractParent parent = AbstractParent.getInstance();
        parent.sayHello();
    }
}