Interface and abstract classes are important in making polymorphism work in object-oriented programming (OOP). They do this in different ways.

First, let's talk about interfaces. An interface is like a rulebook that different classes must follow. It allows different classes to use the same methods, which helps with polymorphism. This means we can use different classes in the same way. For example, if we have an interface called Drawable, both a Circle class and a Square class can follow this interface. So, when we use these classes in a program, we can treat them the same way, like this:

• A Circle and a Square can both be considered Drawable.
• We can use them with a single type of reference, which makes our code more flexible.

Now, let's look at abstract classes. These classes act as a starting point for other classes to build upon. They can have complete methods (which work) and incomplete methods (which need more work). This setup lets subclasses share some code while still needing to create their own versions of certain methods. Here are the benefits:

• We can write common code in the abstract class, so we don’t have to repeat ourselves.
• Each subclass can change the way it works, which also supports polymorphism.

In summary, interfaces set clear rules about how things should behave, while abstract classes let us share some code while still requiring custom changes. Both ideas let us treat objects from different classes like they are part of a common group. This makes it easier to reuse and maintain our code in OOP.