Abstract classes are special classes in Java that can't be instantiated on their own. Instead, you need to create a subclass that extends the abstract class and provides an implementation for the abstract methods. Think of an abstract class as a blueprint for other classes. It can contain both abstract methods and concrete methods (methods with a body). Concrete methods in an abstract class can be used by subclasses as is, but subclasses must override abstract methods. Let's take a look at an example to understand this better. Suppose we have an abstract class Element:

Now, we can create two subclasses Square and Triangle, that extend Element:

You can use the Element class as a blueprint for creating objects of subclasses. The following code will call the drawing method for each object and produce different outputs:

Can you see how abstract classes are making our code more organized and maintainable? By using abstract classes, we can define a common set of methods that all subclasses must implement. There are a few things to know about abstract classes:

• Abstract classes can have non-abstract methods (methods with a body), which subclasses can use;
• In Java, you can't create an object of an abstract class using the new keyword;
• If a class contains at least one abstract method, then the class must be declared as abstract. On the other hand, if a class doesn't contain any abstract methods, then there's no need to declare it as abstract;
• You can extend only one abstract class in Java. However, you can implement multiple interfaces;
• You can also define an abstract class without any abstract methods.

That's it for abstract classes! Let's get to the questions.