What is the role of abstract classes?

Abstract classes are used to capture the common characteristics of subclasses and are used to create templates for subclasses in the inheritance hierarchy. In reality, there is really no need to write some methods in the parent class, because the method in each subclass will definitely be different; instead, write it as an abstract class, so that when you look at the code, you will know that this is an abstract method, and you will know that this method is in the subclass. It is implemented in the class, so it has a prompt effect.

1. What is an abstract class

In the object-oriented concept, all Objects are described by classes, but conversely, not all classes are used to describe objects. If a class does not contain enough information to describe a specific object, such a class is an abstract class.

Except that abstract classes cannot instantiate objects, other functions of the class still exist. Member variables, member methods and constructors can be accessed in the same way as ordinary classes.

Since abstract classes cannot instantiate objects, abstract classes must be inherited before they can be used. For this reason, it is usually decided during the design stage whether to design an abstract class.

The parent class contains common methods of the subclass collection, but because the parent class itself is abstract, these methods cannot be used.

In Java, abstract classes represent an inheritance relationship. A class can only inherit one abstract class, but a class can implement multiple interfaces.

2. Why use abstract classes? What is the use of abstract classes?

I always wonder why we need to reference abstract classes. Aren’t ordinary classes enough? Methods defined in general classes can also be overridden by subclasses. There is no need to define them as abstract.

In fact, this is not to say that abstract classes are of any use. General classes can indeed satisfy applications. However, in reality, there are indeed some methods in parent classes that are not necessary to write, because the methods in each subclass will definitely be different. , so there is no need to write it in the parent class. Of course, you can also write all abstract classes as non-abstract classes, but this is not necessary.

And write it as an abstract class, so that when others see your code, or you see other people's code, you will pay attention to the abstract method and know that this method is implemented in the subclass, so there is Prompt function.

3. The difference between abstract classes and interfaces

Abstract classes

Abstract classes are used To capture the common characteristics of subclasses. It cannot be instantiated and can only be used as a superclass for subclasses. Abstract classes are templates used to create subclasses in the inheritance hierarchy.

Interface

An interface is a collection of abstract methods. If a class implements an interface, then it inherits the abstract methods of this interface. This is like the contract pattern, if you implement this interface, then you must ensure that these methods are used. An interface is just a form, it can't do anything by itself.

Comparison of abstract classes and interfaces

When to use abstract classes and interfaces

• If you have some methods and want some of them to have default implementations, then use an abstract class.
• If you want to implement multiple inheritance, then you must use interfaces. Since Java does not support multiple inheritance, subclasses cannot inherit multiple classes, but they can implement multiple interfaces. So you can use interfaces to solve it.
• If the basic functionality is constantly changing, then you need to use abstract classes. If you keep changing the basic functionality and use an interface, you will need to change all classes that implement the interface.
• If there is an interface and five implementation classes, the current requirement may be to add a method to the interface, which will require changing the five implementation classes, but the requirement only needs to change two of the implementation classes, which can be defined again An abstract class implements this interface, adds this method in the abstract class, and then the other two implementation classes implement this abstract class, or uses the new features in Java 8 to add default methods or static methods in the interface .

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