How do the ways of using abstract classes differ in various versions of Java and how are they different?

Abstract classes in Java are used to create base classes that cannot be instantiated directly but can be inherited from. In earlier versions of Java (before Java 8), abstract classes could contain abstract methods and concrete methods with implementations.

With the introduction of Java 8, default methods appeared in interfaces, which somewhat blurred the lines between interfaces and abstract classes. However, abstract classes can still contain state (fields), which interfaces do not support.

In Java 9, private methods were added to interfaces, improving encapsulation, but this did not fundamentally change the role of abstract classes.

Thus, abstract classes are used for common code and state, while interfaces are used to define contracts. In newer versions of Java, interfaces have become more functional, but abstract classes retain their role for inheritance with implementation and state.