In what cases is it more reasonable to use abstract classes in software system design?

Abstract classes in Java are wisely used when you need to create a base class with common implementation for several subclasses, but the base class itself should not be instantiated directly. They are useful if:

• There are common methods with implementation that all inheritors should use.
• You need to define a behavior template, leaving some methods abstract for implementation in subclasses.
• Partial implementation of an interface is required with the possibility of adding state (fields).

For example, if you have a class Animal with common methods eat() and an abstract method makeSound(), then Animal can be made an abstract class, and specific animals implement makeSound() in their own way.